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UNIVERSITY OF SOUTHAMPTON

FACULTY OF NATURAL AND ENVIRONMENTAL SCIENCES

SCHOOL OF CHEMISTRY

Selective Lithium Extraction from Salt Solutions by Chemical

Reaction with FePO4

by

Noramon Intaranont

Thesis for the degree of Doctor of Philosophy

September 2015

UNIVERSITY OF SOUTHAMPTON

ABSTRACT FACULTY OF NATURAL AND ENVIRONMENTAL SCIENCES

School of Chemistry

Thesis for the degree of Doctor of Philosophy

Selective Lithium Extraction from Salt Solutions by Chemical

Reaction with FePO4

Noramon Intaranont

The spectacular increase in lithium battery applications has raised the question

of whether global lithium resources will be enough in the future. Experts in

the field have estimated that the existing lithium resources will probably be

sufficient to support demands until the year 2100, assuming that lithium

batteries are recycled. Without lithium batteries being recycled, the resources

are expected to be depleted in 50 years’ time. Therefore, there is a great

interest in developing better methods of lithium recycling from batteries, and

also, better methods of lithium extraction from natural resources.

Currently, lithium is extracted from natural brines via the lime soda

evaporation process, i.e. a solar evaporation plus chemical plant process,

which takes between 12 and 24 months. The drawbacks of this process are

that it is complex, slow and inefficient. Also, the currently available methods of

lithium recycling from batteries are too complex and expensive. Thus, the

main objective of this work is to develop a novel, inexpensive and less time-

consuming approach to recover lithium chemically, from the lithium salts

(lithium sources) that contain other metal cations. The new process is also

based on environmental concerns.

A battery material, lithium iron phosphate (LiFePO4) has the olivine structure

and heterosite structure once it discharges to iron phosphate (FePO4). This

structure shows excellent properties of the charge/discharge reversibility. A

few studies on the heterosite FePO4 have reported that it is more selective for

lithium ions (Li+) over other cations. The main advantages of this structure are

the small potential differences of the redox couple, i.e. Fe(II)/Fe(III), and the

stability of LiFePO4 over a wide range of acid-based conditions in an aqueous

solution.

This work investigates a novel process that may be superior to the lime soda

evaporation process for extracting lithium. Heterosite FePO4 was employed to

selectively remove Li+ from lithium sources with the support of a reducing

agent, i.e. sodium thiosulphate (Na2S

2O

3). The resulting LiFePO

4 can be directly

sent not only to lithium battery industries, but also to other industrial uses. In

principle, the other cations could be retrieved back into their sources.

The novel process was examined and demonstrated lithium insertion into a

heterosite FePO4, working as a framework, in aqueous salt solutions. The

evaluation of this process is presented by the Li+ uptake value. The amount of

Li+ uptake can be up to 46 mgLi

+/gsolid

where other cations (i.e. sodium,

potassium, and magnesium) can take less than 3 mg/gsolid,

using this process.

Furthermore. This work could also be developed for future lithium recycling

processes.

i

Table of Contents

ABSTRACT .......................................................................................................................... i

Table of Contents ........................................................................................................... i

List of tables .................................................................................................................... v

List of figures ............................................................................................................... vii

DECLARATION OF AUTHORSHIP ......................................................................... xv

Acknowledgements ................................................................................................. xvii

Definitions and Abbreviations ............................................................................ xix

Chapter 1: Introduction ........................................................................................ 1

1.1 The Availability of Lithium-Present and Future ............................ 1

1.1.1 Consumption, Production and Price of Lithium ................ 1

1.1.2 Resources and Supply ..................................................... 4

1.2 Lithium Extraction from Brine ..................................................... 6

1.2.1 Lime Soda Evaporation .................................................... 6

1.2.2 Ion-sieve spinel type ........................................................ 6

1.2.3 LiFePO4 Electrochemical ................................................... 7

1.3 A Review of LiFePO4 .................................................................... 9

1.4 Thermodynamic Principles for the Selection of Suitable Oxidizing and Reducing Agents ................................................................ 12

1.4.1 Oxidizing Agent ............................................................ 12

1.4.2 Reducing Agent ............................................................. 13

1.5 Outline of the Thesis ................................................................ 18

1.6 References ............................................................................... 18

Chapter 2: Experimental Techniques ......................................................... 23

2.1 Powder X-ray Diffraction ........................................................... 23

2.2 Electrochemical Technique ....................................................... 28

2.2.1 Cyclic Voltammetry ....................................................... 28

2.2.2 Potentiometric Titration ................................................ 33

2.3 Inductively Coupled Plasma Mass Spectroscopy ........................ 35

ii

2.4 References ............................................................................... 38

Chapter 3: Physical and Electrochemical Characterization of the

LiFePO4 and FePO

4 system .................................................................... 41

3.1 Experimental Details ................................................................ 41

3.2 Results and discussion ............................................................. 44

3.3 References ............................................................................... 54

Chapter 4: Test of K2S

2O

8 as an Oxidising Agent of LiFePO

4 to

FePO4 55

4.1 A Study of the Chemical Delithiation of LiFePO4 in Aqueous

Solutions .................................................................................. 55

4.1.1 Introduction .................................................................. 55

4.1.2 Experimental Details ..................................................... 55

4.1.3 Results and Discussion .................................................. 56

4.2 Analysis of the Delithiation rate using a Conductivity Measurement ........................................................................... 66

4.2.1 Introduction .................................................................. 66



4.3 References ............................................................................... 72

Chapter 5: Test of LiI and Na2S

2O

3 as Reducing Agent of FePO

4 to

LiFePO4 ............................................................................................................ 75

5.1 Introduction ............................................................................. 75

5.2 Experimental Details ................................................................ 76

5.3 Results and Discussion ............................................................. 79

5.4 References ............................................................................... 86

Chapter 6: Kinetic Studies of the Chemical Lithiation of FePO4 by

Na2S

2O

3 ............................................................................................................ 87

6.1 A preliminary study of the effect of varying the concentrations of both Li+ and S

2O

3

2- together ....................................................... 87

6.1.1 Introduction .................................................................. 87



iii

6.2 Kinetics of the Chemical Lithiation of FePO4 using Na

2S

2O

3,

varying the concentration of S2O

3

2- and Li+ independently ... 100

6.2.1 Introduction and Theory .............................................. 100

6.2.2 Results and Discussion ................................................ 101

6.3 References ............................................................................. 125

Chapter 7: Selectivity for Li+ versus Other Cations Using the S2O

3

2-

/ FePO4 Reagents .................................................................................... 127

7.1 Chemical Lithiation of FePO4 from Aqueous Solutions Containing

an Excess of Na+ and Mg2+ ....................................................... 127

7.1.1 Experimental Details ................................................... 128


7.2 Chemical Lithiation of FePO4 from Synthetic Brine Solutions.... 134

7.2.1 Experimental details .................................................... 135


7.3 References ............................................................................. 140

Chapter 8: Alternative Reducing and Oxidising Agents ................ 141

8.1 Introduction ........................................................................... 141

8.2 Experimental details ............................................................... 142

8.3 Results and discussion ........................................................... 144

8.4 References ............................................................................. 149

Chapter 9: Conclusions and Future Work .............................................. 151

9.1 Conclusions ............................................................................ 151

9.2 Future work ............................................................................ 153

9.3 References ............................................................................. 155

Appendix A ................................................................................................................. 157

Appendices ................................................................................................................. 159

Bibliography ............................................................................................................... 161

v

List of tables

Table 1.1: Lithium consumption application changes between 2003 and 2011.1

Table 1.2: The comparison of cations between the concentration of cations in

brine and seawater ................................................................... 5

Table 1.3: The comparison of ionic fractions (Li+ vs. other cation from Table

1.2) between the concentration of cations in brine and seawater5

Table 1.4: Properties of possible lithium ion movements. .............................. 12

Table 1.5: Ionic radii of the potential cations that can be intercalated into

FePO449 .................................................................................... 13

Table 1.6: The unit cell parameters of FePO4, LiFePO

4, and NaFePO

4 products

based on XRD data52 ............................................................... 14

Table 2.1: The composition of an active material ink ..................................... 32

Table 2.2: The composition of an electrode ................................................... 34

Table 3.1: The composition of LiFePO4 and FePO

4 as an active material ......... 43

Table 3.2: the lattice parameter of the fitting pattern and the reference2 ....... 47

Table 3.3: Metal ions contained in LiFePO4 and the heterosite FePO

4 .............. 49

Table 3.4: A specific charge of each percentage Li content for validation ...... 52

Table 4.1: Chemical composition of the delithiation experiment by the use of

K2S

2O

8 as an oxidizing agent .................................................... 56

Table 4.2: Stoichiometric coefficient of x in LixFePO

4 samples obtained by

delithiation of LiFePO4 from each experiment at 30 m, 1 h, 2 h, 4

h, and 24 h ............................................................................. 64

Table 6.1: Concentration of Li2SO

4, Na

2S

2O

3, and FePO

4 in each sample .......... 88

Table 6.2: Li and Na concentrations of samples obtained by a 1:2 of [Li+]:[Na+]

solution with 1 g FePO4 at various times. (The data were

published in March 20141). ..................................................... 94



lithiation of FePO4 with a 4-fold excess of reagent (Li

2SO

4+Na

2S

2O

3

in molar ratio 1:2) for different times, using 3 techniques. ...... 95

Table 6.4: The reaction rate of each experiment when x = 1 (LixFePO

4) with

respects the reaction time (estimated data from Figure 6.9-

Figure 6.6) ............................................................................. 99

Table 6.5: Concentrations of the reagents ................................................... 100

vi

Table 6.6: Lithium molar content of LixFePO

4 samples obtained by lithiation

of FePO4 for different times, as estimated from (a) XRD and

(b) ICP measurements. ........................................................ 107

Table 6.7: The reaction rate of each experiment with respect to [Li+] ........... 112

Table 6.8: The reaction rate of each experiment with respect to [S2O

32-] ....... 115

Table 6.9: Values of calculated theoretical rates compares to the experimental

rates ..................................................................................... 118

Table 6.10: Li and Na concentrations of samples obtained by various ratios of

[Li+]:[S2O

3

2- ] solution with 1 g FePO4 at various time. ............... 123

Table 7.1: Ionic radii of the potential cations that can be intercalated into

FePO47 ................................................................................... 128

Table 7.2: Concentration of reagents used in the experiments [Li+]:[Na+] and

[Li+]:[Mg+2] ............................................................................. 129

Table 7.3: The XRD results of lithiated heterosite FePO4 for each experiment133

Table 7.4: Lithium and sodium concentrations found in samples ................. 134

Table 7.5: Chemical compositions of synthetic brine type A/B and the

heterosite FePO4 .................................................................... 135

Table 7.6: Concentration of metals; i.e. Li+, Na+, K+, and Mg2+, that contain the

LiFePO4 samples obtained from type A and type B synthetic

brines. .................................................................................. 137

Table 8.1: Composition of each solution ..................................................... 143

vii

List of figures

Figure 1.1: The world lithium production in 2012, categorised by lithium

mining companies, adapted from Maxwell, 2014.10 ................... 2

Figure 1.2: Estimated lithium carbonate equivalent prices from 1990 to 2013,

adapted from Maxwell, 2015.5 .................................................. 3

Figure 1.3: Cyclic voltammogram of an immobilized LiFePO4 on Pt electrode

(solid line) and bare Pt electrode (dotted line) in 1 M Li2SO

4.

Reproduced with permission from ref. 31. Copyright 2015,

Elsevier. (This figure was published in January 2007 by Mi et

al.31) ........................................................................................ 10

Figure 1.4: a)The structure of a unit-cell of LiFePO4, b) lithium ion migration

path and c) curved trajectories or wavelike path of lithium ion

migration. Reprinted with permission from Ref.34. Copyright

2015 American Chemical Society. (This figure was published in

July 2005 by Islam et al.34) ....................................................... 11

Figure 1.5: The useful range of reducing agent potential (vs. lithium) for FePO4

lithiation. ................................................................................ 15

Figure 2.1: Diagram shows the derivation of Bragg's law ............................... 24

Figure 2.2: The combination of the miller indices of LiFePO4 (blue)3 and FePO

4

(red)4 XRD patterns. ................................................................ 26

Figure 2.3: Binary phase diagram of FePO4-LiFePO

4, obtained from XRD data.

This figure is adapted from Kobayashi et al, 2009.8 ................ 28

Figure 2.4: One-phase CV a) Cyclic votammetry profile presents a peak height

(Ipred) and a peak position (E

pred) b) samples of CV profile

A)reversible, B) quasi-reversible, and C) irreversible electron

transfer. Reprinted with permission from Ref.10. Copyright 2015

Springer London. (This figure was published in July 2005 by

Brownson et al).10 .................................................................... 29

Figure 2.5: A schematic drawing of a two-phase cyclic voltammogram, adapted

from Roberts et. al.11 ............................................................... 30

Figure 2.6: a) before and b) after immersing a Pt mesh electrode into the ink 32

Figure 2.7: Basic diagram of 3 electrode system ............................................ 33

Figure 2.8: the assembled Swagelok cell A) negative and positive connectors,35

viii

Figure 3.1: SEM images of an initial carbon coated lithium iron phosphate

(Tatung), under magnification of a) 2,500x, b) 10,000x and c)

33,000x. The images were recorded with an acceleration voltage

of 15 kV. ................................................................................. 45

Figure 3.2: SEM images of a carbon coated heterosite iron phosphate, which

was obtained from a delithiation process of 5.8 g LiFePO4 + 0.1

M K2S

2O

8 (Chapter 4.1), under magnification of a) 2,500x, b)

10,000x and c) 35,000x. The images were recorded with an

acceleration voltage of 15 kV. ................................................. 46

Figure 3.3: Fit to XRD data of a) LiFePO4 (Tatung): R

wp 1.3%, R

p 1.0% and b)

FePO4:

R

wp 1.5%, R

p 1.1%.

Crosses mark the data points, red line

is the fit and blue line is the difference; a) blue and b) pink tick

marks show the allowed reflection positions for LiFePO4 and

FePO4, respectively. ................................................................. 48

Figure 3.4: Cyclic voltammogram of LiFePO4 in 1.5 M Li

2SO

4 aqueous electrolyte

with a scan rate of 10 mV s-1, where Epa and E

pc are anodic and

cathodic peak potentials, respectively. .................................... 51

Figure 3.5: Potentiometric titration of LixFePO

4 electrodes prepared with the

mixiture of LiFePO4 + FePO

4 = 100%, as indicated. Specific

current: 17 mAh g-1 (at C/10). ................................................. 52

Figure 3.6: A validating graph of lithium content (%) in LiFePO4 and specific

charge .................................................................................... 53

Figure 4.1: The XRD of a LFP sample treated with K2S

2O

8 with a molar ratio of

2:1 for 1 hour. The XRD refinement indicates that the sample

composition is Li0.09

FePO4 ......................................................... 57

Figure 4.2: As in Figure 4.1, but the reaction was left for 2 hours and the XRD

fitting indicates that the sample composition is Li0.03

FePO4 ....... 58


fitting indicates that the sample composition is FePO4. ............ 58


fitting indicates that the sample composition is FePO4 ............. 59


2O


3:2 for 30 min. The XRD refinement indicates that the sample


FePO4 ......................................................... 60

Figure 4.6: As in Figure 4.5, but the reaction was left for 1 hour and the XRD


FePO4. ...... 60

ix


fitting indicates that the sample composition is FePO4. ........... 61




2O


6:2 for 30 min. The XRD refinement indicates that the sample


FePO4 ........................................................ 62

Figure 4.10: As in Figure 4.9, but the reaction was left for 1 hour and the XRD


FePO4. ..... 63



Figure 4.12: As in Figure 4.9, but the reaction was left for 24 hours and the

XRD fitting indicates that the sample composition is FePO4. .... 64

Figure 4.13: Delithiation of LiFePO4 during 24 hours, () 1:2 of K

2S

2O

8 to

LiFePO4, ( ) 3:2 of K

2S

2O

8 to LiFePO

4, ( ) 6:2 K

2S

2O

8 to LiFePO

4

experiments ........................................................................... 65

Figure 4.14: The kinetics delithiation of () 1:2 of K2S

2O

8 to LiFePO

4, ( ) 3:2 of

K2S

2O

8 to LiFePO

4, ( ) 6:2 K

2S

2O

8 to LiFePO

4 experiments ......... 66

Figure 4.15: The conductivity obtained from the mixture of 0.1 M K2S

2O

8 and 5

g LiFePO4 at 25°C with respect to time. .................................... 69

Figure 4.16: The conductivity and temperature obtained from the mixture of

0.2 M K2S

2O

8 and 5 g LiFePO

4 with respect to time. .................. 70

Figure 4.17: The exponential decay of 0.2 M K2S

2O

8 ...................................... 71

Figure 5.1: XRD patterns of 1 M LiI sample obtained after 30 m at room

temperature, compared to FePO4, and LiFePO

4. All diagrams were

indexed in the orthorhombic (Pnma (62)) crystallographic

system. ................................................................................... 79

Figure 5.2: XRD patterns of 2 M LiI obtained after 30 m at room temperature,

compared to FePO4, and LiFePO

4. All diagrams were indexed in

the orthorhombic (Pnma (62)) crystallographic system. ........... 80

Figure 5.3: XRD patterns of 2 M LiI +Zn sample obtained after 30 m at room

temperature, compared to FePO4, and LiFePO


indexed in the orthorhombic (Pnma (62)) crystallographic

system. ................................................................................... 82

x

Figure 5.4: XRD patterns of 2Li2SO

4: 4Na

2S

2O

3: 1FePO

4 sample obtained after 1

h at room temperature, compared to FePO4, and LiFePO

4. All

diagrams were indexed in the orthorhombic (Pnma (62))

crystallographic system. .......................................................... 83

Figure 5.5: XRD patterns of 2LiCl: 2Na2S

2O

3: 0.5FePO

4 sample obtained 1 h at

room temperature, compared to FePO4, and LiFePO

4. All



Figure 5.6: XRD patterns of 2:1 of Na2S

2O

3 :FePO4 ratio sample obtained 1 h at


4. All



Figure 6.1: The XRD fitting obtained from a sample treated with 0.15 M Li2SO

4

+ 0.3 M Na2S

2O

3 (solution 1) for 300 seconds. The result shows

only heterosite, i.e. the FePO4 starting material. ....................... 90


4

+ 0.7 M Na2S

2O


a mixed phase, i.e. partial conversion. Reproduced from Ref. 1

with permission from The Royal Society of Chemistry. (This

figure was published in March 20141) ...................................... 91


4 +

3.0 M Na2S

2O


pure olivine, i.e. LiFePO4 .......................................................... 91

Figure 6.4: A validating graph of percentage lithium content and specific

charge. Reproduced from Ref. 1 with permission from The Royal

Society of Chemistry. (This figure was published in March 20141)92

Figure 6.5: Potentiometric titration of LixFePO4 electrodes prepared with the

reaction product of FePO4 in 0.35 M Li

2SO

4 + 0.7 M Na

2S

2O

3 for

different times, as indicated. Specific current: 17 mA g-1 at C/10

Reproduced from Ref. 1 with permission from The Royal Society

of Chemistry. (This figure was published in March 20141). ...... 93

Figure 6.6: The comparison of XRD, PT, and MS techniques in the extent of

lithiation which was obtained from 0.15 M Li2SO

4+0.3 M Na

2S

2O

3.96



4+0.7 M Na

2S

2O

3.97

xi



4+1.5 M Na

2S

2O

3.97



4+3 M Na

2S

2O

3. . 98

Figure 6.10: Phase fraction of olivine in the heterosite/ olivine composite. .... 98

Figure 6.11: Example of the XRD fitting data for 0.4:4:1 (0.03 M LiCl + 0.3 M

Na2S

2O

3 + 0.075 M FePO

4) for 172800 s (2 days) result in

Li0.25

FePO4 .............................................................................. 102

Figure 6.12: Example of the XRD fitting data for 0.8:4:1 (0.06 M LiCl + 3.0 M

Na2S

2O

3 + 0.075 M FePO

4) for 172800 s (2 days) result in Li

0.80

FePO4) ................................................................................... 103

Figure 6.13: Example of the XRD fitting data for 4:4:1(0.3M LiCl + 0.3 M

Na2S

2O

3 + 0.075 M FePO

4) for 36000 s (10 hours) result in

LiFePO4 ................................................................................. 103

Figure 6.14: Example of the XRD fitting data for 4:1.3:1 (0.3 M LiCl + 0.1 M

Na2S

2O

3 + 0.075 M FePO

4) for 172800 s (2 days) result in

Li0.35

FePO4 .............................................................................. 104

Figure 6.15: Example of the XRD fitting data for 4:8:1 (0.3 M LiCl + 0.6 M

Na2S

2O

3 + 0.075 M FePO

4) for 14400 s (4 hours) result in Li

0.80

FePO4) ................................................................................... 105

Figure 6.16: Rate of the lithiated reaction that was obtained from 0.03 M [Li+],

0.3 M [S2O

32-] and 0.075 [FePO

4] (0.4:4:1 ratio), with respect of

time. ..................................................................................... 108


0.3 M [S2O

32-] and 0.075 [FePO


time. ..................................................................................... 109


0.3 M [S2O

32-] and 0.075 [FePO


time. ..................................................................................... 109


0.3 M [S2O

32-] and 0.075 [FePO

4] (4:4:1 ratio), with respect of

time. ..................................................................................... 110


0.3 M [S2O

32-] and 0.075 [FePO


time. ..................................................................................... 110

xii


0.3 M [S2O

32-] and 0.075 [FePO


time. ..................................................................................... 111


0.3 M [S2O

32-] and 0.075 [FePO


time. ..................................................................................... 111

Figure 6.23: The reaction rates of the kinetic study with respect to the lithium

concentration. ....................................................................... 112


0.1 M [S2O

32-] and 0.075 [FePO

4] (4:1.3:1 ratio), with respect of

time. ..................................................................................... 113


0.3 M [S2O

32-] and 0.075 [FePO


time. ..................................................................................... 113


0.6 M [S2O

32-] and 0.075 [FePO


time. ..................................................................................... 114


0.9 M [S2O

32-] and 0.075 [FePO


time. ..................................................................................... 114


1.2 M [S2O

32-] and 0.075 [FePO


time. ..................................................................................... 115

Figure 6.29: The reaction rates of the kinetic study with respect to the

thiosulphate concentration. ................................................... 116

Figure 6.30: The comparison of the experimental rate and the theoretical rate

of the experiments were obtained from a fixed [S2O

32-] =0.3 M

and [Li+] = 0.03 to 1.2 M. ....................................................... 119

Figure 6.31: The comparison of the experimental rate and the theoretical rate

of the experiments were obtained from a fixed [Li+] =0.3 M and

[S2O

32-] = 0.1 to 1.2 M. ........................................................... 119

Figure 6.32: a schematic current density profile of S2O

32- and LiFePO

4 whenError! Bookmark not defin

Figure 7.1: The XRD pattern obtained from a sample 1:10 of [Li+]:[Na+] treated

with 0.5 M Na2S

2O

3 for 24 h is compared to the initial XRD

pattern of LiFePO4 and the XRD of the de-lithiated sample

(heterosite FePO4) pattern. The result shows only LiFePO

4. ..... 130

xiii

Figure 7.2: The XRD pattern obtained from a sample1:50 of [Li+]:[Na+] treated

with 0.3 M Na2S

2O




4. ..... 130

Figure 7.3: The XRD pattern obtained from a sample 1:100 of [Li+]:[Na+] treated

with 0.3 M Na2S

2O




4. ..... 131

Figure 7.4: The XRD pattern obtained from a sample 1:10 of [Li+]:[Mg+2] treated

with 0.5 M Na2S

2O




4. ..... 132

Figure 7.5: The XRD pattern obtained from a sample 1:20 of [Li+]:[Mg+2] treated

with 0.5 M Na2S

2O




4. ..... 132

Figure 7.6: The XRD pattern obtained from brine type A treated with 0.3 M

Na2S

2O

3 for 24 h is compared to the initial XRD pattern of LiFePO

4

and the XRD de-lithiation (heterosite FePO4) sample pattern. The

result shows only LiFePO4. .................................................... 136

Figure 7.7: The XRD pattern obtained from brine type B treated with 0.3 M

Na2S

2O


4

and the XRD de-lithiation (heterosite FePO4) sample pattern. The

result shows only LiFePO4. .................................................... 137

Figure 7.8: Electrochemical data at rate 0.1C of LiFePO4 prepared by chemical

lithiation of FePO4 in synthetic brines; i.e. type A and B. The

results obtained with the initial LiFePO4 (Tantung) are also

included in the graph for a comparison. (This figure was

published in March 201413). .................................................. 138

Figure 7.9: Electrochemical cycling of LiFePO4 at different cycling rates, as

indicated (0.1C to C). ........................................................... 139

Figure 8.1: Cyclic voltammogram of a Pt working electrode in: ................... 144

Figure 8.2: Cyclic voltammogram of a) a Pt electrode coated with LiFePO

4 in 1.5

M Li2SO

4 (Solution 3*; black), Pt electrode

in 1.5 M Li

2SO

4 and 1.5

M Na2S

2O

3 (Solution 2*; blue), and a Pt electrode

coated with

xiv

LiFePO4 in 1.5 M Li

2SO

4 and 1.5 M Na

2S

2O

3 aqueous electrolyte

(Solution 4*; red). .................................................................. 145

Figure 8.3: Cyclic voltammogram of LiFePO4 deposited on the surface of a Pt

electrode in ........................................................................... 146


electrode in ........................................................................... 147


electrode in ........................................................................... 148


electrode in ........................................................................... 148

Figure 1: The XRD fitting obtained from a sample treated with 0.3 M LiCl + 0.6

M Na2SO

3 for 24 hours. The result shows pure olivine, i.e.

LiFePO4 .................................................................................. 158

xv

DECLARATION OF AUTHORSHIP

I, ....................................................................................... [please print name]

declare that this thesis and the work presented in it are my own and has been

generated by me as the result of my own original research.

[title of thesis] ..................................................................................................

.........................................................................................................................

I confirm that:

1. This work was done wholly or mainly while in candidature for a research

degree at this University;

2. Where any part of this thesis has previously been submitted for a degree or

any other qualification at this University or any other institution, this has

been clearly stated;

3. Where I have consulted the published work of others, this is always clearly

attributed;

4. Where I have quoted from the work of others, the source is always given.

With the exception of such quotations, this thesis is entirely my own work;

5. I have acknowledged all main sources of help;

6. Where the thesis is based on work done by myself jointly with others, I have

made clear exactly what was done by others and what I have contributed

myself;

7. [Delete as appropriate] None of this work has been published before

submission [or] Parts of this work have been published as: [please list

references below]:

Signed: ..............................................................................................................

Date: .................................................................................................................

xvii

Acknowledgements

Firstly, I am grateful to have Professor John R. Owen and Dr. Nuria Garcia-Araez

as my supervisor and co-supervisor, respectively. I would like to thank them for

their supervision and support of the ideas for my work, also their

encouragement as I worked through my PhD life.

I owe several people a debt of gratitude for their help and knowledge.

Professor Andrew L. Hector for his support, especially on my XRD work; Dr. J.

Andy Milton from Ocean and Earth Science, National Oceanography Centre

Southampton, for his kindness and help with the ICP-MS analysis; Dr. Guy

Denuault for his expertise on microelectrodes; also, Heather Simpkins and

Andrew Fleming for their proofreading expertise.

I would like to acknowledge the Ministry of Science and Technology, the Royal

Thai Government for their financial support. I would also like to thank my

superiors and colleagues from FAMC group, National Metal and Materials

Technology Center (MTEC), National Science and Technology Development

Agency (NSTDA), and Office of Educational Affairs (The Royal Thai Embassy): in

particular Dr. Paritud B., Prof. Pramote D., Assoc. Prof.Siriluck N., Dr. Ekkarut

V., Dr. Samerkhae J., Chanida N., Somchai I., and Pakin J.

My acknowledgement section would not be complete without thanking Phra Aj

Professor Khammai D., Phra Aj Uthai Y., Phra Aj Bhikkhu G., Phra Aj. Khoon T.

for teaching me mindfulness and providing me with hospitality.

I would like to thank the members of the Owen Group, past and present, for

their support and friendship (James F, Andy, Jake, Roy, Mike, Saddam, Will, P’

Pan, Kanjiro, Alex, Matt L, Matt R, James D and Louisa). My friends from the UK

and Thailand, i.e.Wai, Nut, Cho, Fon, P’ Toey, R’Tank, P’Ying, P’Pun, N’Eve, the

Fleming Family, P’Luck-P’Pong, Danni, Ashley, P’Khawn, P’Pan, P’Aoot, Dad-

Prasert, P’Nuch, P’Dee, N’Pai, and Luck.

Last, this part of my life could not have happened without my family’s support

in every way, trusting and encouraging me. I am very much obliged to my

family, the late Professor Kitti (Dad), Pilaiwan (Mom), and Manuswi (Sis)-

Intaranont. I would like to send a message to my Dad if only he could

acknowledge me from above. “Daddy, I did it! Hope that you are proud of me.

Love you maxxxx”

xviii

xix

Definitions and Abbreviations

ɑ An activity

c Concentration/ mol per decimetre (mol dm-1)

CE Counter electrode

CV Cyclic voltammogram

d density g cm-3

E Potential/ V

Ee0 The standard potential/ Volt

EV Electric vehicle

F Faraday constant (96485/ C mol-1)

GSAS General Structure Analysis System

∆G Gibbs free energy change/ Joule per mol (J mol-1)

I Current/ Ampere

ICP-MS Inductively coupled plasma mass spectrometer

ICSD Inorganic Crystal Structure Database

and anodic and cathodic rate constants

and standard rate constant of anodic and cathodic

κ conductivity/ Siemens per meter (S m-1)

LCE lithium carbonate equivalent

LFP lithium iron phosphate

LIB Lithium-ion battery

n moles of electrons

NMP N-Methyl-2-pyrrolidone

Mw molecular weight/ gram per mol (g mol-1)

xx

O, R Oxidised, reduced species

Ppb Part per billion

ppm Part per million

PT Potentiometric titration

PTFE Polytetrafluoroethylene

PVDF Polyvinylidene fluoride

R The ideal gas constant (8.314 /J mol-1K-1)

REF Reference electrode

SEM Scanning electron microscopy

SHE Standard hydrogen electrode

SHE Standard hydrogen electrode

T The absolute temperature (Kelvin)

VMP Versatile multichannel potentiostat

WE Working electrode

XRD X-ray diffraction

ᴧ molar conductivity/ Siemens meter square per mol (Sm2 mol-1)

∝ and ∝ anodic and cathodic transfer coefficient (∝ ∝ 1

Chapter 1: Introduction

1


1

Introduction Chapter 1:

The Availability of Lithium-Present and Future 1.1

1.1.1 Consumption, Production and Price of Lithium

Lithium (Li) is one of the most essential elements that we have been using in

everyday life, for example lithium-ion batteries (LIB), glass, ceramics, lubricant

greases, etc. In terms of lithium production and consumption worldwide, in

Table 1.1, the US geological survey reported that the production and

consumption were estimated to have increased 5% and 37%, respectively, from

2011-2014.1-5 The rate of lithium consumption seems to be an exponential

growth due to the expansion of the market for lithium-ion batteries. The

average of 2011-2014 annual growth rate is approximately 11%.

Table 1.1: Lithium consumption application changes between 2003 and 2011.

Year

Worldwide estimated/ KTonnes

Lithium consumption/ % b

Production a Consumption Ceramics and Glass

Batteries Other

20111 34 24 29 27 44 20122,3 35 28 30 22 48 20133 35 30 35 29 36 20144,5 36 33 35 31 34 a Excludes U.S. production b Percent of the worldwide consumption

In 2010, the LIB dominated the worldwide rechargeable battery marketplace,

with around 132 Ktonnes of battery weight sold per year.6 The majority of LIB

are used for laptops and tablets (60.75 Ktonnes of battery weight per year).6

Automotive Li-ion batteries only accounted for 10% of total electric vehicle (EV)

batteries which were produced and used for EV’s in 2008.7 However, Frost &

Sullivan’s research and consulting company predicted that the usage of Li-ion

for EV would be increased to 80% of the total EV batteries in 2015.7 This

information suggests that LIB will continue to grow in the future with greater


2

demand year after year. This indicates that the rising use of LIB will create a

problem of lithium scarcity.

In 2011, the four largest lithium producers were Australia (36%), Chile (35.6%),

China (17%) and Argentina (7.4%).8 However, Chile has switched to a leading

position with Australia since 2012. The top five lithium supplier companies in

2012 are Talison (36%; in Australia), SQM (26%; in Chile), Qinghai CITIC (18%;

in China), Chemetall or Rockwood (12%; in Chile and USA) and FMC Chemical

(7%; in Argentina), as shown in Figure 1.1. 9,10

Figure 1.1: The world lithium production in 2012, categorised by lithium mining companies, adapted from Maxwell, 2014.10

Li2CO

3 has the advantage of a very cheap extraction process which reacts

directly with an acid to produce a suitable salt e.g. LiPF6 for battery production.

Other products of lithium extraction such as lithium hydroxide (LiOH) and

lithium chloride (LiCl) are costed according to their lithium carbonate

equivalent (LCE) which is the mass of Li2CO

3 that would contain the same

amount of lithium as the product.

Talison36%

SQM26%

Qinghai CITIC18%

Rockwood12%

FMC7%

Others1%


3

Figure 1.2 shows prices of lithium in raw materials for many applications

consuming lithium, from 1990-2013. In approximately 1996, the price per LCE

showed a dramatic fall when SQM in Chile, joined the market10 and increased

its full operation capacity9 until it became one of the biggest lithium mining

companies in the world. Moreover, the production in South America was

cheaper than the US, causing many lithium mining companies in the US closing

down. From 2005, the price then increased significantly because of a drop in

supply due to climate impacts on the lithium production in Argentina. Later,

the price started to drop slightly as Chinese producers began to enter the

market (~2010), giving more supply. 5,9 After 2010, there have been two more

producers -Galaxy Resources (Australia) and Canada Lithium (Canada).

Although, some more lithium mining companies started to operate during the

past 10 years, the price of LCE is not as low as it was before 2005. This is

because the demand of lithium is growing whereas supply of lithium resources

is limiting and decreasing.

Figure 1.2: Estimated lithium carbonate equivalent prices from 1990 to 2013, adapted from Maxwell, 2015.5

0

1

2

3

4

5

6

7

8

1990 1995 2000 2005 2010 2015

US Dollars per kg LCE

Year


4

1.1.2 Resources and Supply

Resources and reserves are terminology to classify elements/ minerals.

According to the U.S. Geological survey, resources mean “a concentration of

naturally occurring materials in such form that economic extraction of a

commodity is regarded as feasible, either currently or at some future time”.3

Reserves mean “the resources that could be economically extracted or

produced at the time of determination”.3 Two questions arise from this

definition:

What is the total lithium content on Earth?

How soon will it be before lithium is scarce?

In 2014, the U.S. Geological survey reported that the world’s lithium resources

and reserves are approximately 34 million tonnes and 13 million tonnes,

respectively3. In 2009, 70% of the world’s lithium resources were in South

America; Argentina, Bolivia, Brazil and Chile.7 By 2013, America, Australia, and

China has become one of the main global lithium deposits countries.11 These

resources are expected to be depleted in 65 years’ time assuming a 11%

annual increase in lithium consumption as observed during 2011-2014. The

effect of the scarcity of lithium has promoted lithium recycling technology as

has been commercialised by companies such as Toxco (US), Umicore (Belgium),

and Sumitomo mining and metals (Japan).

Considering the reactivity of lithium with oxygen and water, it is not surprising

that lithium is not found in nature in metallic form. Various compounds of

lithium can be found in the four main deposit types, i.e. salar brines, minerals,

sedimentary rocks and seawater. The most recent economical way to extract

lithium is from salar brines- the second tier being from minerals, i.e.

sedimentary rocks (pegmatite) or clay.8,11,12

As already mentioned, the most concentrated lithium resources are mainly

found in brine lake deposits, or salar, containing lithium ion up to 5x103 ppm,


5

largely located in South America.11,13-17 One of the top three lithium resources is

in Uyuni, Bolivia which has about 27% of the world’s lithium resources.13,18

In Bolivia’s Salar de Uyuni, sodium ion (Na+) is known to have one of the most

concentrated cations, at a concentration of up to 113x103 ppm.13 Lithium (Li+),

magnesium (Mg2+) and potassium (K+) ion concentrations are up to 4.7x103,

75x103 ppm and 30x103 ppm, respectively, as shown in Table 1.2.13 The ratios

of values of lithium ion to some interesting cations such as sodium ion (Li+:Na+)

and magnesium ion (Li+:Mg2+).

The lithium concentration in seawater is comparatively very low approximately

at 0.15 ppm. Other cations such as Na+, K+ and Mg2+ are approximately 1.05 x

104, 4.53 x 102 and 3.08 x 104 ppm, respectively. 11,15-17,19-21

Table 1.3 illustrates how difficult it is to extract Li+ from the sources. The

higher ratios correspond to an easy Li+ extraction. Therefore, Li+ extraction from

brine is easier than extracting from seawater. Lithium from brine can be

extracted by an ion exchange, a solvent extraction, adsorption, and

electrochemistry. 11,14-16

Table 1.2: The comparison of cations between the concentration of cations in brine and seawater

Cations Brine13/ ppm Seawater11,15-17,19-21/ppm Li+ 4.70 x 103 1.50 x 10-1 Na+ 1.13 x 105 1.05 x 104 K+ 3.00 x 104 4.53 x 102

Mg2+ 7.50 x 104 3.08 x 104

Table 1.3: The comparison of ionic fractions (Li+ vs. other cation from Table 1.2) between the concentration of cations in brine and seawater

Cations Brine Seawater Li+/Na+ 4.16 x 10-2 9.43 x 10-6 Li+/K+ 1.57 x 10-1 2.63 x 10-4

Li+/Mg2+ 6.27 x 10-2 7.87 x 10-5


6

Total ratio* 2.61 x 10-1 3.51 x 10-4 *Total ratio: sum up of Li+/Na+, Li+/K+ and Li+/Mg2+

Lithium Extraction from Brine 1.2

1.2.1 Lime Soda Evaporation

Lithium chloride (LiCl) contained in brine is used to make Li2CO

3 as an end

product. The cheapest method to extract lithium from brine is by lime soda

evaporation. This process is done chemically. The most inefficient step is to

evaporate brine water using solar energy for 1-2 years.11,12 Many ponds are built

using salts as a wall and each pond is used to precipitate crystallised salts.

Basically, the purpose of the evaporation process is to crystallise out unwanted

salts and concentrate the more soluble lithium chloride in the brine. The

resultant solution is transported to another pond and lime or calcium

hydroxide (CaOH) is added to the concentrated pond to precipitate out such

compounds as magnesium hydroxide (Mg(OH)2), calcium sulphate (CaSO

4),

calcium borate (Ca3(BO

3)

2), etc. A further liquid transfer takes place and soda

ash or sodium carbonate (Na2CO

3) is added to precipitate the less soluble

Li2CO

3 as the raw material.

Approximately 50% of lithium is extracted from the brine using this process

and the rest of the lithium flows back to the first pond. The Li2CO

3 product is

usually 99.0% purity, which is sufficiently pure for glass and ceramic

applications.12 Therefore, further purification processes are needed to make

lithium acceptable for the grade needed for batteries, i.e. 99.9%.12

1.2.2 Ion-sieve spinel type

There are many spinel types of material that absorb cations from aqueous

solution with a high selectivity of lithium. Examples are λ-MnO2, H

1.33Mn

1.67O

4

H1.6

Mn1.6

O4, which are delithiated from LiMnO

2, and Li

1.33Mn

1.67O

4, Li

1.6Mn

1.6O

4


7

respectively.14,22-24 The recovery of lithium, using a manganese spinel type, can

be done chemically.

The following general scheme illustrates the chemical approach of ion

exchange, where the material is delithiated by an acid treatment. The lithium

ion is then removed from the structure and replaced by a proton, as shown in

Equation 1.1.

Equation 1.1 →

The delithiated material is then used to extract mostly lithium ions from a

lithium salt solution containing several other cations. The reaction should be a

simple ion-exchange, facilitated by an alkaline solution to help remove H+ from

the structure, as shown in Equation 1.2.

Equation 1.2 →

However, this reaction, as reported, can involve complications due to redox

reactions including oxygen evolution (as shown in Equation 1.3) and the

formation of soluble Mn(II) or Mn(VII) compounds which would contaminate the

effluent. Also, another disadvantage is the need to control the pH and redox

conditions simultaneously to preserve the structure.

Equation 1.3 →

1.2.3 LiFePO4 Electrochemical

An electrochemical method to recover lithium from brine using LiFePO4 was

introduced in 2012 by Pasta et al.25 There are four steps in the process, using a

3-electrode system. Both working and counter electrodes are a carbon cloth

covered by drop castinga of LiFePO4 and silver (Ag) particles slurry, respectively.

A silver/silver chloride electrode is used as the reference.

a Drop casting is an electrode preparation by dropping a mixture of active material and solvent solution on the electrode and evaporating the solvent.


8

Step 1, the 3-electrode system is in brine containing lithium as an electrolyte. A

charged working electrode, i.e. heterosite FePO4, is intercalated with lithium

ion (Li+) from the brine by applying a negative current to the WE (cathode), the

chloride ion (Cl-) from brine is captured at the Ag CE (anode). Step 2, the

electrodes are transferred and submerged into a recovery solution, where the

Li+ and Cl- are released by applying a reverse current to the cathode in step 3.

In step 4, the recovered solution is replaced with the new brine and the cycle is

repeated.

This process can recover lithium chloride solution selectively in a sodium-rich

brine, i.e. 1: 100 of Li+: Na+ while consuming only ~1 W h mol-1.25 This is better

than the ion exchange process with the spinel manganese oxide, which

consumes apparently approximately 33 W h mol-1.25 Nevertheless, there is a

drawback due to the high price of silver.

1.2.4 Aims and Objectives of this work

The aim is to investigate and evaluate a new, low cost method for lithium

extraction from lithium solutions contaminated with other metals as described

above.

The objectives are:

1. To demonstrate the effectiveness of the process using different

reagents

2. To assess the quality of the lithium product that can be obtained.

3. To investigate the cost and environmental acceptability of the method,

resource consumption and process time.

The principle of the process is to use FePO4 in conjunction with a reducing

agent as a lithium ion absorbent which can subsequently release lithium ions


9

upon addition of an oxidising agent as shown in Equation 1.4- Equation 1.5,

respectively.

Equation 1.4 → 3.45 . / or0.15 .

Equation 1.5 ←

A Review of LiFePO4 1.3

Since 1997 after the first reports on LiFePO4 by Padhi et al.26-28, there have been

a large number of research studies on this cathode, generating approximately

4200 reports discussing various aspects including its remarkable surface

stability and efforts to overcome its main disadvantage of poor electronic

conductivity that necessitates the application of a carbon coating to aid

electron transport.

Lithium iron phosphate (LiFePO4) is a well-known cathode material for

secondary lithium-ion batteries due to its safety, performance and cost. It has

been used commercially for some time. Heterosite iron phosphate (FePO4)

undergoes a reversible reaction to intercalate lithium at 3.45 V vs. Li/Li+ or

0.15 V vs. SCE, with a redox couple of Fe3+/Fe2+, as shown in Equation 1.4-

Equation 1.5 and Figure 1.3. The theoretical capacity of LiFePO4 is 170 mAh g-1

but in practical use is approximately ~150 mAh g-1.29

The surface stability of LiFePO4 is generally attributed to the strong P-O

covalent bonds in the orthorhombic olivine structure which restrain oxygen

from release, resulting in a high thermal safety battery.27 The strong covalent

bond of PO43- also decreases the iron ion covalent bond, giving a lower Fe3+/Fe2+

redox potential than that of simple hydrated ions in aqueous solution.30

Figure 1.3 shows a cyclic voltammogram (CV) of a two phase material, i.e.

LiFePO4 to FePO

4. The electron transfer occurs in a solid reagent (FePO

4), giving

symmetrical redox peaks on both oxidation and reduction. This characteristic

is distinctive to solid electrochemistry which is described more in Chapter 2.


10

Figure 1.3: Cyclic voltammogram of an immobilized LiFePO4 on Pt electrode

(solid line) and bare Pt electrode (dotted line) in 1 M Li2SO

4. Reproduced with

permission from ref. 31. Copyright 2015, Elsevier. (This figure was published in January 2007 by Mi et al.31)

LiFePO4

adopts an olivine structure type (known as triphylite), orthorhombic

lattice system, space group Pnma with a lattice parameter of a = 10.332(4) Å,

b = 6.010(5) Å and c = 4.787 Å, as shown in Figure 1.4.32 LiFePO4 consists of

PO4

tetrahedra, FeO6 octahedra and a distorted hexagonal close-packed

framework filled with lithium ion. The connections between PO4 tetrahedra and

FeO6

octahedra are presented as sharing oxygen (red circle) and sharing an

edge (blue circle), as shown in Figure 1.4a. When LiFePO4 is charged, the

topotactic phase transformation from olivine to heterosite FePO4 structure will

be performed, giving a volume in unit-cell changes of approximately 6.5%.27,33

The unusually small volume changes during the transformation again

contribute to excellent stability which provides the advantages of a high safety

battery and a suitable performance for the electric vehicle battery.27,30

LiFePO4

Bare Pt

electrode


11

Figure 1.4: a)The structure of a unit-cell of LiFePO4, b) lithium ion migration

path and c) curved trajectories or wavelike path of lithium ion migration. Reprinted with permission from Ref.34. Copyright 2015 American Chemical Society. (This figure was published in July 2005 by Islam et al.34)

Generally in LiFePO4, the lithium ion could migrate in three ways, which are

[010], [001], and [101] along the b, c and a-c axis, respectively, as shown in

Figure 1.4b. Nonetheless, the lithium ion in LiFePO4 is known for migrating

through one-dimension (1D) via the b axis or [010] direction, which indicates

the Miller index of the lattice plain, resulting in low conductivity compared to

structures having 2D or 3D migration.30,35-37 This point was confirmed by the

reports of Morgan et al.36 and Islam et.al.34, as illustrated in Table 1.4. The

average atom positions between lithium ions in each direction were reported,

i.e. 3.00 Å, 4.68 Å and 5.69 Å for the [010], [001] and [101] paths,

respectively.30,34,36,38 The diffusion coefficient (D) of each pathway was calculated

by Morgan et al., resulting in D[010]

> D[101]

> D[001]

. The high diffusion coefficient

implies a faster movement of lithium ion in that direction. The activation

c b

a


12

energy (Ea) indicates the possible diffusion pathway, and lower E

a represents the

favourable pathway. The report showed that Ea [010]

< Ea [101]

< Ea [001]

. The high Ea is

caused by accumulated cation where the FeO6 octahedral face-shared with two

PO4 tetrahedra.35,36 Islam et al. reported the migration energy (E

m) of each

pathway, resulting in Em[010]

< E

m[001] <

E

m[101]. 30,34 The lower E

m of lithium suggests

that lithium ion movement is more preferable. Therefore, these results agree

and confirm that lithium ion in the LiFePO4

[010] pathway is more desirable

than the others.

Table 1.4: Properties of possible lithium ion movements.

Pathway [010] [001] [101] Axis b c a

Atom position Li+-Li+ 34,36/Å 3.00 4.68 5.69 Diffusion coefficient36/cm2 s-1 10-8 10-45 10-19

Activation energy36/ eV 0.27 >2.5 1 Migration energy34/ eV 0.55 2.89 3.36

Thermodynamic Principles for the Selection of 1.4

Suitable Oxidizing and Reducing Agents

1.4.1 Oxidizing Agent

The preliminary research on the subject of an oxidizing agent to delithiate

LiFePO4

was undertaken. There are commonly known oxidizing agents to

extract lithium from LiFePO4,

namely bromine26,39 in acetonitrile and

nitronium40,41 in acetonitrile.42 However, acetonitrile or methyl cyanide is well

known for being hazardous to the human body and environmentally unsafe.

Thus, an aqueous based reagent is considered environmental preferable. At

the start of this project, only two oxidising agents had been reported to give

lithium extraction from LiFePO4, namely hydrogen peroxide42 in acid and

potassium persulphate43-47 in water. Potassium persulphate (K2S

2O

8) was chosen

here because of its neutral pH.


13

Equation 1.6, the redox couple of S2O

82-

/ SO

42- has a standard potential of

+2.05 V vs. SHE48 or +5.10 V vs. Li/Li+, where the hydrogen reference at E = 0 is

+ 3.05 vs. Li/Li+ (2.05+3.05 =5.10). The potential of S2O

82- is higher than the

potential of LiFePO4 which is 3.04 V vs. Li/Li+ and therefore lithium can be

extracted from LiFePO4 using S

2O

82-.46

Equation 1.6 2 2 5.10 . /

The lithium extraction was presented by Ramana et al. (2009). The report

shows a 1:2 molar ratio of K2S

2O

8 to LiFePO

4 for which Equation 1.7 is shown:43

Equation 1.7 2 → 2

1.4.2 Reducing Agent

The following discussion shows the importance of testing the reducing agent

to give the best discrimination between for the absorption of lithium and

rejecting other metals. Among the main small cations in brine, lithium has the

smallest ionic radius which is 59 pm, as shown in Table 1.5.49 Although the

sodium ion has a radius which is 43 pm higher than lithium ion, it can be

inserted into the heterosite FePO4 to form sodium iron phosphate; NaFePO

4.

The lattice parameters of NaFePO4 and LiFePO

4 are very similar34,50-52, as shown

in Table 1.6. The volume of the NaFePO4 unit cell is only slightly bigger than

that of LiFePO4. Although the c parameter of NaFePO

4 is significantly bigger

than the others, the relative sizes of the ionic radii of lithium and sodium are

not considered to be significant factors that could give an enhanced selectivity

for lithium insertion. Selectivity for sodium is particularly desirable because it

is present in larger concentrations than lithium itself.

Table 1.5: Ionic radii of the potential cations that can be intercalated into FePO

449

Ions Lithium

(Li+) Magnesium

(Mg2+) Sodium

(Na+ )

Potassium (K+)

Ionic radii, r/Å 0.59 0.72 1.02 1.38


14

Table 1.6: The unit cell parameters of FePO4, LiFePO

4, and NaFePO

4 products

based on XRD data52

Cathode material

Lattice Parameter/ Å Volume/Å3

a b c FePO

4 9.8152(5) 5.7885(3) 4.7803(3) 271.593(4)

LiFePO4

10.3202(6) 6.0035(4) 4.6928(4) 291.020(8) NaFePO

4 10.4051(4) 6.2216(2) 4.9486(2) 319.933(9)

Given the weakness of size selectivity, an investigation into a new principle for

selectivity was made, based on the different redox potentials at which the

insertion takes place. The operating voltage of LiFePO4 is 3.45 V vs. Li/Li+, as

shown in Equation 1.8. 30,46,50,53,54 Galvanostatic studies by Zaghib et al. reported

an onset potential for sodium insertion at ~ 2.7 V vs. Na/Na+.53 This value can

be calculated to the standard reduction hydrogen reference electrode potential:

SHE, using Equation 1.9 and Equation 1.10.

Equation 1.8 3.45 . /

Equation 1.9 → 2.71 .

Equation 1.10 → 3.05 .

For a meaningful comparison, the above potential for sodium insertion needs

to be converted to the Li/Li+ scale. This may be done by assuming that the

positive shift for sodium ion reduction (Equation 1.9) on conversion from SHE

to the lithium scale is the same for lithium plating (Equation 1.10), i.e. +3.05 V

vs. Li/Li+. Thus, Na/Na+ should occur at E = (-2.71+3.05) = 0.34 V so that E0 for

sodium insertion in FePO4 should be at ~3 V vs. Li/Li+ (2.71+0.34).

Regarding the potential voltage of NaFePO4

and LiFePO4, a useful reducing

agent potential has to be between 3.04 and 3.45 V vs. Li/Li+, respectively, to

give a thermodynamic driving force for Li+ insertion but not for Na+ insertion, as

shown in Figure 1.5. Thiosulfate (S2O

32-) is a soluble reducing agent (Equation

1.11)55, which has a potential of 3.13 V vs. Li/Li+. The fact that thiosulfate is

0.32 V lower than the insertion potential of Li+, means that it should be a

suitable reducing agent, as shown in Figure 1.5. Furthermore, this potential is


15

0.09 V higher than the insertion of Na+; therefore, thiosulfate should exhibit

the lithium selectivity.

Equation 1.11 2 2 3.13 . /

Figure 1.5: The useful range of reducing agent potential (vs. lithium) for FePO4

lithiation.

As an alternative to the standard reduction potentials, the Gibbs free energy

change (∆G/ Joule mol-1) can determine whether a reaction is spontaneous or

not, under a constant pressure and temperature. If the ∆G of the reaction is

more than zero (∆G > 0), this means the chemical reaction is non-spontaneous

or unfavourable. On the other hand, if the ∆G of the reaction is less than zero

(∆G < 0), the reaction is spontaneous or favourable. In other words, the

reaction should go forwards not backwards.

The free energy change (∆G) is related to the standard state free energy (∆G0)

and the activities of reactants and products according to Equation 1.12.56

Equation 1.12 ∆ ∆ ln

Where R = the ideal gas constant (8.314 /J mol-1K-1) T = the absolute temperature (Kelvin) ɑ = the activity of →

The overall reaction of FePO4

and S2O

32- can be written as shown in Equation

1.13. The free energy change of this reaction is shown in Equation 1.14.

Equation 1.13 2 2 2 → 2

0 3.0 3.25 3.50 3.75

LiFePO4

S2O

3

2- NaFePO4

Useful reducing agent potential

Li+ can be inserted

Li+ can be extracted

Na+ can be inserted

Li/ Li+


16

Therefore,


The activity of a pure solid of LiFePO4 and FePO

4 is considered to be unity. The

activity of the soluble reagents and products, i.e. Li+, S2O

32-, and S

4O

62-are

approximated to the molar concentration56. Accordingly, the overall ∆G can be

written as shown in Equation 1.15. Notably, the driving force (∆G) is

proportional to the log .


In this study, therefore, the direction of the reaction driving force (i.e.

Li++FePO4 + S

2O

32-) can be determined from Equation 1.16

Equation 1.16, using the equilibrium potential.56-58 E0 is defined for the case

when all reagents are in the standard state (1 M). Also, E0 is expressing the

activity of the electron.

Equation 1.16 ∆

Where n = moles of electrons F = Faraday’s constant (96485 C mol-1) E0 = the equilibrium potential.

The ∆G0 of the reaction from Equation 1.8 and Equation 1.11 is calculated as

shown below:

∆G0 of Equation 1.8 3.45 . ⁄

∆

∆ 1 96485 3.45

∆ 333

∆G0 of Equation 1.11 2 2 3.13 . /

∆

∆ 2 96485 3.13


17

∆ 604

The overall reaction from Equation 1.8 -Equation 1.9 can be written as Equation

1.13, i.e. (2 x Equation 1.8) - Equation 1.9.

2 2 2 → 2

Therefore, the ∆G0 of Equation 1.13 shows a negative value, as a favourable

reaction :

∆ 2∆ ∆

∆ 2 333 604

∆ 62

The calculation below is to determine whether ∆G0 overall between FePO4

(Equation 1.17) and sodium thiosulphate (Na2S

2O

3) (

Equation 1.18) is positive or negative.

Equation 1.17 3.04 . ⁄

∆

∆ 1 96485 3.04

∆ 293

Equation 1.18 2 2 2 → 2

Therefore,

∆ 2∆ ∆

∆ 2 293 604

∆ 18


18

The result is positive which can be interpreted as an unfavourable chemical

reaction.

Outline of the Thesis 1.5

There are nine chapters in this thesis. Chapter 2 describes techniques which

have been used in each experiment. Chapter 3 describes the electrochemical

and physical characterisation of LiFePO4 and FePO

4. Chapter 4 shows chemical

delithiation experiments using LiFePO4 as a framework and K

2S

2O

8 as an

oxidizing agent. Chapter 5 demonstrates chemical lithiation experiments using

reducing agents, lithium iodide (LiI) and Na2S

2O

3 in aqueous solutions. Once

Na2S

2O

3 was found as the more suitable reducing agent, the kinetics of the

lithiation of FePO4 by Na

2S

2O

3 were studied, as described in Chapter 6. In

Chapter 7, the selectivity of the ion insertion reaction towards Li is assessed

from the composition of the products, using ICP-MS to determine the content

of sodium, magnesium and potassium. Chapter 8 illustrates how to find an

alternative reducing agent by using cyclic voltammetry and all conclusions are

provided in Chapter 9.

References 1.6

(1) United States Geological Survey, Mineral Commodity Summaries 2012. In Lithium; U.S. Government Printing Office: Washington, DC, 2012; pp 94-95. (2) United States Geological Survey, Mineral Commodity Summaries 2013. In Lithium; U.S. Government Printing Office: Washington, DC, 2013; pp 94-95. (3) United States Geological Survey, Mineral Commodity Summaries 2014. In Lithium; U.S. Government Printing Office: Washington, DC, 2014; pp 94-95. (4) United States Geological Survey, Mineral Commodity Summaries 2015. In Lithium; U.S. Government Printing Office: Washington, DC, 2015; pp 94-95. (5) Maxwell, P. Transparent and opaque pricing: The interesting case of lithium. Resources Policy 2015, 45, 92-97. (6) Wiaux, J.-P.: The Impact of New Applications on the European Rechargeable Battery Market. The 16th International Congress for Battery Recycling ICBR: Venice, Italy, 2011. (7) Jawad, I.: Reuse and Recycling to Ensure the Completion of the "Green Car", Analysis of the Global Market for Automotive Lithium-ion Battery


19

Recycling and Second Life (Frost and Sullivan). The 16th International Congress for Battery Recycling ICBR: Venice, Italy, 2011. (8) Speirs, J.; Contestabile, M.; Houari, Y.; Gross, R. The future of lithium availability for electric vehicle batteries. 2014, 35, 183-193. (9) Bauer, D.; Diamond, D.; Li, J.; Sandalow, D.; Telleen, P.; Wanner, B.: Critical Materials Strategy. In Chapter 3. Historical Supply, Demand and Prices for the Key Materials; U.S. Department of Energy: California, USA., 2010; pp 27-52. (10) Maxwell, P. Analysing the lithium industry: Demand, supply, and emerging developments. Mineral Economics 2014, 26, 97-106. (11) Talens Peiró, L.; Villalba Méndez, G.; Ayres, R. Lithium: Sources, Production, Uses, and Recovery Outlook. JOM 2013, 65, 986-996. (12) Moreno, L.: Lithium Industry: A Strategie Energy Metal. In Mineralogy and Resources; Euro Pacific Canada: Toronto, Canada, 2013; pp 3-9. (13) Risacher, F.; Fritz, B. Quaternary geochemical evolution of the salars of Uyuni and Coipasa, Central Altiplano, Bolivia. Chemical Geology 1991, 90, 211-231. (14) Chitrakar, R.; Kanoh, H.; Miyai, Y.; Ooi, K. Recovery of Lithium from Seawater Using Manganese Oxide Adsorbent (H1.6Mn1.6O4) Derived from Li1.6Mn1.6O4. Industrial & Engineering Chemistry Research 2001, 40, 2054-2058. (15) Riley, J. P.; Tongudai, M. The lithium content of sea water. 1964, 11, 563-568. (16) Angino, E. E.; Billings, G. K. Lithium content of sea water by atomic absorption spectrometry. 1966, 30, 153-158. (17) Lindal, B. The production of chemicals from brine and seawater using geothermal energy. 1970, 2, Part 1, 910-917. (18) Gruber, P. W.; Medina, P. A.; Keoleian, G. A.; Kesler, S. E.; Everson, M. P.; Wallington, T. J. Global Lithium Availability. Journal of Industrial Ecology 2011, 15, 760-775. (19) Wang, T.; Kee Lee, H.; Yau Li, S. F. Determination of Sodium, Potassium, Magnesium, and Calcium in Seawater by Capillary Electrophoresis with Indirect Photometric Detection. Journal of Liquid Chromatography & Related Technologies 1998, 21, 2485-2496. (20) Tangen, A.; Lund, W.; Frederiksen, R. B. Determination of Na+, K+, Mg2+ and Ca2+ in mixtures of seawater and formation water by capillary electrophoresis. 1997, 767, 311-317. (21) Kang, K. C.; Linga, P.; Park, K.-n.; Choi, S.-J.; Lee, J. D. Seawater desalination by gas hydrate process and removal characteristics of dissolved ions (Na+, K+, Mg2+, Ca2+, B3+, Cl−, SO

42−). 2014, 353, 84-90.

(22) Ooi, K.; Miyai, Y.; Katoh, S.; Maeda, H.; Abe, M. Topotactic lithium(1+) insertion to .lambda.-manganese dioxide in the aqueous phase. Langmuir 1989, 5, 150-157. (23) Ooi, K.; Miyai, Y.; Sakakihara, J. Mechanism of lithium(1+) insertion in spinel-type manganese oxide. Redox and ion-exchange reactions. Langmuir 1991, 7, 1167-1171. (24) Chitrakar, R.; Makita, Y.; Ooi, K.; Sonoda, A. Selective Uptake of Lithium Ion from Brine by H

1.33Mn

1.67O

4 and H

1.6Mn

1.6O

4. Chemistry Letters 2012,

41, 1647-1649. (25) Pasta, M.; Battistel, A.; La Mantia, F. Batteries for lithium recovery from brines. Energy & Environmental Science 2012, 5, 9487-9491.


20

(26) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. Phospho-olivines as Positive-Electrode Materials for Rechargeable Lithium Batteries. Journal of the Electrochemical Society 1997, 144, 1188-1194. (27) Wang, J.; Sun, X. Olivine LiFePO

4: the remaining challenges for future

energy storage. Energy & Environmental Science 2015, 8, 1110-1138. (28) Tang, P.; Holzwarth, N. A. W. Electronic Structure of FePO

4, LiFePO

4,

and Related Materials. Physical Review B 2003, 68, 1-9. (29) Rangappa, D.; Honma, I.: Designing Nanocrystal Electrodes by Supercritical Fluid Process and Their Electrochemical Properties. In Nanocrystal; 1st ed.; Masuda, Y., Ed., 2011; pp 293-313. (30) Wu, B.; Ren, Y.; Li, N.: LiFePO

4 Cathode Material. In Electric Vehicles -

The Benefits and Barriers, 2011; pp 199-216. (31) Mi, C.; Zhang, X.; Li, H. Electrochemical behaviors of solid LiFePO

4

and Li0.99

Nb0.01

FePO4 in Li

2SO

4 aqueous electrolyte. Journal of Electroanalytical

Chemistry 2007, 602, 245-254. (32) Yakubovich, O. V.; Belokoneva, E. L.; Tsirel'son, V. G.; Urusov, V. S. Electron density distribution in synthetic triphylite LiFePO

4 1990, 45, 93-99.

(33) Okubo, M.; Asakura, D.; Mizuno, Y.; Kim, J. D.; Mizokawa, T.; Kudo, T.; Honma, I. Switching Redox-Active Sites by Valence Tautomerism in Prussian Blue Analogues A

xMn

y[Fe(CN)

6]∙nH

2O (A: K, Rb): Robust Frameworks for

Reversible Li Storage. J. Phys. Chem. Lett. 2010, 1, 2063-2071. (34) Islam, M. S.; Driscoll, D. J.; Fisher, C. A. J.; Slater, P. R. Atomic-Scale Investigation of Defects, Dopants, and Lithium Transport in the LiFePO

4 Olivine-

Type Battery Material. Chemistry of Materials 2005, 17, 5085-5092. (35) Ellis, B. L.; Lee, K. T.; Nazar, L. F. Positive Electrode Materials for Li-Ion and Li-Batteries. Chemistry of Materials 2010, 22, 691-714. (36) Morgan, D.; Van der Ven, A.; Ceder, G. Li Conductivity in Li

x MPO

4 ( M = Mn , Fe , Co , Ni ) Olivine Materials. Electrochemical and Solid-

State Letters 2004, 7, A30-A32. (37) Allen, J. L.; Jow, T. R.; Wolfenstine, J. Kinetic Study of the Electrochemical FePO

4 to LiFePO

4 Phase Transition. Chemistry of Materials

2007, 19, 2108-2111. (38) Rissouli, K.; Benkhouja, K.; Bettach, M.; Sadel, A.; Zahir, M.; Derrory, A.; Drillon, M. Crystallochemical and magnetic studies of LiMi

1−xM

x'PO

4 (M, M' =

Mn, Co, Ni; O ≤ x ≤ 1). 1998, 23, 85-88. (39) Andersson, A. S.; Kalska, B.; Häggström, L.; Thomas, J. O. Lithium extraction/insertion in LiFePO4: an X-ray diffraction and Mössbauer spectroscopy study. Solid State Ionics 2000, 130, 41-52. (40) Rousse, G.; Rodriguez-Carvajal, J.; Patoux, S.; Masquelier, C. Magnetic Structures of the Triphylite LiFePO4 and of Its Delithiated Form FePO4. Chemistry of Materials 2003, 15, 4082-4090. (41) Yu, X.; Wang, Q.; Zhou, Y.; Li, H.; Yang, X.-Q.; Nam, K.-W.; Ehrlich, S. N.; Khalid, S.; Meng, Y. S. High rate delithiation behaviour of LiFePO

4 studied by

quick X-ray absorption spectroscopy. Chemical Communications 2012, 48, 11537-11539. (42) Lepage, D.; Michot, C.; Liang, G.; Gauthier, M.; Schougaard, S. B. A Soft Chemistry Approach to Coating of LiFePO4 with a Conducting Polymer. Angewandte Chemie International Edition 2011, 50, 6884-6887. (43) Ramana, C. V.; Mauger, A.; Gendron, F.; Julien, C. M.; Zaghib, K. Study of the Li-insertion/extraction process in LiFePO4/FePO4. Journal of Power Sources 2009, 187, 555-564.


21

(44) Zaghib, K.; Mauger, A.; Goodenough, J. B.; Gendron, F.; Julien, C. M. Electronic, Optical, and Magnetic Properties of LiFePO4: Small Magnetic Polaron Effects. Chemistry of Materials 2007, 19, 3740-3747. (45) Ait-Salah, A.; Dodd, J.; Mauger, A.; Yazami, R.; Gendron, F.; Julien, C. M. Structural and Magnetic Properties of LiFePO4 and Lithium Extraction Effects. Zeitschrift für anorganische und allgemeine Chemie 2006, 632, 1598-1605. (46) Dodd, J. L.; Yazami, R.; Fultz, B. Phase Diagram of Li

xFePO

4.

Electrochemical and Solid-State Letters 2006, 9, A151-A155. (47) Miao, S.; Kocher, M.; Rez, P.; Fultz, B.; Yazami, R.; Ahn, C. C. Local Electronic Structure of Olivine Phases of LixFePO4. The Journal of Physical Chemistry A 2007, 111, 4242-4247. (48) Atkins, P. W.; De Paula, J.: Standard Potentials. In The elements of physical chemistry; 4th, Ed.; Oxford University Press: Oxford, 2005; pp 612-613. (49) Atkins, P. W.; De Paula, J.: Mettallic, Ionic, and Covalent solids. In The elements of physical chemistry; 4th ed.; Oxford University Press: Oxford, 2005; pp 376-400. (50) Whiteside, A.; Fisher, C. A. J.; Parker, S. C.; Saiful Islam, M. Particle shapes and surface structures of olivine NaFePO4 in comparison to LiFePO4. Phys. Chem. Chem. Phys. 2014, 16, 21788-21794. (51) Tang, P.; Holzwarth, N. A. W.; Du, Y. A. Comparison of the electronic structures of four crystalline phases of FePO

4. Physical Review B 2007, 76, 1-9.

(52) Avdeev, M.; Mohamed, Z.; Ling, C. D.; Lu, J.; Tamaru, M.; Yamada, A.; Barpanda, P. Magnetic Structures of NaFePO4 Maricite and Triphylite Polymorphs for Sodium-Ion Batteries. Inorganic Chemistry 2013, 52, 8685-8693. (53) Zaghib, K.; Trottier, J.; Hovington, P.; Brochu, F.; Guerfi, A.; Mauger, A.; Julien, C. M. Characterization of Na-based phosphate as electrode materials for electrochemical cells. Journal of Power Sources 2011, 196, 9612-9617. (54) Reale, P.; Panero, S.; Scrosati, B.; Garche, J.; Wohlfahrt-Mehrens, M.; Wachtler, M. A Safe, Low-Cost, and Sustainable Lithium-Ion Polymer Battery. Journal of The Electrochemical Society 2004, 151, A2138-A2142. (55) Center, U. o. R. I. C. R.; Company, C. R.: Handbook of chemistry and physics 0363-3055. 70th ed.; Weast, R. C., Lide, D. R., Eds.; CRC Press: Cleveland, Ohio, 1989; pp D155-D158. (56) Atkins, P. W.; Julio, D. P.: Electrochemistry. In The elements of physical chemistry; 4th ed.; Oxford University Press: Oxford 2005; pp 200-224. (57) Pletcher, D.: An introduction to Electrode Reactions. In First Course in Electrode Processes; 2nd ed.; Royal Society of Chemistry: Cambridge, 2009; pp 1-47. (58) Pletcher, D.; Royal Society of, C.: A first course in electrode processes. 2nd ed.; Royal Society of Chemistry: Cambridge, 2009; pp 187-189.


22

Chapter 2: Experimental Techniques

23

Experimental Techniques Chapter 2:

Powder X-ray Diffraction 2.1

The X-ray diffraction (XRD) is one of the most time-efficient and reliable

techniques in identifying the crystal structure of LixFePO

4 sample. This chapter

describes the XRD principles to analyse powders in general as well as to

measure the relative proportions of LiFePO4 and FePO

4.

In principle, the XRD is a non-destructive analytical technique commonly used

to analyse structures of crystalline powder or solids which resulted in a

diffraction pattern. This technique can identify the crystal lattice type and the

separation of planes of lattice points. The XRD starts when the X-ray beams

with wavelength λ are incident onto lattice layers of a crystal sample at an

angle θ, the diffraction occurs once the reflected beams are scattered from the

lattice layers. The diffraction pattern is caused by constructive interference,

meaning the waves are in phase, as shown in Figure 2.1. On the other hand,

destructive interference occurs when the waves are out of phase.1,2 The angle θ

where constructive interference occurs can be calculated by the Bragg

equation, as shown in Equation 2.1.2 The distance that the waves travelled after

the reflection depends on the distance between the atoms. The distances xy

and yz, which travelled by the lower plane, are the extra distances to that

reflected from the upper plane, which is 2dsin θ, as shown in Equation 2.2 and

Equation 2.3.2 To obtain constructive interference, the distance must be equal

to nλ, which corresponds to the diffraction pattern.2 This leads to characterise

the sample.


24

Equation 2.1 2

Equation 2.2

Equation 2.3 2

Where n= an integer λ = a wavelength of X-ray d = a space between lattice planes. h, k, and l = miller indices

For the interpretation, the wavelength of the incidence beam, lattice parameter,

lattice types, and crystal system of the sample can be defined from the

position and the number of reflections on the sample. The position and types

of atom correspond to the intensity of the reflections which are needed to

characterize the crystal structure.

Figure 2.2 shows Miller indices (peaks; hkl) of LiFePO4 and FePO

4 XRD patterns.

For example, the Miller index of LiFePO4 peak at ~17 degree 2 is 200, i.e. h =

2, k = 0, and l = 0 as calculated by Equation 2.1and Equation 2.4.1 (Crystals

exist in a number of different orientations and each crystal has its own specific

orientation; The Rietveld fit assumes that crystal orientations are random. A

Figure 2.1: Diagram shows the derivation of Bragg's law

θ

θ θ

D hkl

x

y

z


25

preferred orientation, e.g. horizontally placed needles, would distort the peak

intensities. )

Equation 2.4

Where h, k, and l are Miller indices

ɑ,b, and c are lattice parameters

XRD software calculates d-spacing from λ and θ which are given and detected,

respectively, by the XRD machine. The θ positions of the diffraction peaks give

a characteristic pattern to a crystalline material. For example, a lattice

parameter of LiFePO4 sample, i.e. a, b and c can be calculated if the angle (θ =

10.218 degree), wavelength (standard Cu radiation λ=1.5406 Å) and miller

indices (hkl, eg. 101) are known from the machine. The calculation can be

carried out as shown below:

2

1.5406 2 10.218

1.54062 0.1774

4.34

The known miller indices are 101,

1

14.34

1 0 1

Therefore, the lattice parameters are 10.332Å, 6.010Å, 4.787Å


26

Figure 2.2: The combination of the miller indices of LiFePO4 (blue)3 and FePO

4

(red)4 XRD patterns.

The verification of the diffraction pattern from known structure patterns was

obtained via the Inorganic Crystal Structure Database (ICSD; Royal Society of

Chemistry). Then, one or more of the known diffraction patterns were fitted

onto the experimental data pattern, using Diffrac.eva, Celref and Rietveld

refinement programs. Diffrac.eva is a simple phase matching program, and

was done prior to each method. The Celref program was used to get an

accurate lattice parameter. The Rietveld refinement was used to analyse and

calculate, by adjusting the parameter of the known structure. This process was

run repeatedly until the calculated diffraction pattern achieved a good fit with

the results of the experimental pattern.

Rietveld refinement was done, using GSAS (General Structure Analysis System)

software, which analyses powder diffraction data obtained from X-rays.5 Three

files were prepared to employ the software, i.e. the XRD diffraction data, a

crystallographic information file (.CIF file) that was obtained from ICSD, and an

instrument parameter that was obtained from the XRD machine (D2 Phaser,

Bruker; .prm file). CIF files should be used which are equal to the known

compound type, and these were observed using the Diffrac.eva program,

giving a space group and lattice parameters of each crystal structure to fit onto

the experimental XRD pattern.


27

Rietveld refinement is a fitting method between the sample XRD pattern, which

was diffracted from Bragg reflections, and a model reference XRD pattern. The

software, then minimises the differences between those patterns. In the fitting,

peak positions, peak intensities and peak widths are important messages that

can characterise and identify a crystal structure. Peak positions are constrained

by the unit cell, corresponding to the size and shape of the unit cell.6 Peak

intensities and peak areas are determined by atom positions in the structure.

The shape of the peak identifies the crystallite size (Lx) and microstrain (Ly).

Thus, a broadened peak corresponds to a small crystallite size/strain and is

determined as a defective crystal structure.

Diffraction peak shape refinement is one of the key parameters in obtaining a

satisfactory agreement between the experiment and the reference intensity,

combining Gaussian and Lorentzian functions. The Gaussian function

corresponds to peak asymmetry, related to the instrumental effects, whereas,

the Lorentzian function corresponds to microstrain and crystallite size which

can be solved from peak broadening problems.

For this case, LiFePO4 and FePO

4 is a two-phase material which is easy to refine.

This is because the GSAS software package calculates the space fraction by

comparing integral characteristics between two patterns. A miscibility gap is

where two patterns (i.e. LiFePO4 and FePO

4) are superimposed, as shown in

Figure 2.3. However, this gap is not as complicated as in a solid solution

region. The software can only calculate the solid-solution region with one

pattern. Nevertheless, the LiFePO4 and FePO

4 solid solution can be determined

by Vegard’s law, by identifying how the lattice parameter changes with

composition within a single phase.7 By changing the value of x in LixFePO

4, the

lattice parameters will shift linearly with the x value.7


28

Figure 2.3: Binary phase diagram of FePO4-LiFePO

4, obtained from XRD data.

This figure is adapted from Kobayashi et al, 2009.8

For its sample preparation, LixFePO

4 was dried at 80°C for ~12 hours to get a

powder product, and ~ 1 g of sample was put on top of an XRD sample holder.

Then, the sample was levelled down to equalize between the surface of the

powder and the sample holder. The holder in the XRD machine was placed and

made ready to analyse. The XRD parameters are provided in each experiment

section.

Electrochemical Technique 2.2

The electrochemical technique is also one of the most efficient methods to

interpret the stoichiometric coefficient. This experiment employed a technique

called cyclic voltammetry (CV) and potentiometric titration (PT). Although, CV

will be shown in Chapter 8 and PT will be shown in Chapter 6. Here, basic

principle of CV and PT should be described before going into details in the

subsequent discussions.

2.2.1 Cyclic Voltammetry

The purpose of CV experiment was to study an alternative of reducing reagent.

The parameters and experimental details that applied with CV will be described

in detail in Chapter 8. CV is a versatile technique used to observe preliminary

electrochemical processes in a system. This section describes a principle of CV,

an electrode preparation and a cell construction for this particular experiment.

0 X in LixFePO

4 1

Solid-solution Solid-solution Miscibility Gap


29

CV is a reversal technique which scans the potential range of an electrode

linearly with time and measures the current, forwards and backwards. The

potential can be swept positive which is the direction of the oxidation reactions

and vice versa, when the negative sweep potential refers to the direction of the

reduction reactions. Normally, the potential scan rate (ν) of CV is in the range

of 25-1000 mV s-1.9 However, the higher potential scan rate results in IR drop

(the voltage drop due to energy losses in a resistor) and charging currents.

When increasing the scan rate, oxidation peaks shift up. In contrast, the

oxidation peaks shift down when decreasing the scan rate. Those peaks

indicates whether the reaction is electrochemical reversible, quasi reversible,

and irreversible, where the electron transfer take place. For electrochemical

reversibility, the electron transfer occurs very fast. Whereas, intermediate rates

and slow rates transfer of electron indicate quasi reversible and irreversible

reaction, respectively.10 As shown in Figure 2.4, Epox and E

pred are a position of an

oxidation peak and a reduction peak, respectively. If the different of these two

peaks (∆Ep= E

pox- E

pred) is less than 59 mV (at 298 K and n = 1), this corresponds

to a reversible reaction. While equal and higher ∆Ep than 59 mV will correspond

to quasi reversible and irreversible reactions. This value obeys the Nernst

equation as shown in Equation 2.5 and Equation 2.6. Plug in the standard

values that are given into Equation 2.6, getting the result of 59 mV as shown in

Equation 2.7.

Figure 2.4: One-phase CV a) Cyclic votammetry profile presents a peak height (I

pred) and a peak position (E

pred) b) samples of CV profile A)reversible, B)

quasi-reversible, and C) irreversible electron transfer. Reprinted with permission from Ref.10. Copyright 2015 Springer London. (This figure was published in July 2005 by Brownson et al).10

Oxidation peak

Reduction peak


30

Equation 2.5

Where O = oxidised species n = number of electrons (e-) R = reduced species

Equation 2.6 .

When E = the potential/ V E

e0 = the standard potential/ V

R = the ideal gas constant (8.314 /J mol-1K-1) T = 25 °C (298.15 /°K)

n = number of e-1 F = Faraday constant (96485/ C mol-1) [ ] = concentration of oxidised species and reduced species/ mol L-1

Equation 2.7

Equation 2.8

Two-phase reaction is different than one-phase reaction as described above.

With the two-phase system, as LiFePO4/FePO

4 material, the CV behaviour shows

a pair of antisymmetrical peaks of a redox couple, as shown in Figure 2.5.

Both anodic and cathodic currents increase and decrease sharply with respect

to the potential.11 Broadening of peaks can indicate resistance and/or diffusion

limiting effects. Chapter 3 describes the two-phase reaction in more details.

Figure 2.5: A schematic drawing of a two-phase cyclic voltammogram, adapted from Roberts et. al.11

+

+

- -


31

In the case of LiFePO4, as presented in Equation 2.8, the Nernst equation can

be written as Equation 2.9 and Equation 2.10 when an activity of solid, i.e. ɑFePO4

and ɑLiFePO4

is equal to one.

Equation 2.9

Equation 2.10 log

Where ɑ = an activity

For an electrode preparation, a lithium ion positive electrode generally consists

of a mixture of an active material (i.e. LiFePO4 and FePO

4), an acetylene black

and a binder. The binder, such as polytetrafluoroethylene (PTFE) or

polyvinylidene fluoride (PVDF), assists the electrode to obtain a plastic–like

material.12

In this experiment, the electrode was produced, using ink deposition

technique. A noble metal mesh such as platinum (Pt) was used as a conductive

based material. The composition of an ink deposition is shown in Table 2.1.

First, the PVDF binder was dissolved in ~4 ml of N-Methyl-2-pyrrolidone (NMP)

and stirred at ~50°C until the binder was dissolved. Then, an active material

and carbon black were mixed in and the ink stirred at ~50°C for ~30 minutes.

The mixture was then sonicated for ~1 hour. Finally, the mixture was

continuously stirred at ~50-60 °C to evaporate all the NMP and allow the ink to

thicken.

Before immersing in the ink, platinum mesh was prepared by cleaning with de-

ionized water and burning residue with a torch. A clean mesh Pt was immersed

into the prepared ink to a depth of ¾ of the electrode, as shown in Figure 2.6.

The electrode was then dried for 24 hours in an oven at ~80°C. The mass of

the Pt electrode was subtracted from the combined mass of the coated

electrode which was weighed before use to obtain the active material weight.


32

Figure 2.6: a) before and b) after immersing a Pt mesh electrode into the ink

Table 2.1: The composition of an active material ink

Ink of active materials/g

Active material (sample) (75%wt) 0.60

Carbon black (15%wt) 0.12

PVDF (10%wt) 0.08

Total (100%wt) 0.80

Once the electrode was ready, a three-electrode cell system: working electrode

(WE), counter electrode (CE), and reference electrode (RE) was constructed as

shown in Figure 2.7. The cell was controlled by the VMP (Various Multi-channel

potentiostat). The working electrode was a Pt mesh coated with an active

material ink. A Pt mesh was used as a counter electrode. Saturated calomel

electrode (SCE; accumet®, Fisher Scientific) was used as the reference

electrode. Electrolytes will be described in Chapter 8.

a) b)

¾ of the electrode


33

2.2.2 Potentiometric Titration

Potentiometric titration (PT) is a common electrochemical technique to

characterise battery material and to measure its performance. In this section

the principle of PT, an electrode preparation, and a cell construction are

described.

A battery cell’s capacity by PT is measured by a constant current which is

repeatedly charged and discharged with a potential limitation. The current

relates to the specific C-rate of the cell. The C-rate means the current is

required to charge and discharge the battery within one hour. Normally, charge

and discharge rates are applied in fractions of the C-rate. Thus, a 0.1 C-rate

means the current is required to charge and discharge completely in 10 hours.

The applied current can be calculated by Equation 2.11 and Equation 2.12.

Equation 2.11

Equation 2.12

Where Q

A = Capacity of the active material in the electrode/ mAh

mA = Mass of the active material in the electrode/ g

QT = Theoretical capacity of the active material/ mA h g-1

I = Applied current/ A C

r = C-rate

Figure 2.7: Basic diagram of 3 electrode system


34

The theoretical capacity can be calculated by using Equation 2.13.13

Equation 2.13 1000

11

3600

.

Where n = number of electrons F = Faradic number (96485)/ C mol-1

M = molecular mass / g For example, calculating the theoretic capacity of LiFePO

4 is done as follows:

. . 169.887 170

An electrode pellet is used for this type of experiment. The composition of a

pellet is shown in Table 2.2. The constituents were mixed well with a pestle

and mortar for ~20 minutes and pressed to a thickness ~200-100 microns.

Then, the mixed material was punched out into a 1 cm diameter disc. The

pellets were dried under vacuum (Buchi® tube) at 120°C overnight to remove

water from the material and to be ready to assemble into a cell on the next

day.

The active material (LixFePO

4) is coated with approximately 1-3% carbon.

However, carbon black is added into LiFePO4 as a conductive additive,

providing electronic pathways for lithium intercalation to take place. PTFE

(DuPont®) is used as a binder to provide mechanical strength.

Table 2.2: The composition of an electrode

Cathode-working electrode/g

Active material (sample) (75%wt) 0.225

Carbon black (15%wt) 0.045

PTFE (10%wt) 0.030

Total (100%wt) 0.300

A sample pellet was assembled, using the Swagelok® cell which is worked as a

2-electrode cell, as shown in Figure 2.8. The sample pellet was placed at the

cathode as a positive electrode and is followed by a 1.4 cm diameter separator


35

(0.67 mm glass microfiber filters; Whatman®). Eight drops of 1 M LiPF6 were

added as an electrolyte. A 1 cm diameter disc of Lithium foil was used at the

anode as a negative and reference electrode. Finally, a current collector, a

spring, a connector, and perfluoroalkoxy (PFA) tube were placed and tightened.

The cell was assembled within the glove box and wrapped with Parafilm® to

prevent oxygen penetration. The cell was then connected the VMP (varied

multi-potentiostat; Bio-Logic instrument) ready for an experiment.

Inductively Coupled Plasma Mass Spectroscopy 2.3

An inductively coupled plasma mass spectrometer (ICP-MS) is one of efficient

analytical techniques to determine trace of metal in concentration of liquid,

solid, or gas sample. The ICP-MS is a combination of an extremely high

temperature ICP source equipped with MS. The advantages of ICP-MS such as

low detection limits for most elements, high selectivity, and high accuracy. In

A A B

Negative and reference electrode

B C D B

Positive electrode Separator

Figure 2.8: the assembled Swagelok cell A) negative and positive connectors,

B) perfluoroalkoxy (PFA) tube fittings, C) spring, D) current collector


36

this section, a principle, a digestion procedure, and calculations to interpret

the sample should be described.

The principle is that each isotope has a unique mass-to-charge ⁄ ratio. This

ratio is analysed by mass spectrometry. Inductive coupled plasma (ICP) is argon

plasma at extremely high temperature (~10,000° K). ICP is used to convert the

sample into ions. The mass spectrometer then separates the ions by ⁄ ratio.

The ions that match the selected ⁄ ratio will be delivered to a detector. The

detector will determine the presence of a number of ions proportional to the

concentration.14,15

Here, a digestion acid solution and a procedure were carried out, as described

in Delacourt et al. (2005) and Dean et al. (2002)15,16. A stock solution of 20% wt

HCl (23 ml.), 20% wt HNO3

(10 ml.), and De-ionized (DI) water (17 ml.) was

mixed to give a volume of 50 ml of a yellow colour solution. Firstly,

approximately 0.1 g of each sample was weighed to an accuracy of 10 % in a

sample vial. Then, 2 ml of the stock solution was added into the vail and

covered the vial with a watch glass. The sample was heated to approximately

50-60°C and stirred for two hours on a hot plate. During this time, the solution

turned more yellow except for a black precipitate, assuming to be the carbon

coating. After that, the vial was removed from the hot plate and added DI water

to a total volume of 10 ml. Then, the digested sample was filtered out the

black precipitate with a Gooch funnel, rinse the sample vial with 5 ml of DI

water, and filter with the funnel. Lastly, the filtered sample was collected (15

ml. in total).The sample were sent for trace elements analysis by ICP-MS at

Ocean and Earth Science, University of Southampton, (NOC) for results of the

ICP analysis.

An X-SERIES 2 ICP-MS (Thermo Fisher Scientific, Bremen, Germany) was setup in

standard mode using an impact bead/cyclonic spray chamber and concentric

nebuliser. The instrument was tuned for optimum sensitivity, stability and low

oxide formation using a 1 ppb multi-element tuning solution. Data was then

acquired for all isotopes of interest in peak-jumping mode (4 x 30 second


37

repeats per sample). After each sample analysis, a wash solution containing 3%

HNO3 was run until background levels were achieved (typically 3 minutes). All

samples, standards and blanks were spiked with internal standard elements

beryllium (Be), indium (In) and rhenium (Re).

The data quality was monitored throughout the run by examination of the

statistics produced after each analysis. Within the run the reproducibility was

typically better than 1% relative standard deviation (RSD) for the 4 repeats. The

data processing was carried out using the Plasmalab software. Raw data were

blank sample and internally corrected and then calibrated against matrix

matched synthetic standards

The X-Series 2 ICP-MS instrument configuration and settings are listed below: Radio frequency (RF) Power: 1.40kW Forward, <1W Reflected. Sample Introduction System: Concentric nebulizer with low-volume impact

bead spray chamber Torch: Standard one-piece quartz torch with Plasma Screen Interface: Standard Xt (Ni sample/Ni Skimmer) Cool Gas Flow (L/min) 13

Auxiliary Gas Flow (L/min) 0.8 Nebuliser Gas Flow (L/min) 0.90 Sample Uptake

Rate (mL/min) 0.4 approx. Detector Simultaneous pulse/analogue Wash Time Monitored, minimum 60 seconds, maximum 300 seconds Detection limits are element and run specific, typically <0.1ppb.

The results were returned in part per million (ppm) units and needed to

calculate further for the interpretation. Percentages of lithium were desired to

be reported for comparison with the other two methods. The ppm units were

converted into the percentage of element content: i.e. lithium, using Equation

2.14.

Equation 2.14

%


38

.

.

Some calculations were done using results from ICP analysis to study the ratio

of lithium ion concentration to other cation: i.e. Na+, K+, and Mg2+,

concentration uptake. For example, these are determined as follows: Li+ and

Na+ uptake, the ratio of lithium ion concentration to sodium ion concentration

as in solid ([Li+]:[Na+] solid), and lithium selectivity are shown in Equation 2.15-

Equation 2.17.

Equation 2.15

Equation 2.16 :

Equation 2.17 :

:

References 2.4

(1) Atkins, P. W.; De Paula, J.: The elements of physical chemistry. 4th ed.; Oxford University Press: Oxford, 2005; pp 394-395. (2) Zumdahl, S.; DeCoste, D. J.: Chemical Principles. 7th ed.; Cengage Learning, 2013; pp 791-793. (3) Yakubovich, O. V.; Belokoneva, E. L.; Tsirel'son, V. G.; Urusov, V. S. Electron density distribution in synthetic triphylite LiFePO

4 1990, 45, 93-99.

(4) Andersson, A. S.; Kalska, B.; Häggström, L.; Thomas, J. O. Lithium extraction/insertion in LiFePO4: an X-ray diffraction and Mössbauer spectroscopy study. Solid State Ionics 2000, 130, 41-52. (5) Larson, A. C.; Dreele, R. B. V.: GSAS Manual. 26 September 2004 ed.; Los Alamos National Laboratory: New Mexico, 2004; pp 1. (6) Langford, J. I.; Louër, D. Powder diffraction. Reports on Progress in Physics 1996, 59, 131-234. (7) Jacob, K. T.; Raj, S.; Rannesh, L.: Vegard's Law: A Fundamental Relation or an Approximation? ; International Journal of Materials Research, 2007; pp 1-7. (8) Kobayashi, G.; Nishimura, S.-i.; Park, M.-S.; Kanno, R.; Yashima, M.; Ida, T.; Yamada, A. Isolation of Solid Solution Phases in Size-Controlled LixFePO4 at Room Temperature. 2009, 19, 395-403. (9) Pletcher, D.; Royal Society of, C.: A first course in electrode processes. 2nd ed.; Royal Society of Chemistry: Cambridge, 2009; pp 187-189.


39

(10) Brownson, D. A. C.; Banks, C. E.: Interpreting Electrochemistry. In The Handbook of Graphene Electrochemistry; 1st ed.; Springer-Verlag London, 2014; pp 23-77. (11) Roberts, M. R.; Vitins, G.; Denuault, G.; Owen, J. R. High Throughput Electrochemical Observation of Structural Phase Changes in LiFe1 − xMnxPO4 during Charge and Discharge. Journal of The Electrochemical Society 2010, 157, A381-A386. (12) Prosini, P. P.: Iron Phosphate Materials as Cathodes for Lithium Batteries: The Use of Environmentally Friendly Iron in Lithium Batteries. In Iron Phosphate Materials as Cathodes for Lithium Batteries: The Use of Environmentally Friendly Iron in Lithium Batteries; Springer, 2011; pp 57. (13) Vincent, C. A.; Scrosati, C. A. V.: 2 - Theoretical background. In Modern Batteries; 2 nd ed.; Butterworth-Heinemann: Oxford, 1997; pp 18-64. (14) de Hoffmann, E.; Stroobant, V.: Mass Spectrometry: Principles and Applications. In Mass Spectrometry: Principles and Applications; 3rd ed.; Wiley, 2007; pp 69-71. (15) Dean, J. R.; Jones, A. M.; Holmes, D.; Reed, R.; Weyers, J.; Jones, A.: Practical Skills in Chemistry. 1st ed.; Pearson Education Limited: Dorset, 2002; pp 175-179. (16) Delacourt, C.; Poizot, P.; Tarascon, J.-M.; Masquelier, C. The existence of a temperature-driven solid solution in LixFePO4 for 0 ≤ x ≤ 1. Nat Mater 2005, 4, 254-260.


40

Chapter 3: Physical and Electrochemical Characterization of the LiFePO4 and

FePO4 system

41

Physical and Electrochemical Chapter 3:

Characterization of the LiFePO4 and FePO

4

system

In this chapter, LiFePO4 and FePO

4 were characterised via scanning electron

microscopy (SEM), X-ray diffraction (XRD), mass spectroscopy (MS) and

electrochemistry techniques.

Experimental Details 3.1

In this study, LiFePO4

and FePO4

were mainly examined by using three

techniques: scanning electron microscopy (SEM), X-ray diffraction (XRD), and

inductively coupled plasma-mass spectroscopy (ICP-MS). Cyclic voltammetry

(CV) was used to observe LiFePO4 cycling in lithium salt. Potentiometric titration

(PT) was employed to calibrate a curve of the stoichiometric coefficient x of

LixFePO

4 against specific charge. The overview of each technique was previously

described in Chapter 2. These measurements were performed in order to

calibrate results for kinetics experiments in Chapter 6.

Scanning Electron Microscopy (SEM)

For sample preparation, 0.001 g of 3 % carbon coated LiFePO4 (Tatung) was

transferred to a 10 ml sample vial, and acetone was added to cover the sample

(1-2 ml). The mixture was sonicated in an ultrasonic bath for approximately 1

hour. Then, 1-2 drops of suspension were transferred on an SEM specimen

stub using a disposable pipette. Once the sample had dried, it was ready to

examine using SEM. The same procedure was applied to the FePO4 sample,

which obtained from Chapter 4.1(1:2 of K2S

2O

8 to LiFePO

4 ratio).


FePO4 system

42

X-ray Diffraction (XRD)

Samples of LiFePO4

and FePO4 were examined using XRD (Bruker D2 Phaser),

and scanned from 15 to 50 degrees of XRD pattern with an increment of

0.0081 degrees with a time per step of two seconds, for approximately three

hours for the total scan. The reference XRD patterns of LiFePO4 were obtained

via the Inorganic Crystal Structure Database (ICSD; RSC). The diffrac.eva

program was employed to observe, roughly, the result of the sample XRD data.

Then, the GSAS (General structure analysis system) program was applied to

complete the Rietveld refinement of the experimental XRD pattern.

Cyclic voltammetry (CV)

The purpose of CV experiment is to study the behaviour of LiFePO4 and FePO

4.

A standard three-electrode cell was used. LiFePO4 ink coated on platinum mesh

(Pt), Pt mesh and a saturated calomel electrode (SCE) were used as a working, a

counter, and a reference electrode, respectively. A solution of 1.5 M Li2SO

4 was

made by adding 12.37 g of Li2SO

4 (0.1125 mol) (Sigma-Aldrich, ≥98.5 %) to 75

ml of de-ionized water (DI water) which was added and used as an electrolyte.

The electrode preparation and cell setup were as described in the previous

chapter. The cell was argon purged for ~1 hour. The cell were cycled between

-1.2 V and 1.2 V with a scan rate of 10 mV s-1 for 20 cycles.

Potentiometric Titration

Another electrochemical technique is the validation method, using Pt to

identify the stoichiometric coefficient x of LixFePO

4 for a series of pellets,

containing various percentages of LiFePO4 mixed with FePO

4. (FePO

4 was

obtained by delithiating LiFePO4 which will be described in Chapter 4.1.) Table

3.1 shows the actual masses of LiFePO4 initially and FePO

4 used. Each sample

pellet was incorporated into the Swagelok® cell, adding the mixture pellet of


FePO4 system

43

LiFePO4, separator, and lithium foil. Potentiometric cycling with potential

limitation was applied to the cells at a rate of 0.1C while monitoring the

potential in order to observe charge and discharge of the cells. More

information about the main pellet compositions, cell setup and the current

calculations were given in Chapter 2.

For example, %Li = 0

Equation 3.1

= (0.0173 g x 75%) x170 mA h g-1

= 2.205 mA h

Where active material is 75% mass of the pellet.

Equation 3.2

= 0.1 h-1 x 2.205 mA h

= 0.2205 mA

Table 3.1: The composition of LiFePO4 and FePO

4 as an active material

%Li

Active material mass/g Pellet

LiFePO4

(MW=157.76 g/mol) FePO

4

(MW=150.82 g/mol) Total

Average mass/g

I/mA

100 0.1578 0 0.1578 0.0234 0.2983

75 0.1578 x 0.75 =

0.1184 0.1508 x 0.25 =

0.0377 0.1561 0.0323 0.4120

50 0.1578 0.1508 0.3086 0.0252 0.3206

25 0.1578 x 0.25 =

0.0395 0.1508 x 0.75 =

0.1131 0.1526 0.0314 0.3998

0 0 0.1508 0.1508 0.0173 0.2205


FePO4 system

44

Mass-spectrometry

Lithium content in LiFePO4 and delithiated sample of LiFePO

4 was determined

by mass-spectrometery. Approximately 0.1 g samples containing both LiFePO4

and FePO4

were dissolved in an aqueous solution containing 20% wt HCl and

20% wt HNO3, and then the solution samples were sent to the National

Oceanography Centre Southampton (NOCS) for analysis. The analysis and

preparation methods were described in details in Chapter 2.

Results and discussion 3.2

The results of SEM, XRD, ICP-MS, CV and GM are shown below in order.

SEM

Morphological image analyses of the LiFePO4 sample are shown in Figure 3.1a-

c. Large agglomerates of large irregular needle and potato shapes are found

with dimensions of approximately 2 μm long by 0.5 μm thick (blue arrows) and

1 μm long by 0.2 μm thick (red arrows), respectively. All small sphere

agglomerates are presumably carbon coating.


FePO4 system

45

Figure 3.1: SEM images of an initial carbon coated lithium iron phosphate (Tatung), under magnification of a) 2,500x, b) 10,000x and c) 33,000x. The images were recorded with an acceleration voltage of 15 kV.

Figure 3.2 shows the morphology of the heterosite FePO4

obtained from the

delithiated LiFePO4 sample for 24 hours (see Chapter 4.1). The morphology of

heterosite FePO4

looks similar to LiFePO4, with large agglomerates of large

irregular needle and potato shapes. The agglomerates of the irregular needle

shape are approximately 2 μm long and 0.8 μm wide (blue arrows). For the

irregular potato shape, the sizes are approximately 1 μm long and 0.3 μm wide

(red arrows). All small sphere agglomerates are presumably carbon coating.

a) b)

c)


FePO4 system

46

From the morphological studies, it can be concluded that there are no

significant changes in terms of shapes and dimensions of both samples, with

respect to a micron scale. This demonstrates that the extent of corrosion was

insignificant, despite the fact that the delithiation process (heterosite FePO4

sample) involved a strongly oxidizing solution with agitation for 24 hours.

Figure 3.2: SEM images of a carbon coated heterosite iron phosphate, which was obtained from a delithiation process of 5.8 g LiFePO

4 + 0.1 M K

2S

2O

8

(Chapter 4.1), under magnification of a) 2,500x, b) 10,000x and c) 35,000x. The images were recorded with an acceleration voltage of 15 kV.

a) b)

c)


FePO4 system

47

XRD

Figure 3.3a-b shows the Rietveld refinement patterns obtained from the initial

LiFePO4 and FePO

4 samples, where the black cross marks represent the

experimental XRD pattern, red lines define the refined or calculated pattern,

blue-pink tick marks show the reflection position of LiFePO4- FePO

4 and below

the patterns is the difference plot shown in a dark blue line.

As shown in Figure 3.3a-b, the calculated patterns do not completely fit on the

experimental data. However, their lattice parameters are in agreement with the

references as shown in Table 3.2. Also, both X2 values are close to 1 which

indicates an ideal fitting.1

Table 3.2: the lattice parameter of the fitting pattern and the reference2

Sample Lattice Parameter/ Å X2

a b c

LiFePO4

Reference3 10.332(4) 6.010(5) 4.787 - Experiment 10.3154(2) 5.99938(12) 4.6889(1) 1.9

FePO4

Reference2 9.8142(2) 5.7893(2) 4.7820(2) - Experiment 9.8173(3) 5.7879(1) 4.7815(2) 2.8


FePO4 system

48

Figure 3.3: Fit to XRD data of a) LiFePO

4 (Tatung): R

wp 1.3%, R

p 1.0% and b)

FePO4:

R

wp 1.5%, R

p 1.1%.

Crosses mark the data points, red line is the fit and

blue line is the difference; a) blue and b) pink tick marks show the allowed reflection positions for LiFePO

4 and FePO

4, respectively.

a)

b)


FePO4 system

49

ICP-MS

Table 3.3 shows a list of metal ions, i.e. Li+, Na+, Mg2+ and K+, which are

contained in the initial sample of LiFePO4 and the heterosite FePO

4 , by using

ICP-MS. The results from LiFePO4

show approximately 300 ppm of lithium

content. For FePO4, a lithium contained of approximately 18 ppm was found,

which indicates that the sample was not completely delithiated. Due to the

solid solution as described in Chapter 2, XRD could not detect the small

amount of lithium in the heterosite FePO4 structure, as resulted in Figure 3.3b.

The report shows some contamination of Na+ and Mg2+ in both samples, but the

quantity is not significantly high. K+ was found in FePO4 to be approximately 15

ppm compared to LiFePO4, owing to K

2S

2O

8 working as an oxidizing agent for

the delithiation process.

Table 3.3: Metal ions contained in LiFePO4 and the heterosite FePO

4

Compound Metal ions/ ppm

Li+ Na+ Mg2+ K+

LiFePO4 306.65 0.99 2.16 *bd

FePO4 18.10 1.70 2.28 15.12

*bd = below detection

Cyclic voltammetry

A cyclic voltammogram of LiFePO4 with a Pt counter electrode versus SCE in 1.5

M Li2SO

4 at the scan rate of 10 mV s-1 is shown in Figure 3.4. The CV profile is

associated with lithium ion extraction and insertion towards the oxidation and

reduction of the Fe2+/Fe3+ redox couple. The red linear dashed line in the profile

represents a constant slope on both sides (i.e. oxidation and reduction) which

extrapolates to the same value of E0, i.e. 0.2 V. This characteristic of one pair

of symmetrical redox peaks is peculiar to a two-phase system such as LiFePO4

and heterosite FePO4.4


FePO4 system

50

The profile starts at E0, scanning towards positive potential with a full

conversion to Fe3+. Position “a” shows a current that is limited by resistance

according to Equation 3.3-Equation 3.4.

Equation 3.3

Equation 3.4

Where i = current/ A E = the potential/ V E0 = the standard potential/ V

R = resistance/ Ω

The drop in the current after position “b” suggests a limitation by diffusion

within the LiFePO4 particles. Lithium extraction at the surface was almost

completed at “c” after starting a backward scan. Lithium insertion was then

started at E0 to “d”, with a gradient depending on the resistance, in the same

way as “a”. Position “e” indicates the accumulation of lithium in the surface

until saturation when the scan reaches the potential limit of -1.2 V. Then, the

forward scan starts with no reaction occurs at “f”, until the scan reaches E0 as

for a cycle.


FePO4 system

51

`

Figure 3.4: Cyclic voltammogram of LiFePO4

in 1.5 M Li2SO

4 aqueous

electrolyte with a scan rate of 10 mV s-1, where Epa and E

pc are anodic and

cathodic peak potentials, respectively. Deviations due to uncompensated Ohmic drops can be observed by measuring potentials from the blue lines. At the top left corner shows the corrected potential value.

The CV voltammogram is characteristic of a phase-change material, because of

the very small difference between Epa and E

pc after IR compensation (R=0.088

Ω), so could be described as essentially reversible with fast kinetics at the

given scan rate of 10 mV s-1. However, the main purpose of the

electrochemistry was to determine E0 and the reaction free energy in the

forthcoming chapters. Cyclic voltammograms of a LiFePO4 electrode in the

presence of several reduction agents were also recorded in order to evaluate

the rate of chemical lithiation of FePO4 by those reducing agents, as described

in Chapter 8.


A calibration curve was made by galvanostatic charge-discharge of the

prepared cells at the rate of 0.1C to calibrate the LixFePO

4 samples in the

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

-4

-2

0

2

4

6

Ec

p

E0

I/ m

A m

g-1

E/ V vs. SCE

E0

a

b

c

d

e

f

Li+ extraction

Li+ insertion

Ea

p

-1.0 -0.5 0.0 0.5 1.0

-4

-2

0

2

4

6

Ecorr/ V vs. SCE


FePO4 system

52

kinetics study in Chapter 6. The first cycle of each cell extraction was used to

calculate a specific charge, as shown in Figure 3.5. Generally, a high specific

charge has a high percentage of Li content which corresponds to the value in

Table 3.4 and Figure 3.6.

Table 3.4: A specific charge of each percentage Li content for validation

%Li Specific Charge /mAh g-1

Initial extraction

100 154 75 120 50 82 25 42 0 4

Figure 3.5: Potentiometric titration of LixFePO

4 electrodes prepared with the

mixiture of LiFePO4 + FePO

4 = 100%, as indicated. Specific current: 17 mAh g-1

(at C/10).

0 20 40 60 80 100 120 140 1603.4

3.6

3.8

4.0

4.2

4.4100%75%25% 50%

E/ V

vs

Li/L

i+

Specific charge/ mAh g-1

0%


FePO4 system

53

Figure 3.6: A validating graph of lithium content (%) in LiFePO4 and specific

charge

In summary, this chapter demonstrated as shown below;

The morphological images of LiFePO4 and FePO

4 showed a similar

agglomerates of irregular needle and potato shapes, with a dimension of

approximately 4 μm long and 0.5 μm wide.

The Rietveld refinements of LiFePO4 and FePO

4 were illustrated, showing

lattice parameters derived from the experimental results similar to those in

the references.

Contamination with metal ions was less than 1% in LiFePO4. FePO

4 showed

some traces of lithium approximately 6% with the other metal ions, due to

the delithiation process.

The cyclic voltammetry of LiFePO4 showed a pair of symmetrical redox

peaks.

A calibration curve determination of the percentage of lithium content in

LiFePO4 is shown for the kinetics study in Chapter 6.

0 20 40 60 80 100

0

20

40

60

80

100

120

140

160

Sp

ecif

ic c

har

ge/

mA

h g

-1

% Li LiFePO4 FePO

4


FePO4 system

54

References 3.3

(1) McCusker, L.; Von Dreele, R.; Cox, D.; Louer, D.; Scardi, P. Rietveld refinement guidelines. Journal of Applied Crystallography 1999, 32, 36-50. (2) Andersson, A. S.; Kalska, B.; Häggström, L.; Thomas, J. O. Lithium extraction/insertion in LiFePO

4: an X-ray diffraction and Mössbauer

spectroscopy study. Solid State Ionics 2000, 130, 41-52. (3) Yakubovich, O. V.; Belokoneva, E. L.; Tsirel'son, V. G.; Urusov, V. S. Electron density distribution in synthetic triphylite LiFePO

4 1990, 45, 93-99.

(4) Mi, C.; Zhang, X.; Li, H. Electrochemical behaviors of solid LiFePO4

and Li0.99

Nb0.01

FePO4 in Li

2SO


Chemistry 2007, 602, 245-254.


2O


4 to FePO

4

55

Test of K2S

2O

8 as an Oxidising Chapter 4:

Agent of LiFePO4 to FePO

4

Chapter 3 studied the properties of LiFePO4

and heterosite FePO4, which are

selective to lithium. Heterosite FePO4 is used for lithium recovery and can be

obtained by extraction of lithium from LiFePO4. Therefore, the next step is to

find an oxidizing agent to act as an electron accepter, to remove lithium from

LiFePO4. In this chapter, we studied K

2S

2O

8 as an oxidising agent to obtain

FePO4 from LiFePO

4.

A Study of the Chemical Delithiation of LiFePO4 in 4.1

Aqueous Solutions

4.1.1 Introduction

The preliminary research of an oxidizing agent to delithiate LiFePO4 was

obtained by potassium persulphate (K2S

2O

8)1, using a 2:1 molar ratio of LiFePO

4

to K2S

2O

8 for which Equation 4.1is shown:1

Equation 4.1 2 → 2

This process stirred the aqueous mixture of K2S

2O

8 and LiFePO

4 at room

temperature for 24 hours. However, the literature did not indicate the

concentration of K2S

2O

8 used. Therefore, a small concentration of 0.1 M K

2S

2O

8

was used in this experiment. Three experiments were made, i.e. 1:2

(stoichiometric molar ratio), 3:2 and 6:2 of K2S

2O

8 to LiFePO

4 ratio.

4.1.2 Experimental Details

Five grams of K2S

2O

8 (Sigma-Aldrich, ACS reagent, ≥99.0%) was dissolved in

0.184 litres of de-ionized water (DI water; Purite) to obtain 0.1 M solution.


2O


4 to FePO

4

56

Then, 5.805 grams LiFePO4 (Tatung, ~3% carbon coated) with a molar ratio of

1:2 K2S

2O

8 to LiFePO

4 was added to the 0.1 M solution. The mixture was stirred

for approximately 24 hours at room temperature (~25°C). The samples were

collected at 1 h, 2 h, 4 h and 24 h after the experiment started. Each sample

was filtered with qualitative grade No. 1 (110 mm diameter) filter paper,

washed with DI water, and dried at 80 °C in an oven for approximately 12 h.

The samples were examined using X-ray diffraction technique (XRD; D2

Pharser) to measure the extent of conversion to FePO4 heterosite. The sample

was scanned from 15 to 50 degrees of XRD pattern. The reference XRD

patterns of LiFePO4 and FePO

4 were obtained via the Inorganic Crystal Structure

Database (ICSD; RSC). Celref and Diffrac.eva programs were used to analyse

the XRD patterns.

For the second and the third experiments, 3:2 and 6:2 molar ratio of LiFePO4 to

K2S

2O

8 were employed. These ratios were chosen to enhance a conversion rate

of FePO4

heterosite, as compared to that in the literature.1-5 The chemical

compositions of each experiment are shown in Table 4.1. The process of

collecting samples and analysis was the same as for the previous experiment.

Table 4.1: Chemical composition of the delithiation experiment by the use of K

2S

2O

8 as an oxidizing agent

Molar ratio of

K2S

2O

8:LiFePO

4

Chemical composition/g DI

water/L

Molar

K2S

2O

8/ M K

2S

2O

8 LiFePO

4

1:2

(stoichiometric) 5 5.805 0.184 0.1

3:2 10 3.881 0.369 0.1

6:2 10 1.94 0.369 0.1

4.1.3 Results and Discussion

Three types of molar ratio of K2S

2O

8 to LiFePO

4, i.e. 1:2, 3:2 and 6:2 are shown

below.


2O


4 to FePO

4

57

The K2S

2O

8 to LiFePO

4 ratio of 1:2 (stoichiometric molar ratio)

The samples were taken after 1-hour-long and 2-hour-long experiment, the

XRD patterns indicated a two-phase mixture of LiFePO4 and FePO

4 heterosite in

both samples, as shown in Figure 4.1 and Figure 4.2. The fitting to the XRD

patterns illustrated that these samples contain 9% and 3% of lithium.

The XRD fitting of the De-LiFePO4 at 4 hours (h) was not sufficient to identify

lithium content; however, the XRD pattern was found to show a very small

trace of LiFePO4 by visual examination as shown in Figure 4.3, whereas the

fully converted FePO4 result was indicated by XRD from the sample taken after

24 h. Figure 4.4 shows a single-phase FePO4 heterosite of the De-LiFePO

4 24 h

sample.


2O


2:1 for 1 hour. The XRD refinement indicates that the sample composition is Li

0.09FePO

4

The crosses shown are the data points, the red line is the fit and the blue line is the difference. The upper blue tick marks show the allowed reflection positions for LiFePO

4 and the lower pink tick marks are for heterosite FePO

4.


2O


4 to FePO

4

58

Figure 4.2: As in Figure 4.1, but the reaction was left for 2 hours and the XRD fitting indicates that the sample composition is Li

0.03FePO

4

Figure 4.3: As in Figure 4.1, but the reaction was left for 4 hours and the XRD fitting indicates that the sample composition is FePO

4.


2O


4 to FePO

4

59


4

3:2 of K2S

2O

8 to LiFePO

4 ratio

These experiments were done by adding a three times higher concentration

than the stoichiometric molar ratio experiment (1:2 of K2S

2O

8 to LFP). The

results from the sample taken after 30 minutes and 1 hour were found to be a

two-phase mixture. The XRD fitting from De-LiFePO4 for 30 min and 1 h

indicated 7% and 6% of lithium content, respectively, as shown in Figure 4.5

and Figure 4.6. The sample collected after 2 h to 24 h resulted as fully

converted to FePO4 heterosite as shown in Figure 4.7 and Figure 4.8.


2O


4 to FePO

4

60


2O


3:2 for 30 min. The XRD refinement indicates that the sample composition is Li

0.07FePO

4



4.

Figure 4.6: As in Figure 4.5, but the reaction was left for 1 hour and the XRD fitting indicates that the sample composition is Li

0.06FePO

4.


2O


4 to FePO

4

61


4.


4.


2O


4 to FePO

4

62

6:2 of K2S

2O

8 to LiFePO

4 ratio

To improve the technique with the possibility of a faster yet complete

conversion of FePO4, a six times higher K

2S

2O

8 was used. The results from 30

min and 1 h were found to be a two-phase mixture, which were similar to the

3:2 of K2S

2O

8 to LiFePO

4 ratio experiment. However, both of the results which

samples were taken at 30 m and 1 h (6:2 ratio), contained lower lithium

content than the 3:2 of K2S

2O

8 to LiFePO

4 ratio experiment, i.e. 5% and 4% Li,

respectively, as illustrated in Figure 4.9 and Figure 4.10. All samples taken

after 2 h resulted as fully delithiated LiFePO4, which was similar to the 3:2 of

K2S

2O

8 to LiFePO

4 ratio experiment, as reported in Figure 4.11and Figure 4.12.

The stoichiometric coefficient of lithium content from samples in each

experiment is shown in Table 4.2.


2O


6:2 for 30 min. The XRD refinement indicates that the sample composition is Li

0.05FePO

4



4.


2O


4 to FePO

4

63

Figure 4.10: As in Figure 4.9, but the reaction was left for 1 hour and the XRD fitting indicates that the sample composition is Li

0.04FePO

4.


4.


2O


4 to FePO

4

64


4.



delithiation of LiFePO4 from each experiment at 30 m, 1 h, 2 h, 4 h, and 24 h

K2S

2O

8: LiFePO

4

Time / h

0.5 1 2 4 24

1:2 (stoichiometric)

- Li0.09

FePO4 Li

0.03FePO

4 FePO

4 FePO

4

3:2 Li0.07

FePO4 Li

0.06FePO

4 FePO

4 FePO

4 FePO

4

6:2 Li0.05

FePO4 Li

0.04FePO

4 FePO

4 FePO

4 FePO

4

As shown in Table 4.2, in the 1:2 of K2S

2O

8 to LiFePO

4 ratio experiment, the

LiFePO4 sample was fully converted into FePO

4 heterosite after 4 h, whereas in

the other two experiments were fully converted after 2 h.

Figure 4.13 shows a preliminary analysis of the kinetic data. The graphs show

that the experiment using 1:2 of K2S

2O

8 to LiFePO

4 ratio completed the chemical

oxidation last, while the other experiments were faster. However, the kinetics

study cannot be defined due to the shortage of the collected data vs. time.


2O


4 to FePO

4

65

As shown in Figure 4.14, not all data show a linear relationship. The graph of

1:2 of K2S

2O

8 to LiFePO

4 ratio experiment shows a rough estimation of first

order reaction in the concentration of the reactant, i.e. K2S

2O

8 where the plot at

2 h indicates a depletion of the reactant. For the other two experiments, the

reactions almost completed after 30 min and continue for another 30 min,

which might be due to the residual reaction, i.e. reaction with large particles.

The large particles react to the reactant slowly; whereas, the small particles

react fast. In fact, these graphs show a distribution of particle. The main

limitation to the reaction rate is the concentration of S2O

82-. Low concentration

of S2O

82- slows down the reaction rate, as shown in the graph of 1:2 of K

2S

2O

8 to

LiFePO4 ratio experiment in Figure 4.14. If the concentration of S

2O

82- is high,

the concentration of S2O

82- is not a limiting step but the depletion of FePO

4.

0 5 10 15 20 25

0.0

0.2

0.4

0.6

0.8

1.0

x in

Li (1

-x)F

eP

O4

Time/ h

1K2S

2O

8:2 LiFePO

4

3K2S

2O

8:2 LiFePO

4

6K2S

2O

8:2 LiFePO

4

Figure 4.13: Delithiation of LiFePO4 during 24 hours, () 1:2 of K

2S

2O

8 to

LiFePO4, ( ) 3:2 of K

2S

2O

8 to LiFePO

4, ( ) 6:2 K

2S

2O

8 to LiFePO

4 experiments


2O


4 to FePO

4

66

Figure 4.14: The kinetics delithiation of () 1:2 of K2S

2O

8 to LiFePO

4, ( ) 3:2 of

K2S

2O

8 to LiFePO

4, ( ) 6:2 K

2S

2O

8 to LiFePO

4 experiments

Analysis of the Delithiation rate using a Conductivity 4.2

Measurement

4.2.1 Introduction

The extent of the reaction from the previous section 4.1 can be monitored by

measuring the conductivity of the solution. Assuming the molar conductivity of

all ions are similar, the conductance is expected to be double in the reaction,

as presented in Equation 4.2.

Equation 4.2 2 2 → 2 2 2 2

3 ions 6 ions


2O


4 to FePO

4

67

Dilute solutions of salts usually have an ionic conductivity that is proportional

to the concentration. Conductivity can be used to monitor the ionic

concentration in the solution according to Equation 4.36;

Equation 4.3 ᴧ

Where κ = conductivity/ S m-1

ᴧ= molar conductivity/ Sm2 mol-1

c = concentration/ mol dm-1

The conductivity is calculated from measurements of the conductance and

dimensions of a material as shown in Equation 4.4.

Equation 4.4

Where G = conductance/ S (Siemen) l = length of the current path/ m A = area/ m-2

Conductance is a reciprocal of resistance and measured from Equation 4.5 as

follows;

Equation 4.5

Equation 4.6 is a result of substitution of Equation 4.3 and Equation 4.4 in

Equation 4.5. Therefore, the current is proportional to the concentration if all

the other terms remain constant as refer in Equation 4.6.

Equation 4.6 ᴧ

Where I = current/ A V= potential/ V

Here, two experiments are undertaken in order to study the rate of the reaction

depending on the concentration of K2S

2O

8. In each case, the conductivity and

temperature of the solution were measured versus time.

The reason for measuring the temperature is that the temperature indicates a

measure of the energy spent in heating or cooling the solution. Therefore, the


2O


4 to FePO

4

68

extent of the reaction is reflected in the temperature increase or decrease

according to the sign of the enthalpy change, ∆H, which is usually negative,

indicating heating. In either case, the change in temperature will reflect the

extent of the reaction.


Two experiments were completed, using 0.1 M K2S

2O

8 and 0.2 M K

2S

2O

8 as

reactants. The first of these, 0.1 M K2S

2O

8 was used and measured the

conductivity with time, before and after the addition of LiFePO4. For the second

experiment, 0.2 M K2S

2O

8 was used, and both conductivity and the temperature

were measured as a function of time.

0.1 M K2S

2O

8 (measuring conductivity versus time)

A solution of 0.1 M K2S

2O

8 was prepared by adding 5.40 g of K

2S

2O

8 (0.02 mol)

to 200 ml of DI water in a 250 ml Erlenmeyer flask. Then, the solution was

stirred and the conductivity of the solution was measured by conductivity

meter (Hanna). 5 g or 0.03 mol LiFePO4 was then added to 0.02 mol K

2S

2O

8 to

give a small excess of K2S

2O

8 with respect to the stoichiometry described in

Equation 4.2. Under this condition the rise in conductance was defined by the

amount of LiFePO4.

The timer was started after adding LiFePO4. Readings of the conductivity of the

solution were taken at 2, 5, 10, 15, 20, 30, 60, 1080, 1140, and 1440 minutes

after adding LiFePO4. The solution was stirred continuously except when

collecting data at which times the stirrer was turned off.

0.2 M K2S

2O


The procedure for this experiment was similar to the one for experiment 1.

However, a solution of 0.2 M K2S

2O

8 was made by adding 5.40 g (0.02 mol)


2O


4 to FePO

4

69

K2S

2O

8 to 100 ml of DI water in a 250 ml beaker (to add a thermometer to the

glass container). The temperature and conductivity of the solution were

measured then the solution was stirred while adding, 5 g (0.03 mol) LiFePO4

quickly while the timer was started. Readings of the conductivity and

temperature of the solution were taken at 2, 5, 10, 15, 20, 30, 60, 160, 180,

1080, 1140,1200, 1260 and 1440 minutes


0.1 M K2S

2O


Figure 4.15 shows a rapid increase in the conductivity of 0.1 M K2S

2O

8 during

the first 10 minutes, from ~20 μS cm-1 to ~28 μS cm-1 and then it falls to a

plateau ~26 μS cm-1. The peak indicates a completed reaction between 10-30 m

possibly due to the residual reaction or the reduced temperature. While the

experiment of 0.2 M K2S

2O

8 started at ~30 μScm-1 as shown in Figure 4.16. It is

noted that the initial conductivity of 0.2 M K2S

2O

8 solution is ~10 μS cm-1 higher

than the 0.1 M K2S

2O

8 one, as expected due to the higher K

2S

2O

8 concentration.

Figure 4.15: The conductivity obtained from the mixture of 0.1 M K2S

2O

8 and 5

g LiFePO4 at 25°C with respect to time.

0 200 400 600 800 1000 1200 1400

0

10

20

Co

nd

uct

ivit

y/

S c

m-1

time/ min

0 10 20 30 40 50 60

20

22

24

26

28

time/ min

Co

nd

uct

ivit

y/

S c

m-1


2O


4 to FePO

4

70

Figure 4.16: The conductivity and temperature obtained from the mixture of 0.2 M K

2S

2O

8 and 5 g LiFePO

4 with respect to time.

Figure 4.16 shows a rapid rise in both conductivity and temperature followed

by a fall in the temperature according to an exponential decay at a rate of -

0.053 m-1, as shown in Figure 4.17. The initial rise in both before and after

adding LiFePO4 is attributed to the concentration and temperature effects as

shown in Equation 4.7 and Equation 4.8.

Equation 4.7 ᴧ

Equation 4.8 ᴧ

When Equation 4.8-Equation 4.7 = Equation 4.9

Equation 4.9 ∆ ᴧ

Where κ = conductivity/ S m-1

c = concentration/ mol dm-1

ᴧ25

= molar conductivity at room temperature/ Sm2 mol-1

α = temperature coefficient T = actual temperature/ °C T

25 = room temperature (25°C)

0 200 400 600 800 1000 1200 1400

0

10

20

30

40

50

time/ min

Co

nd

uct

ivit

y/

S c

m-1

0

5

10

15

20

25

30

Conductivity/ S cm-1

Temperature/ °C

Tem

per

atu

re/ °

C

0 20 40 60 80 100 120

30

35

40

45

50

55

Time/ min

Co

nd

uct

ivit

y/

S c

m-1

16

18

20

22

24

26

28

Te

mp

erat

ure

/ °C


2O


4 to FePO

4

71

The contribution due to ᴧ25

(c2-c

1) is shown by the change in conductivity at the

end of the experiment after the solution cooled, as shown in Equation 4.9. The

reason for the initial peak rises is due to the second term that reflect the effect

of a temporary temperature rises before the solution cool to room

temperature.

Figure 4.17: The exponential decay of 0.2 M K2S

2O

8

where T= temperature

Tinitial

= temperature at start

Tfinal

= temperature at final

The summary of Chapter 4 are shown as follows;

90% conversion to FePO4 was obtained within an hour for all reagent

concentrations.

Almost 99% conversion to FePO4 occurred after 30 minutes for the 3:2

and 6:2 of K2S

2O

8 to LFP ratio, after 2 hours for the 1:2 of K

2S

2O

8 to LFP

ratio.

The delithiation results showed that a depletion of FePO4 occurred with

an excessive amount of S2O

82- in the solution.

The conductivity result showed that the reaction completed in 10

minutes.

A high conductivity reflects a high concentration, and so does the

temperature.

0 10 20 30 40 50

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

ln (

T-T

fin

al)/

(Tin

itia

l-T

fin

al)

time/ min


2O


4 to FePO

4

72

References 4.3

(1) Ramana, C. V.; Mauger, A.; Gendron, F.; Julien, C. M.; Zaghib, K. Study of the Li-insertion/extraction process in LiFePO

4/FePO

4. Journal of Power

Sources 2009, 187, 555-564. (2) Zaghib, K.; Mauger, A.; Goodenough, J. B.; Gendron, F.; Julien, C. M. Electronic, Optical, and Magnetic Properties of LiFePO4: Small Magnetic Polaron Effects. Chemistry of Materials 2007, 19, 3740-3747. (3) Ait-Salah, A.; Dodd, J.; Mauger, A.; Yazami, R.; Gendron, F.; Julien, C. M. Structural and Magnetic Properties of LiFePO4 and Lithium Extraction Effects. Zeitschrift für anorganische und allgemeine Chemie 2006, 632, 1598-1605. (4) Dodd, J. L.; Yazami, R.; Fultz, B. Phase Diagram of Li

xFePO

4.

Electrochemical and Solid-State Letters 2006, 9, A151-A155. (5) Miao, S.; Kocher, M.; Rez, P.; Fultz, B.; Yazami, R.; Ahn, C. C. Local Electronic Structure of Olivine Phases of LixFePO4. The Journal of Physical Chemistry A 2007, 111, 4242-4247. (6) Atkins, P. W.; Julio, D. P.: Electrochemistry. In The elements of physical chemistry; 4th ed.; Oxford University Press: Oxford 2005; pp 200-224.


2O


4 to FePO

4

73


2O

3 as a Reducing Agent of FePO

4 to LiFePO

4

75

Test of LiI and Na2S

2O

3 as Chapter 5:

Reducing Agent of FePO4 to LiFePO

4

Introduction 5.1

The previous chapter, studied on the oxidizing agent to extract lithium to

obtain the heterosite FePO4

as a framework. Here, this section discusses the

choice of a suitable reducing agent to selectively insert lithium into the

heterosite FePO4 framework without inserting other cations, such as sodium,

calcium and magnesium. These cations are all present in brine in a larger

concentration than lithium.

There are several reports of lithiation of FePO4 from solution of lithium iodide

(LiI) in acetonitrile.1-4 In this work, an aqueous reducing agent was preferred for

environmental reasons and therefore in the first approach, LiI was studied in

aqueous solution instead, Equation 5.1

Equation 5.1 2 2 → 2

A high concentration of LiI was used to make the reaction faster, according to

Le Chatelier’s principle. Another way is to use an activating agent, i.e. zinc, to

aid an increase in the rate of the reaction, as shown in Equation 5.2. Zinc

reacts with iodine (I2) which removes I

2 from the reaction in Equation 5.1.

Equation 5.2 →

Here, this experiment is considered to examine LiI, LiI+Zn and S2O

32- in aqueous

solutions as a possible reducing agent. Nevertheless, lithium salt (e.g. LiCl or

LiSO4) would be added to the solution as a lithium source.


2O


4 to LiFePO

4

76

Experimental Details 5.2

Lithium iodide was used twice with the aid of a higher concentration (1M LiI

and 2 M LiI). The last experiment with LiI also obtained Zn as an activating

agent. Then, thiosulphate was introduced, with and without the lithium salts in

order to investigate sodium behaviour from Na2S

2O

3.

The products from each experiment were examined using X-ray diffraction

(XRD; D2 Pharser) to characterize the LiFePO4

olivine structure or FePO4

heterosite structure. The sample was scanned from 15 to 45 degrees of XRD

pattern. The reference XRD patterns of LiFePO4 and FePO

4 were obtained via the

Inorganic Crystal Structure Database (ICSD; RSC). Celref and Diffrac.eva

programs were used to analyse the sample XRD patterns.

LiI as a reducing agent

a) 1 M LiI

A molar ratio of 3:1 of LiI to FePO4 was used. A solution of 1 M LiI was made by

adding 5 g of LiI (0.0373 mol) (Sigma-Aldrich, ≥99.0 %) to 37.3 ml of de-

ionized water (DI water). Then, 1.875 g of FePO4 (from the previous delithiation

LiFePO4 experiments) was added. While LiI and FePO

4 were mixed by stirring on

a hotplate for approximately for 30 minutes at room temperature, the reaction

was observed. The product was then filtered, washed with DI water, and dried

at 80°C for approximately 12 hours. The product was examined using XRD.

b) 2 M LiI, 2 M LiI +Zn

A molar ratio of 3:1 of LiI to FePO4 was used in the same way as the experiment

above. The same amount of LiI and FePO4 weight was used. A solution of 2 M

LiI was made in 18.65 ml of DI water. The mixing procedure for collecting and

analysing the product was done in the same way as previously.


2O


4 to LiFePO

4

77

Zn was used as an activating agent in 2 M LiI. According to Equation 5.2,

stoichiometry molar ratio is 1:2:2 (Zn: FePO4: LiI). However, there were an

excess molar ratio of Zn to LiI which was adjusted to 3:1:3. A solution of 2 M

LiI was used by adding 5 g of LiI (0.0373mol) in 18.65 ml of DI water. One

2.443 g purified Zn granule (0.0373 mol) (BDH Chemicals, Ltd., 99.8 %) and

1.875 g of FePO4 (0.0124 mol from the previous delithiation LiFePO

4

experiments) were prepared.

The FePO4 and the Zn granules were added to the LiI solution. The mixture was

combined by stirring on a hotplate for approximately 30 minutes at room

temperature. The procedure for collecting and analysing the product was

similar to the previous experiment.

Using Na2S

2O

3 as a reducing agent with/without lithium salts

a) With lithium salts

According to Equation 5.3, lithium sulphate (Li2SO

4) and lithium chloride (LiCl)

were used as lithium salts as shown in Equation 5.4 and Equation 5.5.

Equation 5.3 2 2 2 → 2


Equation 5.5 2 2 2 → 2 2

Equation 5.4 shows a molar ratio of 1:2:2 of Li2SO

4: Na

2S

2O

3: FePO

4. However, a

molar ratio of 2:4:1 of Li2SO

4:Na

2S

2O

3:FePO

4 was used. The excess of reducing

agent, i.e. Na2S

2O

3, was applied. A solution of 1 M Li

2SO

4 was made by adding 4

g of Li2SO

4 (0.0363 mol) (Sigma-Aldrich, ≥98.5 %) in 36.38 ml of DI water. The

solution was stirred and heated to approximately 50°C on a hotplate to aid its

dissolution. The solution was allowed to cool down to room temperature.

Then, 18.021 g of Na2S

2O

3 (0.0728 mol) (Na

2S

2O

3·5H

2O, Timstar Laboratory

Supplier, Ltd.) was added, stirred, and heated. After Na2S

2O

3 was dissolved,

2.719 g of FePO4 (0.0182 mol) was then mixed into the solution and continued

to be stirred for approximately for 1 hour at room temperature. The process of


2O


4 to LiFePO

4

78

collecting and analysing the product was done in the same way as the

procedure above.

Equation 5.5 shows a molar ratio of 2:2:2 of LiCl: Na2S

2O

3: FePO

4. A molar ratio

of 2:2:0.5 of LiCl: Na2S

2O

3: FePO

4 was used. The excess of reducing agent, i.e.

Na2S

2O

3, was applied. A solution of 1 M LiCl was made by adding 5 g of LiCl

(0.1179mol) (Sigma-Aldrich, ≥ 99.9%) in 117.9 ml of DI water and stirred at

room temperature. After the LiCl was dissolved, 29.594 g of Na2S

2O

3 (0.1179

mol) was mixed into the solution and continued to be stirred at room

temperature. 4.44 g of FePO4 (0.0294 mol) was added to the solution when

Li2SO

4 and Na

2S

2O

3 were dissolved. The mixture was stirred on a hotplate for

approximately 1 hour at room temperature. The process of collecting and

analysing the product was done in the same way as the procedure above.

b) Without lithium salts


The purpose of this experiment is to observe whether sodium can be inserted

into FePO4 by S

2O

32- reagent. A possible reaction in the absence of lithium salt is

shown in Equation 5.6. A molar ratio of 2 Na2S

2O

3: 1 FePO

4 was used. A

solution of 1 M of Na2S

2O

3 was made by adding 10 g of Na

2S

2O

3 (0.0403 mol) in

40.29 ml of DI water. Heating (~50°C) and stirring were applied on a hotplate

to dissolve the Na2S

2O

3. Then, after the solution cooled down, 3.037 g of FePO

4

(0.020 mol) was added after Na2S

2O

3 had dissolved. The mixture was

continuously stirred for 1 hour then filtered, washed and dried at 80°C. The

product was analysed in the same as previously mentioned.


2O


4 to LiFePO

4

79

Results and Discussion 5.3

The XRD pattern samples were intended to match approximately with the

indexed pattern of LiFePO4, FePO

4, and NaFePO

4, which were obtained via the

Inorganic Crystal Structure Database (ICSD; RSC).

LiI as a reducing agent

a) 1M LiI

During this experiment, a yellowish colour formed in the solution. This

indicates the colour of iodine (I2) which corresponds to Equation 5.1. However,

the XRD pattern of 1 M LiI sample showed no conversion from FePO4 to LiFePO

4

as shown in Figure 5.1. The figure shows the combination of the Miller indices

of the FePO4 XRD pattern and LiFePO

4 XRD pattern. The sample XRD pattern

matched the FePO4 XRD pattern. The conclusion was that 1 M LiI in aqueous

solution did not react with FePO4.

Figure 5.1: XRD patterns of 1 M LiI sample obtained after 30 m at room temperature, compared to FePO

4, and LiFePO

4. All diagrams were indexed in the

orthorhombic (Pnma (62)) crystallographic system.


2O


4 to LiFePO

4

80

a) 2M LiI Throughout 2 M LiI experiment, a yellow-brownish colour formed in the

solution which indicates the colour of iodine (I2), again in the Equation 5.1. This

occurred the same way as in the 1 M LiI experiment. However, the XRD pattern

of 2 M LiI sample still showed no trace of olivine structure in the sample as

shown in Figure 5.2.

15 20 25 30 35 40 450

1

2

3

4

XR

D In

ten

sity

/CP

S X

103

2Theta/ Degree

FePO4 pattern

LiFePO4 pattern

40

0

41

1

112

31

1

10

1

21

0

01

1 111

020

112

121

10

1

21

0

01

1

20

2

00

2121

212

321

202

401

22141

0102

311

22030

1

020

211

201

111

011

101

210

200

200

10

1

21

0

01

1

201

21

10

20

30

1

220 102

41

0

221

401

212

15 20 25 30 35 40 450

1

2

3

4

5

6

7

112

2M LiI

200

101

210

011

111

201

211

020

301

311

121

102 410

221

401

202

41

1

212

Figure 5.2: XRD patterns of 2 M LiI obtained after 30 m at room temperature, compared to FePO

4, and LiFePO



2M LiI + Zn

The last attempt was made using Zn as an activating agent in 2 M LiI solution.

The colour of the solution changed to a yellow- brownish colour indicating I2 as

shown in Equation 5.1. In the reaction, LiI was oxidized to form I2, giving the

brownish colour. Lithium ion might have remain as Li+ in the solution because

the potential from LiI is not enough to provide for Li+ insertion into FePO4. Zinc

weights before and after the experiment were 2.5815 g and 2.5742 g.

Therefore, approximately 0.0073 g of Zn was dissolved in the solution.


2O


4 to LiFePO

4

81

2.443 g of Zn is expected to dissolve in the solution as shown in the

calculation below.

.

65.39 2.443

However, 0.0073 g of Zn was used in the solution which is equivalent to 1.116

x10-4 mole as shown below

0.0073 65.39

1.116 10

This corresponds to the product of LiFePO4, as shown in the calculation below.

2 1.116 10 157.76

0.0352

Therefore, the amount of Zn consumed in the experiment was little compared

to the amount expected from the reaction. Also, the XRD result did not show

any trace of LiFePO4, as expected in Figure 5.3.

However, the concentration of LiI may need to be more than 2 M in order to

convert FePO4 to LiFePO

4. Since 2 M LiI is quite high concentration, therefore

another reducing agent is used.


2O


4 to LiFePO

4

82

15 20 25 30 35 40 450

1

2

3

4

LiFePO4 pattern

XR

D In

ten

sity

/CP

S X

103

2Theta/ Degree40

0

411

11

2

311

10

1

210

011

11

1

112

121

10

1

210

011

202

00

2

121

212

321

202

401

22141

0102

311

220301

020

211

201

111

011

101

210

200

20

0

10

1

210

011

20

1

211

020

301

22

0

10

24

10

22

14

01

21

2

FePO4 pattern

15 20 25 30 35 40 45

3

4

5

6

7 2M LiI +Zn

112

200 10

1

210

011

111

201

211

020

301

311

121

102

410

221

401 20

2

41

12

12

Figure 5.3: XRD patterns of 2 M LiI +Zn sample obtained after 30 m at room temperature, compared to FePO

4, and LiFePO



Using Na2S

2O

3 as a reducing agent with/without lithium salts

Lithium salts, i.e. Li2SO

4 and LiCl, were used as a lithium source in the lithiation

experiment. Na2S

2O

3 acted as a reducing agent as shown in Equation 1.13. The

results showed a total conversion of FePO4 to LiFePO

4. A control experiment,

i.e. no lithium salts, was performed using Na2S

2O

3. The result found no trace of

NaFePO4 in its sample. The result of each experiment is shown in the

following.

a) With lithium salts

Li2SO

4

Approximately 30 minutes into the experiment, the optical observation of the

lithiated material suspension was shown to have darker colour. This evidence

indicated a change of the sample. According to Equation 5.4, the sample

molecule was expected to show a conversion of FePO4 to LiFePO

4. As the result


2O


4 to LiFePO

4

83

shown in Figure 5.4 indicates fully lithiated FePO4 was found in the XRD

sample pattern.

15 20 25 30 35 40 450

1

2

3

4

2Theta/ Degree

FePO4 pattern

LiFePO4 pattern

400

411

112

311

101

210

011 11

1

112

121

101

210

011

202

002

121

212

321

20240

122

141010

2

311

22030

1

020

211

201

111

011

101

210

200

200

101

210

011

201

211

020

301

220

102

410

221

401

212

15 20 25 30 35 40 450

1

2

3

4

112

202

32122

14

01

102410

002

121

220

311

301

201

111

0112

10

101

200

XR

D In

ten

sity

/CP

S X

103

2Li2SO

4: 4Na

2S

2O

3: 1FePO

4

211

020

Figure 5.4: XRD patterns of 2Li2SO

4: 4Na

2S

2O

3: 1FePO

4 sample obtained after

1 h at room temperature, compared to FePO4, and LiFePO

4. All diagrams

were indexed in the orthorhombic (Pnma (62)) crystallographic system.

LiCl

The reaction of this experiment (Equation 5.5) was similar to the experiment

using Li2SO

4. A black colour of the solution was found during the reaction. The

sample was analysed and showed the result given in Figure 5.5. A total

conversion LiFePO4 was found for the XRD sample pattern.


2O


4 to LiFePO

4

84

15 20 25 30 35 40 450

1

2

3

4

101

2Theta/ Degree

FePO4 pattern

400

411

112

311

111

112

121

202

00212

1

212

3212 0

2401

22141

0

102

311

220301

020

211

2 01

111

0 11

101

210

2 00

200

101

210

011 2

01

211

020

301 220

102

410

221

401

21

2

LiFePO4 pattern

15 20 25 30 35 40 450

1

XR

D In

ten

sity

/CP

S X

103

002

102

112 20

2

21

2

121

220

311

301

020

21101

1210

200

2LiCl: 2Na2S

2O

3: 0.5FePO

4

111

201

401

221

Figure 5.5: XRD patterns of 2LiCl: 2Na2S

2O

3: 0.5FePO

4 sample obtained 1 h

at room temperature, compared to FePO4, and LiFePO


indexed in the orthorhombic (Pnma (62)) crystallographic system.

b) Without lithium salts

The re-insertion without lithium salts (sodiation or sodium insertion) was done

using 2:1 of Na2S

2O

3 to FePO

4 ratio. The main reason was to see if sodium ion is

more difficult to be inserted into the heterosite structure than lithium ion, as in

Equation 5.6, and to see whether Na2S

2O

3 acted as a reducing agent for

NaFePO4 or not.

During the experiment, there was no evidence of a colour change in the

suspension. This suggests that Fe2+ compounds were not formed. In Figure 5.6,

the XRD sample pattern found no trace of NaFePO4. Therefore, the sodium ion

is harder to insert into the heterosite structure than lithium ion by stirring at

room temperature.


2O


4 to LiFePO

4

85

15 20 25 30 35 40 450

1

2

3

4

5

6

401

4 10

400

220

2 20

301

0 20

211

1 01

2 00

2 02

2 12

321

411

2Theta/ Degree

NaFePO4 pattern

FePO4 pattern

101

011 22

140

1 1 12

212

410

1 0212

13 1

1

301

020

211

201

210

1 11

200

112

2 02

210

011

201

111

311 1 2

100

210

2

15 20 25 30 35 40 450

1

2

3

4

XR

D I

nte

nsi

ty /

CP

S X

103

101

011

221

401

112

212

410

10212

13 1

1

301

0 20

211

20121

0

1112 0

0

2Na2S

2O

3 : 1FePO

4

Figure 5.6: XRD patterns of 2:1 of Na2S

2O

3 :FePO4 ratio sample obtained 1 h at


4. All diagrams were indexed

in the orthorhombic (Pnma (62)) crystallographic system.


Aqueous LiI, by contrast with LiI in acetonitrile, does not give any trace

of lithium insertion into FePO4, even when Zn is added to remove the I

2

product.

Na2S

2O

3 reduces FePO

4 completely in the presence of excess lithium salt,

resulting in fully lithiated FePO4. Therefore, Na

2S

2O

3 is considered to be a

suitable reducing agent for lithium insertion in heterosite FePO4.

Na2S

4O

6 was formed not to react with S

2O

32- in the absence of lithium salt.

This means that NaFePO4

will not be produced by S2O

32- as a reducing

agent.


2O


4 to LiFePO

4

86

References 5.4

(1) Prosini, P. P.; Carewska, M.; Scaccia, S.; Wisniewski, P.; Passerini, S.; Pasquali, M. A New Synthetic Route for Preparing LiFePO

4 with Enhanced

Electrochemical Performance. Journal of The Electrochemical Society 2002, 149, A886-A890. (2) Shiratsuchi, T.; Okada, S.; Yamaki, J.-i.; Yamashita, S.; Nishida, T. Cathode performance of olivine-type LiFePO

4 synthesized by chemical

lithiation. Journal of Power Sources 2007, 173, 979-984. (3) Prosini, P. P.; Carewska, M.; Scaccia, S.; Wisniewski, P.; Pasquali, M. Long-term cyclability of nanostructured LiFePO

4. Electrochimica Acta 2003, 48,

4205-4211. (4) Galoustov, K.; Anthonisen, M.; Ryan, D. H.; MacNeil, D. D. Characterization of two lithiation reactions starting with an amorphous FePO

4

precursor. Journal of Power Sources 2011, 196, 6893-6897.

Chapter 6: Kinetic Study of the Chemical Lithiation of FePO4 by Na

2S

2O

3

87

Kinetic Studies of the Chemical Chapter 6:

Lithiation of FePO4 by Na

2S

2O

3

A preliminary study of the effect of varying the 6.1

concentrations of both Li+ and S2O

3

2- together

6.1.1 Introduction

As already mentioned, thiosulphate is a suitable reducing reagent for the

lithiation of FePO4. However, a kinetic study of the lithiation reaction is

important to understand the lithiation mechanism. It is also significant to

identify an optimal concentration of the reducing agent. That is because high

cost and waste management problems may arise if too much concentrated

reducing agent is consumed. On the other hand, a sufficiently high

concentration may be required to provide a rapid reaction and thus a clear

advantage over the original process, lime soda evaporation, which takes 1-2

years to produce Li2CO

3 as the raw product of lithium-ion battery industry.

Therefore, this section discusses the kinetic mechanism and the choice of an

optimal concentration of thiosulphate as the reducing agent to selectively

insert lithium into the heterosite FePO4 framework.

The following studies, the kinetics of chemical lithiation of FePO4

are

investigated with respect to variations in both lithium ion (Li+) and thiosulphate

ion (S2O

32-) concentrations together, and independently. All experiments were

performed using Na2S

2O

3 as the reductant.


2S

2O

3

88


The stoichiometry of the chemical reaction is shown in Equation 6.1. A molar

ratio of 2 Li2SO

4: 4 Na

2S

2O

3: 1FePO

4 was used, i.e. with a 4 excess of reducing

agent (Na2S

2O

3) and lithium salt to aid faster conversion. A solution preparation

and a sample collection procedure are described in detail below.

Equation 6.1 2 2 2

Solution preparation

Solutions of 0.15 M, 0.35 M, 0.75 M, and 1.5 M Li2SO

4 (Sigma-Aldrich, ≥98.5 %)

in de-ionized (DI) water were made. A solution of 0.15 M Li2SO

4 was made by

adding 1.50 g (0.0136 mol) of Li2SO

4 (Sigma-Aldrich, ≥98.5 %) in 90.93 ml of DI

water and the solution was stirred until all the salt dissolved. Then, 6.77 g of

Na2S

2O

3 (0.0273 mol) (Na

2S

2O

3•5H

2O, Timstar Laboratory Supplier, Ltd.) was

added, then the solution was stirred, and heated to approximately 50°C to

dissolve the Na2S

2O

3, then cooled to room temperature before adding 1.028 g

of FePO4 (0.00682 mol). The concentration of substances is shown in Table

6.1.

For 0.75 M, 0.35 M, and 0.15 M Li2SO

4, the amounts of Li

2SO

4, Na

2S

2O

3, and

FePO4 used were the same; however, the volumes of DI water were different.

The volumes of DI water used for 0.35 M, 0.75 M, and 1.5 M Li2SO

4 were 38.97

ml, 18.19 ml, and 9.10 ml, respectively.

Table 6.1: Concentration of Li2SO

4, Na

2S

2O

3, and FePO

4 in each sample

Solution Li2SO

4 /M Na

2S

2O

3 /M FePO

4 /M*

1 0.15 0.3 0.075

2 0.35 0.7 0.175

3 0.75 1.5 0.375

4 1.5 3 0.75

Note: *Average concentration in the suspension in mol/cm3

1 ml = 1 cm3


2S

2O

3

89

Sample collection procedure

Once the solutions were dissolved and cooled to room temperature, the

insoluble FePO4 was added to form a suspension, as the timer was started. The

suspensions were stirred and samples were taken for filtration at 300, 1200,

3600, 72000, 14400, 36000 and 86400 seconds (5 min, 20 min, 1 h, 2 h, 4 h,

10 h, and 24 h, respectively). The filtration was done using a vacuum pump

with grade 1 qualitative filter paper diameter 110 mm. Each sample was then

washed with DI water, and dried at 80°C for approximately 12 h. Samples were

analysed using X-ray diffraction (XRD), galvanostatic cycling, and inductive

coupled plasma-mass spectrometry (ICP-MS).

For each sample the extent of conversion from the LiFePO4 (olivine) structure to

FePO4 (heterosite) structure, was determined by XRD, using the Bruker D2

Phaser. Samples were scanned from 15 to 50 degrees of XRD pattern. The

reference XRD patterns of LiFePO4 and FePO

4 were obtained via the Inorganic

Crystal Structure Database (ICSD; RSC).

Samples were also fabricated into composite electrodes in lithium-ion cells to

determine the lithium content by potentiometric titration at a rate of 0.1 C and

the results were compared with the calibration curve from Chapter 3.

Some samples: i.e. 0.15 M Li2SO

4 at 300, 36000 and 86400 seconds, 0.35 M

Li2SO

4 at 3600 and 7200 seconds, 0.75 M Li

2SO

4 at 1200 seconds, 1.5 M Li

2SO

4

at 300, 1200 and 36000 seconds were digested and sent to the National

Oceanography Centre Southampton (NOCS) for ICP-MS analysis. The analysis

and preparation method are described more in Chapter 2.


2S

2O

3

90


Examples of results from XRD, potentiometric titration and ICP-MS are listed

below. Table 6.2 shows preliminary results on lithium selectivity. Table 6.3

illustrates the stoichiometric coefficient of x in LixFePO

4 that was found by all

methods.

X-ray Diffraction

Figure 6.1-Figure 6.3 show examples of the XRD pattern of heterosite FePO4,

and fully and partially lithiated LiFePO4, that were collected from the

experiments. Rietveld refinement of the data provides an estimate of the

extent of lithiation, defined as the % phase of the olivine vs. the heterosite

structure. The refinement method is described in details in Chapter 2.

Figure 6.1: The XRD fitting obtained from a sample treated with 0.15 M Li

2SO

4 + 0.3 M Na

2S

2O

3 (solution 1) for 300 seconds. The result shows only

heterosite, i.e. the FePO4 starting material.

The crosses shown are the data points, the red line is the fit and the blue line is the difference. The pink tick marks show the allowed reflection positions for heterosite FePO

4.


2S

2O

3

91


2SO

4 + 0.7 M Na

2S

2O

3 (solution 3) for 7200 seconds. The result shows a

mixed phase, i.e. partial conversion. Reproduced from Ref. 1 with permission from The Royal Society of Chemistry. (This figure was published in March 20141)


2SO

4

+ 3.0 M Na2S

2O

3 (solution 1) for 3600 seconds. The result shows pure

olivine, i.e. LiFePO4

The crosses shown are the data points, the red line is the fit and the blue line is the difference. The upper blue tick marks show the allowed reflection positions for olivine LiFePO

4 and the lower pink tick marks are for heterosite

FePO4.


2S

2O

3

92


As already mentioned, electrochemical potentiometric titration was used to

determine the redox capacity, and hence the molar lithium deficiency with

respect to LiFePO4 according to Faraday's Law. This could be done by either

oxidative removal of lithium or by the reductive insertion of lithium - both

methods providing consistent results. The specific charge results were

compared with the specific charge in the calibration graph, which was given in

Chapter 3 (reproduced here as Figure 6.4). The comparison gives a percentage

of lithium content to compare with the other methods, i.e. XRD and MS (in

Table 6.3). Figure 6.5 illustrates an example of the potentiometric removal of

lithium, where the higher values of the specific charge are associated with a

higher molar lithium content.

0 20 40 60 80 100

0

20

40

60

80

100

120

140

160

Sp

ecif

ic c

har

ge/

mA

h g

-1

% Li

Figure 6.4: A validating graph of percentage lithium content and specific charge. Reproduced from Ref. 1 with permission from The Royal Society of Chemistry. (This figure was published in March 20141)


2S

2O

3

93

Figure 6.5: Potentiometric titration of LixFePO4 electrodes prepared with the reaction product of FePO

4 in 0.35 M Li

2SO

4 + 0.7 M Na

2S

2O

3 for different times,

as indicated. Specific current: 17 mA g-1 at C/10 Reproduced from Ref. 1 with permission from The Royal Society of Chemistry. (This figure was published in March 20141).

Inductively Coupled Plasma-Mass Spectrometry

The third estimation of the lithium content in the LixFePO

4 reaction product was

found by ICP-MS. The lithium mole fraction in the solid samples were

calculated from the ion concentrations found by ICP-MS to determine lithium

and sodium uptake, and lithium selectivity using Equations 2.14-2.17 from

Chapter 2, as shown in Table 6.2.


2S

2O

3

94

Table 6.2: Li and Na concentrations of samples obtained by a 1:2 of [Li+]:[Na+] solution with 1 g FePO

4 at various times. (The data were published in March

20141).

Solution

[Li2SO

4]

+[Na2S

2O

3]

/M

Result* Time/

s

Uptake /mg g-1 [Li+]:[Na+]soli

d

Lithium selectivit

y [Li+] [Na+

]

1 0.15 +

0.3

Li0.01

FePO

4

300 2.8 0.7 13.3 26.6

Li0.50

FePO

4

36000 25.8

1.4 59.5 118.9

LFP 86400 45.1

1.7 89.0 178.0

2 0.35 +

0.7

Li0.30

FePO

4

3600 15.5

1.5 34.3 68.6

Li0.47

FePO

4

7200 22.8

1.1 67.9 135.8

3 0.75 +

1.5 Li

0.25FeP

O4

1200 14.3

1.3 37.4 74.9

4 1.5 + 3.0

Li0.09

FePO

4

300 5.9 1.9 10.1 20.2

Li0.39

FePO

4

1200 20.7

3.3 21.0 42.0

LFP 36000 44.7

2.2 66.3 132.7

*average result from Table 6.3 *LFP =LiFePO

4

Table 6.3 summarizes the results for the lithium contents as determined by

XRD, potentiometric, and ICP measurements vs. the treatment time and

condition. The area between the linear orange trend lines in Table 6.3 shows

the range of conversion from FePO4 to LiFePO

4. The lowest concentration, i.e.

solution 1, showed a trace of about 50% Li and a fully insertion at 36000 s (10

h) and 86400 s (24 h), respectively. For solution 2, the sample results were

found to be about 30% Li after 3600 s (1 h) and 100% Li after 14400 s (4 h).

The sample from solution 3 at 1200 s (20 min) started to show the insertion of

Li in the FePO4 structure as an average of 25% of Li. The suspension from

solution 3 completed the reduction after 3600 s. The highest concentration,

i.e. solution 4, starts to convert into an average of 9% LFP at 300 seconds (5

min) and the samples converted almost completely to LiFePO4 after 3600

seconds, similarly to solution 3.


2S

2O

3

95



lithiation of FePO4 with a 4-fold excess of reagent (Li

2SO

4+Na

2S

2O

3 in molar ratio

1:2) for different times, using 3 techniques.

Solution

[Li2SO

4]+

[Na2S

2O

3]/

M

Techniques

Time/ s

300 1200 3600 7200 14400 36000 86400

1 0.15 + 0.3

XRD 0.00 0.00 0.00 0.00 0.00 0.48 1.00

PT 0.02 0.01 0.05 0.04 0.12 0.50 1.01 ICP-MS 0.03 - - - - 0.54 0.98

2 0.35 + 0.7

XRD 0.00 0.00 0.31 0.44 1.00 1.00 1.00 PT 0.02 0.04 0.28 0.50 1.00 1.01 -

ICP-MS - - 0.32 0.47 - - -

3 0.75 + 1.5

XRD 0.00 0.22 1.00 1.00 1.00 1.00 1.00 PT 0.02 0.25 0.86 1.00 1.00 1.02 -

ICP-MS - 0.29 - - - - -

4 1.5 + 3.0

XRD 0.08 0.28 1.00 1.00 1.00 1.00 1.00 PT 0.09 0.41 0.82 1.02 1.02 1.10 -

ICP-MS 0.10 0.48 - - - 0.95 -

For the electrochemical and ICP-MS analysis, the lithium percentage of each

sample is similar to the results from the XRD analysis except for the sample of

solution 1 at 1200 s, which shows almost a 50 percentage difference.

Therefore, it was concluded that XRD was a less reliable measurement of the

conversion to LiFePO4

than the electrochemical and ICP methods. Fitting the

XRD pattern to a two-phase mixture was difficult when one of the phases was

present in a small proportion, e.g. in solution 2-3 at 300-1200 s, in which case

the XRD patterns were easier to fit with a single-phase model.

Figure 6.6-Figure 6.9 show estimations of the extent of lithiation according to

the three methods. In all cases the data show an approximately linear

dependence with time at a reaction extent up to about 0.8 and then a tapering

off towards 1.0 as the reaction nears completion. Also, the rates of lithium

uptake clearly increase from the lowest to the highest reagent concentrations.

However, there are some subtle differences in the data obtained according to

the method used and some subtle differences are discussed as follows.


2S

2O

3

96

Mass spectrometry (MS) measures the mole fraction of lithium in the product,

and in most cases shows higher values of lithium than other measurements as

shown in Figure 6.6-Figure 6.9. This is due to the fact that MS is a very

sensitive method to analyse. Also, it is possibly enhanced by some lithium ion

absorption at surface sites without redox activity or structural change, similarly

to larger ionic radii ions (i.e. Na+, K+, Mg2+) behaviour.2 Potentiometric titration

(PT) measures the extent of reduction of Fe(III) to Fe(II) as facilitated by the

thiosulphate reducing agent. It does not distinguish between the cations that

may counterbalance the electron charge during the reduction process, which

are Li+, Na+ and H+ in this case. XRD simply measures the phase fraction of

olivine, which is only approximately related to the lithium uptake according to

the phase diagram shown schematically below. We note that during the course

of the reaction, the heterosite phase should still exist up to a limiting x value

corresponding to the maximum supersaturation before nucleation of olivine;

similarly the Olivine phase can fully consume the heterosite at x values less

than 1.0, as shown in Figure 6.10. Therefore the structural measurement using

Rietveld refinement based on the phase ratio is not an accurate measure of the

x value, particularly in the dilute heterosite and dilute Olivine regions.

Figure 6.6: The comparison of XRD, PT, and MS techniques in the extent of lithiation which was obtained from solution 1 (0.15 M Li

2SO

4+0.3 M Na

2S

2O

3).

0 20000 40000 60000 80000 100000

0.0

0.2

0.4

0.6

0.8

1.0

0.15 M Li2SO

4+ 0.3 M Na

2S

2O

3

XRD PT MS

x in

Li x

FeP

O4

Time/ s


2S

2O

3

97

0 2000 4000 6000 8000 10000 12000 14000 16000

0.0

0.2

0.4

0.6

0.8

1.0

0.35 M Li2SO

4+ 0.7 M Na

2S

2O

3

XRD PT ICP

x in

Li x

FeP

O4

Time/ s


2SO

4+0.7 M Na

2S

2O

3).

0 2000 4000 6000 8000

0.0

0.2

0.4

0.6

0.8

1.0

0.75 M Li2SO

4+ 1.5 M Na

2S

2O

3

XRD PT MS

x in

Li x

Fe

PO

4

Time/ s


2SO

4+1.5 M Na

2S

2O

3).


2S

2O

3

98


2SO

4+3 M Na

2S

2O

3).

Figure 6.10: Phase fraction of olivine in the heterosite/ olivine composite.

0 2000 4000 6000 8000

0.0

0.2

0.4

0.6

0.8

1.0x

in L

i xF

ePO

4

Time/ s

XRD PT MS

1.5 M Li2SO

4+ 3.0 M Na

2S

2O

3


2S

2O

3

99

Discounting the XRD results, which errors in the determination of low degrees

of lithiation, the above data show an approximately constant rate during the

progress of the reaction until the FePO4 is almost fully lithiated. Therefore each

plot can be considered to represent pseudo zero order kinetics due to the fact

that both reagents are present in large excess, i.e. the reagent concentrations

are almost constant during each reaction and measured in M s-1 with respect to

the effective molarity of FePO4. We define the reaction rate as the change in the

molar lithium content in LixFePO

4 with time:

Equation 6.2

where x = the stoichiometric coefficient in LixFePO

4.

Since the present results show a pseudo-zero order behaviour, the reaction

rate can be calculated as the inverse of the reaction time, where the reaction

time corresponds to the time when all FePO4 has been converted into LiFePO

4.

The rates are determined in Table 6.4.

It is obvious that increasing the Li+ concentration and the S2O

32- concentration

rises the reaction rate. The following subchapter studies the effect of

increasing Li+ concentrations at a constant S2O

32- concentration and of

increasing S2O

32- concentrations at a constant Li+ concentration.

Table 6.4: The reaction rate of each experiment when x = 1 (LixFePO

4) with

respects the reaction time (estimated data from Figure 6.9-Figure 6.6)

[FePO4]

/M

[Li2SO

4]

/M [Na

2S

2O

3]

/M [Li

2SO

4]x[Na

2S

2O

3]

/M2

Reaction time/s (xrd)

Reaction rate/s-1

0.075 0.15 0.3 0.045 85000 1.2x10-5 0.175 0.35 0.7 0.245 14000 7.1x10-5

0.375 0.75 1.5 1.125 4200 2.4x10-4

0.75 1.5 3 4.5 3900 2.6x10-4


2S

2O

3

100

Kinetics of the Chemical Lithiation of FePO4 using 6.2

Na2S

2O

3, varying the concentration of S

2O

3

2- and Li+

independently

6.2.1 Introduction and Theory

The stoichiometry of this chemical reaction is shown in Equation 6.3. Lithium

chloride (LiCl) was used in this experiment instead of lithium sulphate (Li2SO

4),

as in the previous experiment. This was to show that there is no significant

difference between lithium salts.

Equation 6.3 2 2 2 → 2 2

The order of reaction and rate constant with respect to the concentration of Li+

were studied by fixing the concentration of S2O

32- and varying the concentration

of Li+; similarly the kinetics of the reaction with respect to S2O

32- concentration

were studied by fixing the Li+ concentration and varying the concentration of

S2O

32-. The FePO

4 itself was considered as a minority component so that the rate

of reaction was measured by the rate of conversion of FePO4 to LiFePO

4 rather

than the rate of consumption of the solution reagents, which was considered

to be negligible.


The preparation process was done the same way as in Chapter 5.1. 0.03 M,

0.06 M, 0.1 M, 0.3 M, 0.6 M, 0.9 M, and 1.2 M of LiCl (Sigma-Aldrich, ≥ 99.9%)

were used for studying the kinetics of Li+ concentration by fixing the S2O

32-

concentration and fixed FePO4 concentration at 0.3 M and 0.075 M,

respectively. The kinetics study of S2O

32- concentration was done by using 0.1

M, 0.6 M, 0.9 M, and 1.2 M Na2S

2O

3 with the fixed concentration of 0.3 M LiCl

and 0.075 M FePO4 as shown in Table 6.5.

Table 6.5: Concentrations of the reagents


2S

2O

3

101

Solution Substrate / M

Volume/ L Ratio [Li+]:[S2O

3

2-]:[FePO4]

LiCl Na2S

2O

3

1 0.03

0.3

0.884

0.4 : 4 : 1 2 0.06 0.8 : 4 : 1 3 0.1 1.3 : 4 : 1 4 0.3 4 : 4 : 1 5 0.6 8 : 4 : 1 6 0.9 12 : 4 : 1 7 1.2 16 : 4 : 1 8

0.3

0.1 4 :1.3 : 1 9 0.6 4 : 8 : 1

10 0.9 4 :12 :1 11 1.2 4: 16 :1


The processes of mixing the substrates were similar to experiment 5.1.

Nevertheless, for each solution, samples were made individually and collected

for filtration at 1200, 3600, 7200, 14400, 36000, 86400, and 172800 seconds

(20 m, 1 h, 2 h, 4 h, 10 h, 1 day and 2 days, respectively) after adding FePO4.

The samples were made separately due to the fact that a sufficient amount ~1

g of samples was needed for each analysis. Thus, 77 samples were made. Each

sample was filtered using grade No. 1 qualitative filter paper with 110 mm

diameter, washed with DI water, and dried at 80°C for approximately 12 hours.

Samples were analysed using XRD (scanned from 15 to 50 degrees) and

inductive coupled plasma-mass spectrometry (ICP-MS) as described in Chapter

2. The results are reported in terms of stoichiometric coefficient of x in

LixFePO

4.


Two studies are observed which are the effect of lithium ion concentration with

respect to thiosulphate ion concentration and the effect of thiosulphate ion

concentration with respect to lithium ion concentration. The results of LixFePO

4

are shown in Table 6.6. The reaction rates are shown in Table 6.7-Table 6.9.

The uptake of Li+ and Na+ of each sample was calculated and is shown in

Table 6.10.


2S

2O

3

102

The effect of Li+ concentration

In this experiment, the various Li+ concentrations were mixed with a fixed

[S2O

32-] and a fixed [FePO

4]. The various concentrations of Li+ consisted of 0.03

M, 0.06 M, 0.1 M, 0.3 M, 0.6 M, 0.9 M, and 1.2 M LiCl. The fixed

concentrations were 0.3 M [Na2S

2O

3] and 0.075 M [FePO

4] (added as a

suspension). The samples were first examined by XRD which indicated the

amount of each phase by XRD fitting. Examples of XRD fitting profiles are

shown in Figure 6.11- Figure 6.13.

Figure 6.11: Example of the XRD fitting data for 0.4:4:1 (0.03 M LiCl + 0.3 M Na

2S

2O

3 + 0.075 M FePO


0.25FePO

4



4.


2S

2O

3

103

Figure 6.12: Example of the XRD fitting data for 0.8:4:1 (0.06 M LiCl + 3.0 M Na

2S

2O

3 + 0.075 M FePO


0.80 FePO

4)

Figure 6.13: Example of the XRD fitting data for 4:4:1(0.3M LiCl + 0.3 M Na

2S

2O

3 + 0.075 M FePO

4) for 36000 s (10 hours) result in LiFePO

4



4

(in Figure 6.12).


2S

2O

3

104

The kinetics study of S2O

32-

In a similar way, the various [S2O

32-] were mixed with a fixed [Li+] and a fixed

[FePO4]. The various concentrations of S

2O

32- consisted of 0.1 M, 0.3 M, 0.6 M,

0.9 M, and 1.2 M. The fixed concentrations were 0.3 M [LiCl] and 0.075 M

[FePO4]. Samples were done individually for the times that were mentioned

earlier. The samples were first examined by XRD which indicated the amount of

each phase by XRD fitting. Examples of XRD fitting profiles are shown in

Figure 6.14 and Figure 6.15.

Figure 6.14: Example of the XRD fitting data for 4:1.3:1 (0.3 M LiCl + 0.1 M Na

2S

2O

3 + 0.075 M FePO


0.35FePO

4



4.


2S

2O

3

105

Figure 6.15: Example of the XRD fitting data for 4:8:1 (0.3 M LiCl + 0.6 M Na

2S

2O

3 + 0.075 M FePO

4) for 14400 s (4 hours) result in Li

0.80 FePO

4)



4.

Table 6.6 shows the conversion of FePO4

to LiFePO4 for both studies; i.e.

kinetics of Li+ and S2O

32- concentrations. The linear orange trend lines show a

range of starting points of the conversion. The starting point of the conversion

is more extended at the lowest lithium concentration ([Li+] = 0.03 M at 4 hours)

than the lowest concentration of thiosulphate ([S2O

32-] = 0.1 M at 2 hours). The

fully lithiated FePO4 result was not found at 0.4: 4: 1 and 0.8: 4: 1 after 2 days.

Presumably, this means the ratio of [Li+] was not enough for the full

conversion. However, 1.3-16: 4: 1 at 10 hours results indicated a 100% LiFePO4.

For the S2O

32- kinetics study, the starting point of the conversion at the least

S2O

32- concentration was indicated at 4 hours and the full conversion was found

after 2 days. The 4: 4 to 8: 1 ratio started to convert after ~ 2 hours and

completed its conversion at 10 hours. The last two ratios, i.e. 4: 12: 1 and 4:

16: 1, started to convert before 1 hour and finished the conversion at 4 hours.


2S

2O

3

106

These results correspond to the kinetics models, i.e. diffusion limited model

and surface-reaction limited model.


2S

2O

3

107

Table 6.6: Lithium molar content of LixFePO

4 samples obtained by lithiation of FePO

4 for different times, as estimated from

(a) XRD and (b) ICP measurements.

[Li+]:[S

2O

3

2-]:[FePO4]

Concentration/ M Time/s [LiCl] [Na

2S

2O

3 ] [FePO

4] 1200 3600 7200 14400 36000 86400 172800

0.4 : 4 : 1 0.03

0.3

0.075

0.00a 0.00a 0.00a 0.00a

0.04b

0.07a

0.09b

0.11a

0.14b

0.25a

0.23b

0.8 : 4 : 1 0.06 0.00a 0.00a 0.00a 0.00a

0.04b

0.09 a

0.13b

0.28 a 0.26b

0.80 a 0.70b

1.3 : 4 : 1 0.1 0.00a

0.00b

0.00a

0.07b 0.00a

0.07b 0.01 a

0.12b 0.30 a

0.40 b 0.91 a

0.79 b 1.00a

1.00 b

4 : 4 : 1 0.3 0.00a

0.05b 0.10a

0.10b 0.18a

0.25b 0.36a

0.44b 1.00a

1.00b - -

8 : 4 : 1 0.6 0.00a

0.05b

0.16a

0.15b

0.22a

0.20b

0.46a

0.43b

1.00a

1.00b - -

12 : 4 : 1 0.9 0.00a

0.10b 0.00a

0.17b 0.24a

0.30b 0.56a

0.64b 1.00a

1.10b - -

16 : 4 : 1 1.2 0.00a

0.10b 0.17a

0.23b 0.34a

0.44b 0.58a

0.78b 1.00a

1.18b - -

4 : 1.3 : 1

0.3

0.1 0.00a

0.06b 0.00a

0.07b 0.00a

0.06b 0.00a

0.06b 0.08a

0.13b 0.33a

0.47b

0.35a

0.32b

4 : 4 : 1 0.3 0.00a

0.05b 0.10a

0.10b 0.18a

0.25b 0.36a

0.44b 1.00a

1.00b - -

4 : 8 : 1 0.6 0.00a

0.10b 0.21a

0.23b 0.40a

0.46b 0.70a

1.08b 1.00a

1.26b 1.00a

1.04b -

4 : 12 : 1 0.9 0.00a

0.13b 0.29a

0.36b 0.87a

1.12b 1.00a

1.16b - - -

4 : 16 : 1 1.2 0.00a

0.13b 0.54a

0.68b 0.98a

1.13b

1.00a

1.23b - - -


2S

2O

3

108

Figure 6.16-Figure 6.22 show a graph of the stoichiometric coefficient x in

LixFePO

4 with respect to time. The graphs were obtained with various

concentrations of Li+ and fixed concentrations of S2O

32- and FePO

4. Each graph

presents a linear relationship of the x-y axis. Each slope indicates the reaction

rate. For the reaction rate evaluation, the XRD data that corresponds to nearly

fully lithiated or delithiated sample (i.e. x values were close to zero or one)

were discarded. The reaction rate can be defined as a derivative of x with

respect to time where x is a stoichiometric coefficient for lithium as in LixFePO

4.

Figure 6.23 and Table 6.7 show the relationship between the reaction rate and

the concentration of lithium. It is shown that the reaction rate varies with the

square root of the lithium concentration.

Figure 6.16: Rate of the lithiated reaction that was obtained from 0.03 M [Li+], 0.3 M [S

2O

32-] and 0.075 [FePO

4] (solution 1; 0.4:4:1 ratio), with respect of

time.

0 50000 100000 150000 200000

0.00

0.05

0.10

0.15

0.20

0.25

y=1.4310-6 x

XRD MS

x in

Li x

FeP

O4

Time/ s

0.03 M [Li+]+ 0.3 M [S2O2-

3]+0.075 M [FePO

4]


2S

2O

3

109


2O

32-] and 0.075 [FePO


time.


2O

32-] and 0.075 [FePO


time.

0 40000 80000 120000 160000

0.0

0.2

0.4

0.6

0.8

y=4.0610-6 x

0.06 M [Li+]+ 0.3 M [S2O2-

3]+0.075 M [FePO

4]

XRD MS

x in

Li x

FeP

O4

Time/ s

0 20000 40000 60000 80000 100000

0.0

0.2

0.4

0.6

0.8

1.0

y=9.6510-6 x

0.1 M [Li+]+ 0.3 M [S2O2-

3]+0.075 M [FePO

4]

Time/ s

x in

Li x

FeP

O4

XRD MS


2S

2O

3

110


2O

32-] and 0.075 [FePO

4] (solution 4; 4:4:1 ratio), with respect of time.


2O

32-] and 0.075 [FePO


0 5000 10000 15000 20000 25000 30000 35000 40000

0.0

0.2

0.4

0.6

0.8

1.0

y=2.7810-5 x

0.3 M [Li+]+ 0.3 M [S2O2-

3]+0.075 M [FePO

4]

Time/ s

x in

Li x

FeP

O4

XRD MS

0 5000 10000 15000 20000 25000 30000 35000 40000

0.0

0.2

0.4

0.6

0.8

1.0

y=2.8410-5 x

0.6 M [Li+]+ 0.3 M [S2O2-

3]+0.075 M [FePO

4]

x in

Li x

FeP

O4

XRD MS

Time/ s


2S

2O

3

111


2O

32-] and 0.075 [FePO



2O

32-] and 0.075 [FePO


0 5000 10000 15000

0.0

0.2

0.4

0.6

y=4.1110-5 x

XRD MS

x in

Li x

FeP

O4

0.9 M [Li+]+ 0.3 M [S2O2-

3]+0.075 M [FePO

4]

Time/ s

0 5000 10000 15000 20000

0.0

0.2

0.4

0.6

0.8

y=4.9010-5 x

1.2 M [Li+]+ 0.3 M [S2O2-

3]+0.075 M [FePO

4]

x in

Li x

FeP

O4

XRD MS

Time/ s


2S

2O

3

112

Table 6.7: The reaction rate of each experiment with respect to [Li+]

Ratio [Li+]/M [S2O

3

2-]/M Reaction rate /s-1

0.4:4:1 0.03

0.3

1.43 x 10-6

0.8:4:1 0.06 4.06 x 10-6 1.3:4:1 0.1 9.65 x 10-6 4:4:1 0.3 2.78 x 10-5 8:4:1 0.6 2.84 x 10-5 12:4:1 0.9 4.11 x 10-5 16:4:1 1.2 4.90 x 10-5

Figure 6.23: The reaction rates of the kinetic study with respect to the lithium concentration.

For the kinetic study of S2O

32- concentration, Figure 6.24-Figure 6.28 show a

graph of the stoichiometric coefficient of x in LixFePO

4 with respect to time. The

graphs were obtained with various concentrations of S2O

32- and fixed

concentrations of Li+ and FePO4. Each graph also presents a linear relationship

between the x-y axis, similar to the kinetic study of Li+ concentration’s. Figure

6.29 and Table 6.8 show the relationship between the reaction rate and the

concentration of thiosulphate. This result indicates that the reaction rate is

proportional to the S2O

32- concentration.

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0

1x10-5

2x10-5

3x10-5

4x10-5

5x10-5

Rea

ctio

n r

ate/

s-1

[Li+]/ M

[S2O2-

3]=0.3 M


2S

2O

3

113


2O

32-] and 0.075 [FePO

4] (solution 8; 4:1.3:1 ratio), with respect of

time.


2O

32-] and 0.075 [FePO


0 20000 40000 60000 80000 100000

0.0

0.1

0.2

0.3

0.4

0.5

y=4.7910-6 x

x in

Li x

FeP

O4

XRD MS

Time/ s

0.3 M [Li+]+0.1 M [S2O2-

3]+0.075 M [FePO

4]

0 5000 10000 15000 20000 25000 30000 35000 40000

0.0

0.2

0.4

0.6

0.8

1.0

y=2.7810-5 x

0.3 M [Li+]+ 0.3 M [S2O2-

3]+0.075 M [FePO

4]

Time/ s

x in

Li x

FeP

O4

XRD MS


2S

2O

3

114


2O

32-] and 0.075 [FePO



2O

32-] and 0.075 [FePO

4] (solution 10; 4:12:1 ratio), with respect of

time.

0 5000 10000 15000 20000

0.0

0.2

0.4

0.6

0.8

y=5.3110-5 x

0.3 M [Li+]+0.6 M [S2O2-

3]+0.075 M [FePO

4]

Time/ s

x in

Li x

FeP

O4

XRD MS

0 2000 4000 6000 8000

0.0

0.2

0.4

0.6

0.8

1.0

y=1.1010-4 x

0.3 M [Li+]+0.9 M [S2O2-

3]+0.075 M [FePO

4]

Time/ s

x in

Li x

FeP

O4

XRD MS


2S

2O

3

115


2O

32-] and 0.075 [FePO

4] (solution 11; 4:16:1 ratio), with respect of

time.

Table 6.8: The reaction rate of each experiment with respect to [S2O

32-]

Ratio [Li+]/M [S2O

3

2-]/M Reaction rate/ s-1

4:1.3:1

0.3

0.1 4.79 x 10-6

4:4:1 0.3 2.78 x 10-5

4:8:1 0.6 5.31 x 10-5 4:12:1 0.9 1.10 x 10-4

4:16:1 1.2 1.48 x 10-4

0 2000 4000 6000 8000

0.0

0.2

0.4

0.6

0.8

1.0

y=1.4810-4 x

0.3 M [Li+]+1.2 M [S2O2-

3]+0.075 M [FePO

4]

Time/ s

x in

Li x

FeP

O4

XRD MS


2S

2O

3

116

Figure 6.29: The reaction rates of the kinetic study with respect to the thiosulphate concentration.

In order to analyse the effect of Li+ and S2O

32- concentration on the reaction

rate, we developed two models: diffusion limited model and surface-reaction

limited model, to interpret the kinetics of Li+ and S2O

32- concentrations, using

the data graphs from Figure 6.23- Figure 6.29.

Diffusion Limited Model

The FePO4 particle is considered as a microelectrode. So, we want to calculate

the rate at which ions can diffuse towards the surface of the FePO4 particle. The

rate can be defined as a derivative of x in respect of time where x is a

stoichiometric coefficient for lithium as in LixFePO

4, as shown in Equation 6.4.

The rate unit is s-1.

Equation 6.4

The flux of ions diffusing towards a FePO4 particle that behaves as a micro

electrode is3:

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0.0

2.0x10-5

4.0x10-5

6.0x10-5

8.0x10-5

1.0x10-4

1.2x10-4

1.4x10-4

1.6x10-4

Rea

ctio

n r

ate/

s-1

[S2O2-

3]/ M

[Li+]=0.3 M


2S

2O

3

117

Equation 6.5 4

Where D = diffusion coefficient of the ion in solution (D of Li+ in solution 4~ 1.0

x 10-5/ cm2s-1, D of S2O

32- in solution 5 ~ 6.46 x 10-6/ cm2s-1)

c = the concentration of Li+ or S2O

32- / mol cm-3

r = the radius of agglomerate FePO4 ~ 1.0 x 10-4 /cm (data from Tatung

in Appendix) In order to obtain the reaction reate, the number of Li+ sites on the FePO

4

particle

Equation 6.6 43

3

Where d = density of FePO

4 ~ 3.4/ g cm-3

Mw = molecular weight of FePO4 ~ 151/ g mol-1

π = 3.1415, 1 mol L-1 = 1.0 x 10-3 mol cm-3

The reaction rate ( unit s-1) was obtained by dividing the flux by the number of

Li+ sites, since the rate has been defined as the derivative of the stoichiometric

coefficient x in LixFePO

4 with time, as shown in Equation 6.7.

Equation 6.7 43

3

The idea is to compare the theoretical reaction rate model to the experimental

reaction rate, considering a diffusion limited process.

The reaction rate of the experiment can calculate from a slope of a graph of x,

LixFePO

4 in respect of time (s). For example, from the kinetics of the Li+

concentration graph, the rate of experiment that obtained from 0.03 M Li+

concentration, 0.3 M S2O

32- concentration, and 0.075 M FePO

4 concentration is

1.43 x 10-6 s-1.

The theoretical model,

443

4 10 3 10 10

43 10 3.4 1

151


2S

2O

3

118

1.20 10

9.43 10

1.27

The calculation shows that the experimental rate is not limited by the diffusion

of Li+ ions in the solution. The experimental rate is 106 times lower, compared

to the theoretical rate.

Table 6.9: Values of calculated theoretical rates compares to the experimental rates

[Li+]/M [S2O

32-]/M Experimental rate (slope) /s-1 Theoretical rate/ s-1

0.03

0.30

1.43 x 10-6 1.27 0.06 4.06 x 10-6 2.54 0.10 9.65 x 10-6 4.24 0.30 2.78 x 10-5 1.27 x 101 0.60 2.84 x 10-5 2.54 x 101 0.90 4.11 x 10-5 3.82 x 101 1.20 4.90 x 10-5 5.09 x 101

0.30

0.10 4.79 x 10-6 2.74 0.30 2.78 x 10-5 8.22 0.60 5.13 x 10-5 1.64 x 101 0.90 1.10 x 10-4 2.47 x 101 1.20 1.48 x 10-4 3.29 x 101


2S

2O

3

119

Figure 6.30: The comparison of the experimental rate and the theoretical rate of the experiments were obtained from a fixed [S

2O

32-] =0.3 M and [Li+] = 0.03

to 1.2 M.

Figure 6.31: The comparison of the experimental rate and the theoretical rate of the experiments were obtained from a fixed [Li+] =0.3 M and [S

2O

32-] = 0.1 to

1.2 M.

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0

1x10-5

2x10-5

3x10-5

4x10-5

5x10-5

Reaction rate Theoretical rate

Re

acti

on

ra

te/ s

-1

[Li+]/ M

[S2O2-

3]=0.3 M

Th

eore

tica

l rat

e/ s

-1

0

10

20

30

40

50

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0.0

2.0x10-5

4.0x10-5

6.0x10-5

8.0x10-5

1.0x10-4

1.2x10-4

1.4x10-4

1.6x10-4

Experimental rate Theoretical rate

[S2O2-

3]/ M

Exp

erim

enta

l rat

e/ s

-1

0

10

20

30

Th

eore

tica

l rat

e/ s

-1

[Li+]=0.3 M


2S

2O

3

120

Surface-reaction limited model

This model is based on a simple electron transfer redox couple (Equation 6.8)

which occurred at the FePO4 surface.

Equation 6.8

Where O = oxidised species n = number of electrons (e-) R = reduced species

For kinetics, the rate of oxidation and reduction can be written as shown in

Equation 6.9 - Equation 6.106,7.

Equation 6.9

Equation 6.10

Where and = anodic and cathodic rate constants [R] and [O] = the concentration of the reduced species and the oxidised species.

Both and depends on the potential gradient at the surface of FePO4

as

shown in Equation 6.11 and Equation 6.12.

Equation 6.11 ∝

Equation 6.12 ∝

Where and = standard rate constant of anodic and cathodic

∝ and ∝ = anodic and cathodic transfer coefficient (∝ ∝ 1 n = number of e-1 F = Faraday constant 96485/ C mol-1 E0 = the standard potential of the redox reaction/ V E = the applied potential/ V R = the gas constant (8.314 / J K-1 mol-1) T = the temperature/ K


2S

2O

3

121

Thus, the rate of oxidation and reduction can also be defined as in Equation

6.13 and Equation 6.14.

Equation 6.13 ∝

Equation 6.14 ∝

At equilibrium, when the steady state is reached, the potential and the rates

are equal. According to the Butler-Volmer equation, the potential value will

adjust itself until the rate is equal to another.6,7 If there is no mass transport

limitation, the concentration at the surface is proportional to the concentration

at the bulk. For example, when the concentration at the bulk is doubled, the

concentration at the surface is expected to be twice as much.

In this experiment, the thiosulphate reaction (oxidation reaction) is assumed as

the rate determination step (Equation 6.15), meaning that once this equation

occurs, the rest of the reaction will happen.

Equation 6.15 → 1

The rate of thiosulphate reaction can be defined as shown in Equation 6.16. At

the equilibrium, is equal to zero, due to the fact that any

number to the zero power is one. Hence, in this case, the rate of the

thiosulphate reaction is proportional to S2O

32- concentration. Likewise, the

lithium concentration is proportional to the rate reaction with respect to the Li+

concentration. The rate will increase exponentially by increasing the potential

if the potential depends on the concentration. In other words, if the potential

does not depend on the concentration, the rate is proportional to the

concentration, which corresponds to this case. However, this theory only

applies when S2O

32-> Li+ concentrations.


2S

2O

3

122

Equation 6.16 ∝

Where E0 = the standard potential of the redox reaction/ V E = the applied potential/ V

R = the gas constant (8.314 J/ K mol) F = the Faraday constant (96485/ C mol-1) T = the temperature/ K n = the number of electrons transferred k = the rate constant α = transfer coefficient T = the temperature/ K.

In the case of Li+>S2O

32- concentrations, the experimental rate of Li+

concentration is limited by the diffusion of Li+ ion in the solution. The potential

does not depend on S2O

32 concentration but Li+ concentration. The reaction of

thiosulphate oxidation is very slow, whereas, the reaction of lithium reduction

is not. This means that the insertion of lithium ion into FePO4 framework is

assumed to be fast. Therefore, the electrode potential will vary with Li+

concentration in a Nernstian way (Equation 6.18). Plug Equation 6.18 into

Equation 6.16 and simplify the equation.

Equation 6.17 →

Equation 6.18 /

ln

If ∝ =0.5, the model is shown to predict that the reaction will be first order

with respect to the S2O

32- concentration. The reaction will also vary with [Li+]0.5,

as observed experimentally.


2S

2O

3

123

Table 6.10: Li and Na concentrations of samples obtained by various ratios of

[Li+]:[S2O

3

2- ] solution with 1 g FePO4 at various time.

ratio [LiCl] [Na

2S

2O

3] time/s

uptake/ mg g-1 [Li+]:[Na+] solid

lithium selectivity [Li+]:[S

2O

32-]: [FePO

4] Li+ Na+

0.4:4:1 0.03

0.3

14400 0.5 1.6 1.0 20.4

36000 2.6 2.2 3.9 77.4

86400 5.2 2.5 6.9 137.3

172800 9.3 2.7 11.7 233.0 259200 8.6 2.3 12.3 246.7

0.8:4:1 0.06

14400 0.9 1.8 1.5 15.5 36000 4.9 2.3 6.9 69.2 86400 10.7 2.8 12.6 126.3

172800 30.9 3.1 32.8 327.5

1.3:4:1 0.1

3600 2.0 3.2 2.1 12.5 7200 1.9 5.0 1.3 7.8

14400 4.4 3.2 4.6 27.3 36000 17.1 3.7 15.3 91.8 86400 34.7 2.7 43.6 261.3

4:4:1 0.3

1200 1.0 1.8 2.0 3.9 3600 3.1 1.9 5.5 11.1 7200 10.2 3.5 9.8 19.6

14400 19.0 2.9 22.1 44.1 36000 44.6 4.4 33.8 67.7

8:4:1 0.6

1200 1.1 2.1 1.7 1.7 3600 5.5 2.0 8.9 8.9 7200 7.9 2.1 12.8 12.8

14400 18.3 2.1 29.2 29.2 36000 39.4 3.1 42.7 42.7

12:4:1 0.9

1200 3.2 2.5 4.1 2.8 3600 6.3 2.6 7.9 5.3 7200 12.4 2.3 18.0 12.0

14400 27.9 3.1 30.4 20.3 36000 49.2 3.5 47.0 31.4

16:4:1 1.2

1200 3.1 1.4 7.4 3.7 3600 9.1 1.8 16.8 8.4 7200 18.7 1.9 33.5 16.8

14400 34.6 2.2 52.7 26.4 36000 52.6 2.5 69.4 34.7

4:1.3:1 0.3 0.1 3600 1.6 1.7 3.1 2.1 7200 1.8 2.1 2.9 1.9

14400 1.3 1.2 3.6 2.4


2S

2O

3

124

36000 4.8 1.2 13.3 8.8 86400 20.3 2.5 27.1 18.1

172800 13.4 1.8 24.5 16.4

4:8:1 0.6

1200 3.0 3.8 2.7 10.6 3600 9.3 4.0 7.8 31.4 7200 19.8 4.2 15.9 63.5

14400 48.2 5.9 27.3 109.4 36000 56.6 6.2 30.3 121.2 86400 46.5 5.2 29.7 118.8

4:12:1 0.9

1200 4.6 4.4 3.6 21.3 3600 14.9 4.1 12.2 72.9 7200 50.2 5.9 28.4 170.3

14400 51.9 6.0 28.7 172.1

4:16: 1 1.2

1200 4.5 4.0 3.7 30.0 3600 29.9 5.1 19.7 157.8 7200 50.6 5.3 32.0 256.4

14400 55.2 6.2 29.8 238.7

Table 6.10 shows values of Li+ or Na+ in milligrams that absorbed into 1 g of

FePO4 (uptake) and selectivity of lithium which obtained from MS analysis. The

maximum Li+ uptake value is approximately 56 mg g-1 which is higher than

some advanced manganese oxide ion sieves; i.e. they absorbed Li up to 38-46

mg g-1. 1,8-11

The maximum Na+ uptake value is approximately 6 mg g-1, with respect to the

maximum of [Na2S

2O

3] (1.2 M). A high lithium selectivity value indicates that the

ratio of lithium to sodium in solution is lower than the ratio in solid. Therefore,

a fully converted product does not have to be the same value.


The kinetic studies of chemical lithiation of FePO4 with respect to both

Li+ and S2O

32- concentrations demonstrates a pseudo-zero order reaction,

calculated by using the inverse of the reaction time.

The fully lithiated FePO4 can be obtained from as low as 0.3 M Na

2S

2O

3

with 0.15 M Li2SO

4 for 24 h.


2S

2O

3

125

Potentiometric titration and mass spectrometry are considered to be the

most reliable to find the coefficient x in LixFePO

4 techniques. However,

XRD technique is very beneficial for rapid crystal structure identification.

Among all experiments, the thiosulphate reaction is the rate

determining step. The surface-reaction limited model is considered to

be a suitable model to interpret the kinetics with respect to Li+ and S2O

32-

concentrations, defining a first order rate reaction when S2O

32-> Li+

concentrations. Inversely, S2O

32-< Li+ concentrations, the rate of the

experiment is limited by Li+ diffusion. This agrees to the reaction rate

with respect to Li+ concentration, which the rate increases with the

square root of the lithium concentration.

References 6.3

(1) Intaranont, N.; Garcia-Araez, N.; Hector, A. L.; Milton, J. A.; Owen, J. R. Selective lithium extraction from brines by chemical reaction with battery materials. Journal of Materials Chemistry A 2014, 2, 6374-6377. (2) Feng, Q.; Kanoh, H.; Miyai, Y.; Ooi, K. Hydrothermal Synthesis of Lithium and Sodium Manganese Oxides and Their Metal Ion Extraction/Insertion Reactions. Chemistry of Materials 1995, 7, 1226-1232. (3) Denuault, G.; Mirkin, M. V.; Bard, A. J. Direct determination of diffusion coefficients by chronoamperometry at microdisk electrodes. 1991, 308, 27-38. (4) Lepage, D.; Sobh, F.; Kuss, C.; Liang, G.; Schougaard, S. B. Delithiation kinetics study of carbon coated and carbon free LiFePO4. Journal of Power Sources 2014, 256, 61-65. (5) Sabzi, R. E. Electrocatalytic oxidation of thiosulfate at glassy carbon electrode chemically modified with cobalt pentacyanonitrosylferrate. Journal of the Brazilian Chemical Society 2005, 16, 1262-1268. (6) Spiro, M. Polyelectrodes: the behaviour and applications of mixed redox systems. Chemical Society Reviews 1986, 15, 141-165. (7) Pletcher, D.: An introduction to Electrode Reactions. In First Course in Electrode Processes; 2nd ed.; Royal Society of Chemistry: Cambridge, 2009; pp 1-47. (8) Zhang, Q.-H.; Li, S.-P.; Sun, S.-Y.; Yin, X.-S.; Yu, J.-G. Lithium selective adsorption on 1-D MnO

2 nanostructure ion-sieve. Advanced Powder Technology

2009, 20, 432-437. (9) Chitrakar, R.; Kanoh, H.; Miyai, Y.; Ooi, K. Recovery of Lithium from Seawater Using Manganese Oxide Adsorbent (H

1.6Mn

1.6O

4) Derived from

Li1.6

Mn1.6

O4. Industrial & Engineering Chemistry Research 2001, 40, 2054-2058.

(10) Chitrakar, R.; Makita, Y.; Ooi, K.; Sonoda, A. Selective Uptake of Lithium Ion from Brine by H

1.33Mn

1.67O

4 and H

1.6Mn

1.6O


41, 1647-1649. (11) Yu, Q.; Sasaki, K.; Hirajima, T. Bio-templated synthesis of lithium manganese oxide microtubes and their application in Li+ recovery. Journal of Hazardous Materials 2013, 262, 38-47.


2S

2O

3

126


32-/ FePO

4

Reagents

127

Selectivity for Li+ versus Other Chapter 7:

Cations Using the S2O

3

2-/ FePO4 Reagents

The studies of lithium absorption into a framework, such as manganese oxide,

manganese dioxide and lithium iron phosphate, as an adsorbent have been

reported in many research studies.1-6 Most were reported using lithium uptake

to interpret how much lithium can be inserted or absorbed into a particular

framework. Other metal ions were reported to be absorbed, along with lithium

ions.

The aims in this chapter are to demonstrate thiosulphate/ FePO4 reagent for Li+

sequestering and testing the selectivity for Li versus other metals. Therefore,

the study of metal uptake, namely lithium, sodium, magnesium and potassium,

into the framework, i.e. FePO4, using thiosulphate as a reducing agent are

reported. Also, the selectivity of lithium ions are calculated to show the

efficiency of the framework.

Chemical Lithiation of FePO4 from Aqueous Solutions 7.1

Containing an Excess of Na+ and Mg2+

Lithium (Li+), sodium (Na+), and magnesium (Mg2+) ions are considered in this

section. Owing to sodium being known as one of dominant cations in brine,

though Na+ is almost double size to Li+, as shown in Table 7.1. The selectivity

between lithium and sodium is thus interesting. For magnesium, although the

concentration of magnesium is lower than sodium in brine, Mg2+ has a smaller

size to Na+.7 Therefore, Na+ and Mg2+ are considered to have a high probability

for insertion and contamination in an FePO4 lithiation process.


32-/ FePO

4

Reagents

128

Table 7.1: Ionic radii of the potential cations that can be intercalated into FePO

47

Ions Lithium

(Li+) Magnesium

(Mg2+) Sodium

(Na+ )

Potassium (K+)

Ionic radii, r/pm 59 72 102 138

Here, experiments compare Li+ to Na+ insertion ratios ([Li+]:[Na+]) in response to

a reducing agent and the FePO4 in aqueous solutions containing both lithium

and sodium salts. Three different ratio of concentrations of lithium to sodium

were employed; i.e. 1:10, 1:50, and 1:100 of [Li+]:[Na+]. Likewise, for

magnesium, experiments compared the lithium ion to magnesium ion

concentration ratios ([Li+]:[Mg+2]). In the case of magnesium, the ratios of the

interferent ion to lithium varied from 1:10 to 1:20.


Equation 7.1 2 2 2 → 2 2

The stoichiometry of this chemical reaction is shown in Equation 7.1. Table 7.2

shows the concentrations of chemical compositions in each experiment. The

concentration of 0.3 M Na2S

2O

3 was mainly used because this concentration

was found to be optimal for the 24 hour experiment, as described in Chapter

6. Another Na2S

2O

3 solution of 0.5 M was also included in the study. An excess

of lithium in solution compared with the amount of FePO4 solid with respect to

Equation 7.1 was used in all cases to ensure full lithiation.

LiCl (Sigma-Aldrich, ≥99.0 %), Na2S

2O

3 (Sigma-Aldrich, ≥99.5 %), and other salts,

i.e. NaCl (Sigma-Aldrich, ≥99.0 %), MgCl2 · 6H

2O (Fisher Scientific, ≥99.0 %)

were dissolved in deionized water. The salts, which were added for each

experiment, are shown in Table 7.2. FePO4, which was prepared by delithiation

of LiFePO4, using 0.1 M K

2S

2O

8 as mentioned in Chapter 4, was then added to

the solution and stirred at room temperature for 24 hours. Each of the

products was filtered, washed and dried at 80°C for 24 hours. All powder


32-/ FePO

4

Reagents

129

samples were examined by X-ray Diffraction (XRD). The lithiated samples from

experiments [Li+]:[Na+] and [Li+]:[Mg2+] were examined further by inductively

coupled plasma mass spectrometry analysis (ICP-MS) to check for

contaminations and to measure the lithium- sodium uptake (Equations in

Chapter 2) into a solid structure (FePO4).

Table 7.2: Concentration of reagents used in the experiments [Li+]:[Na+] and [Li+]:[Mg+2]

Experiment Ratio [Chemical composition]/M

DI/L Na

2S

2O

3 LiCl NaCl MgCl

2·6H

2O FePO

4

[Li+]:[Na+]

1:10 0.30 0.06 - - 0.03 0.201

1:10 0.50 0.10 - - 0.05 0.121

1:50 0.30 0.06 2.4 - 0.03 0.201

1:100 0.30 0.06 5.4 - 0.03 0.201

[Li+]:[Mg+2]

1:10 0.30 0.06 - 0.60 0.03 0.201 1:10 0.50 0.10 - 1.00 0.05 0.121

1:20 0.30 0.06 - 1.20 0.03 0.201

1:20 0.50 0.10 - 2.00 0.05 0.121


The XRD patterns results are presented under the lithium to sodium ratio and

lithium to magnesium ratio experiment headings. ICP results are shown at the

end.

Lithium to Sodium Ratio Experiment

The XRD sample patterns of the totally converted LiFePO4, obtained from the

1:10 (at 0.5M Na2S

2O

3), 1:50, 1:100 of [Li+]:[Na+] experiments, are shown in

Figure 7.1 - Figure 7.3. The XRD sample patterns were roughly compared with

the LiFePO4 and FePO

4 initial patterns using the position of peaks that indicate

the Miller indices (numbers in blue and red). These imply that with the effect of

high sodium ion concentration in the solution, the results show no sodium

contamination in the crystal structure.


32-/ FePO

4

Reagents

130

15 20 25 30 35 40 45

15 20 25 30 35 40 45

15 20 25 30 35 40 45

2/ degree

De-lithiation

Inte

nsi

ty/ A

.U.

LiFePO4 initial

1:10 defect FePO4 2nd run

200

200

200

101

101

101

210

011

111

201

210

011

111

201

210

111

201

211

020

020

211

211

020

301

301

301

311

311

311

121

410

121

410

121

102

102

102

221

401

112 20

2

212

221

401

112 20

2

212

410

221

112

212

401

Figure 7.1: The XRD pattern obtained from a sample 1:10 of [Li+]:[Na+] treated with 0.5 M Na

2S

2O


4

and the XRD of the de-lithiated sample (heterosite FePO4) pattern. The result

shows only LiFePO4.

15 20 25 30 35 40 45

15 20 25 30 35 40 45

15 20 25 30 35 40 45

2/ degree

De-lithiation

Inte

nsi

ty/ A

.U.

LiFePO4 initial

011

210

101

200

1:50 defect FePO4

200

200

101

101

210

011

111

201

210

111

201

020

211

211

020

301

301

311

311

121

410

121

102

102

221

401

112 20

2

212

410

221

112

212

401

111

201

020

211

301

311

121

410

102 22

14

01

112 20

2

212

Figure 7.2: The XRD pattern obtained from a sample1:50 of [Li+]:[Na+] treated with 0.3 M Na

2S

2O


4


shows only LiFePO4.


32-/ FePO

4

Reagents

131

15 20 25 30 35 40 45

15 20 25 30 35 40 45

15 20 25 30 35 40 45

2/ degree

De-lithiation

Inte

nsi

ty/ A

.U.

LiFePO4 initial

1:100 defect FePO4

200

200

200 10

110

11

01

210

011

11

1

201

210

011

111

201

210

111

201

211

020

020

211

211

020

301

301

301

311

311

311

121

410

121

410

121

102

102

102

221

401

112

202 2

12

221

401

112

202

212

410

221 11

2

212

401

Figure 7.3: The XRD pattern obtained from a sample 1:100 of [Li+]:[Na+] treated with 0.3 M Na

2S

2O


4


shows only LiFePO4.

Lithium to Magnesium Ratio Experiment

As shown in Figure 7.4 - Figure 7.5, results showing fully converted LiFePO4

were obtained from 1:10 and 1:20 of lithium ion to magnesium ion ratio with

0.5 M Na2S

2O

3 for 24 hours. This suggests that there is no major effect in the

LiFePO4 results with the presence of an excess of magnesium. Besides, full

intercalation of magnesium into FePO4 has not been reported in any literature

yet. This might be due to the size of magnesium ion, which is slightly bigger

than lithium ion.


32-/ FePO

4

Reagents

132

15 20 25 30 35 40 45

15 20 25 30 35 40 45

15 20 25 30 35 40 45

2/ degree

De-lithiation

Inte

nsi

ty/ A

.U.

LiFePO4 initial

1Li:10Mg defect FePO4

011

210

101

200

200

200 10

110

1

210

011

111

201

210

111

201

020

211

211

020

301

301

311

311

121

410

121

102

102

221

401

112 20

2

212

410

221

112

212

401

111

201

020

211

301

311

121

410

102 22

140

1

112 20

2

212

Figure 7.4: The XRD pattern obtained from a sample 1:10 of [Li+]:[Mg+2] treated with 0.5 M Na

2S

2O


4


shows only LiFePO4.

15 20 25 30 35 40 45

15 20 25 30 35 40 45

15 20 25 30 35 40 45

2/ degree

De-lithiation

Inte

nsi

ty/ A

.U.

LiFePO4 initial

1Li:20Mg defect FePO4

011

210

101

200

200

200 10

110

1

210

011

111

201

210

111

201

020

211

211

020

301

301

311

311

121

410

121

102

102

221

401

112

202

212

410

221

112

21240

1

111

201

020

211

301

311

121

410

102 22

14

01

112 20

2

212

Figure 7.5: The XRD pattern obtained from a sample 1:20 of [Li+]:[Mg+2] treated with 0.5 M Na

2S

2O


4


shows only LiFePO4.


32-/ FePO

4

Reagents

133

The majority of the results show only LiFePO4, as shown in Table 7.3. These

confirm that the full conversion to LiFePO4 occurred with 0.5 M Na

2S

2O

3 and

that partial lithiation sometimes occurred with 0.3 M Na2S

2O

3. Most

significantly, no crystalline NaFePO4 is shown and there is no evidence of

magnesium intercalation.

Table 7.3: The XRD results of lithiated heterosite FePO4 for each experiment

Name [Na2S

2O

3]/M ratio XRD Result

[Li+]:[Na+]

0.3 1:10 LFP+FePO4

0.5 1:10 LFP 0.3 1:50 LFP 0.3 1:100 LFP

[Li+]:[Mg+2]

0.3 1:10 LFP+FePO4

0.5 1:10 LFP 0.3 1:20 LFP+FePO

4

0.5 1:20 LFP *LFP=LiFePO

4

ICP results for the dissolved products are shown in Table 7.4. The lithium

uptake of 40-46 mg g-1 for the samples found by XRD to be fully lithiated

agrees well with the theoretical value of ~46 mg g-1. Comparing to other

literatures, the heterosite FePO4 structure absorbs lithium higher than other

types of absorbent, except for some advanced manganese oxide ion-sieves

which absorb up to 38-40 mg g-1. 1,3,8-11

The maximum sodium uptake is 3.8 mg of sodium per gram of FePO4 solid,

obtained from 1:100 of lithium to sodium ratio, resulting in the fully lithiated

sample. For the Li+ to Mg2+ concentrations ratio experiment, the results suggest

that magnesium uptake is very small compared to the lithium uptake, about

1/40 of Mg2+ uptake to Li+ uptake, as shown in Table 7.4.

It is surprising that the contaminant levels in the lithiated samples were

relatively constant, and did not increase significantly with the concentrations of

contaminant in the initial solution. This finding suggested a more useful


32-/ FePO

4

Reagents

134

measure of the efficiency, in terms of a selectivity coefficient defined as the

lithium/ contaminant ratio for the product divided by the same ratio for the

original solution. The final column in the table shows a dramatic selectivity

increase as the concentration of lithium in solution is made much smaller than

that of other ions. The high values suggest that S2O

32-/FePO

4 reagent for Li+

sequestering could be used in brine solutions.

Table 7.4: Lithium and sodium concentrations found in samples

Solution Ratio [Na

2S

2O

3]/

M Result

Uptake/mg g-1 Solid

[Li+]:[metal] Li selectivity

Li Na Mg

[Li+]:[Na+]

1/10 0.3 LFP+ FePO

4 35.1 3.0 - 39 390

1/10 0.5 LFP 40.4 2.8 - 47 470 1/50 0.3 LFP 44.8 3.0 - 48 2400

1/100 0.3 LFP 45.3 3.8 - 40 4000

[Li+]:[Mg2+] 1/10 0.5 LFP 46.9 1.5 1.8 112 1123 1/20 0.5 LFP 44.5 0.8 1.4 111 2221

Note: The formulas of the metal uptake, the solid ratio and lithium selectivity are in Chapter 2.3

Chemical Lithiation of FePO4 from Synthetic Brine 7.2

Solutions

The purpose of the present study was to confirm the reaction of lithium

insertion into FePO4 by S

2O

32- in a synthetic brine environment and to study the

selectivity of the interested metal ions, i.e. Li+, Na+, Mg2+ and K+, where Na+,

Mg2+ and K+ are highly concentrated in brine, with their ion sizes being in the

same range compared to Li+.


32-/ FePO

4

Reagents

135

7.2.1 Experimental details

Two types of brine were made mimicking those in the Bolivia’s Salar de Uyuni

and called type A and type B.12 The chemical compositions of both brine types

are shown in Table 7.5.

Table 7.5: Chemical compositions of synthetic brine type A/B and the heterosite FePO

4

Synthetic

Brine

[Chemical composition]/ M DI/L

Na2S

2O

3 LiCl NaCl MgCl

2 ∙ 6H

2O KCl K

2SO

4 FePO

4

Type A 0.3 0.06 4 0.3 0.2 - 0.03 0.134

Type B 0.3 0.20 2.4 1.3 - 0.3 0.10 0.067

The procedure was similar to experiment 7.1. LiCl (Sigma-Aldrich, ≥99.0 %),

Na2S

2O

3 (Sigma-Aldrich, ≥99.5 %), and other salts, i.e. NaCl (Sigma-Aldrich,

≥99.0 %), MgCl2 ∙ 6H

2O (Fisher Scientific, ≥99.0 %) and KCl (Sigma-Aldrich,

≥99.5 %) / K2SO

4 (Fisher Scientific, ≥99.0 %) were dissolved in deionized water.

The salts, which were added for each experiment, are shown in Table 7.5.

FePO4, which was prepared by delithiated LiFePO

4, using 0.1 M K

2S

2O

8 as

mentioned in Chapter 4, was then added to the solution and stirred at room

temperature for 24 hours. Each of the products was filtered, washed and dried

at 80°C for 24 hours. All powder samples were examined by XRD to confirm

the crystal structure, ICP-MS analysis to check for contaminations and to

measure the lithium-sodium uptake into a solid structure (FePO4), and

galvanostatic measurement to measure the capacities and cycles of the

products.


Figure 7.6 - Figure 7.7 show the XRD data of the samples that were obtained

from synthetic brine type A and B with 0.3 M Na2S

2O

3 after 24 hours of reaction

time. It is observed that the reaction results in full lithiation of FePO4, with no

major presence of any contaminant. This confirms that Na2S

2O

3 is a suitable


32-/ FePO

4

Reagents

136

reducing agent to extract Li+ from salt based solutions into the heterosite

FePO4. The samples were examined by ICP and found to have a very small

contamination of metal ions compared to Li+, as shown in Table 7.6. Again,

this method confirms that the heterosite FePO4 can absorb up to 45-46 mg of

lithium per gram of absorbent with the full insertion of lithium, and with a

similar value to experiment 7.1. This process has a very high selectivity

towards lithium, leading to an enrichment in lithium concentration vs. other

ions of more than 500 under the conditions relevant to lithium extraction from

brines.

15 20 25 30 35 40 45

15 20 25 30 35 40 45

15 20 25 30 35 40 45

2/ degree

De-lithiation

Inte

nsi

ty/ A

.U.

LiFePO4 initial

Brine Normal 1 Day

200

200

200 10

110

110

1

210

011

111

201

210

011

111

201

210

111

201

211

020

020

211

211

020

301

301

301

311

311

311

121

410

121

410

121

102

102

102

221

401

112 20

2

21

2

221

401

112 20

2

21

2

410

221

112

212

401

Figure 7.6: The XRD pattern obtained from brine type A treated with 0.3 M Na

2S

2O


4 and the

XRD de-lithiation (heterosite FePO4) sample pattern. The result shows only

LiFePO4.


32-/ FePO

4

Reagents

137

15 20 25 30 35 40 45

15 20 25 30 35 40 45

15 20 25 30 35 40 45

2/ degree

De-lithiation

Inte

nsi

ty/

A.U

.

LiFePO4 initial

Brine with K2SO

4 1 Day

200

200

200 10

110

110

1

210

011

111

201

210

011

111

201

210

111

201

211

020

020

211

211

020

301

301

301

311

311

311

121

410

121

410

121

102

102

102

221

401

112 20

2

212

221

401

112 20

2

212

410

221

112

212

401

Figure 7.7: The XRD pattern obtained from brine type B treated with 0.3 M Na

2S

2O


4 and the

XRD de-lithiation (heterosite FePO4) sample pattern. The result shows only

LiFePO4.

Table 7.6: Concentration of metals; i.e. Li+, Na+, K+, and Mg2+, that contain the LiFePO

4 samples obtained from type A and type B synthetic brines.

Metal

Uptake/mg g-1 [Li+]:[Me]solution

[Li+]:[Me]solid

Lithium

selectivity

Type A Type B Type

A

Type

B

Type

A

Type

B Type A Type B

Li+ 45.7 46.4 - - - - - -

Na+ 2.5 0.4 1/77 1/15 61 370 4700 5550

K+ 1.5 1.5 1/3 1/3 170 180 510 540

Mg2+ 0.8 0.2 1/5 1/6.5 200 870 1000 5600

Note: Me = metal The formulas for the calculations are in Chapter 2.3

Galvanostatic measurements were taken to compare the character of the fully

lithiated FePO4 samples that were obtained from both synthetic brines, type A

and B, with the commercial LiFePO4

from Tatung. The results are shown in

Figure 7.8. The first extraction of the samples (LiFePO4) at a rate of 0.1C


32-/ FePO

4

Reagents

138

demonstrates a similar specific charge (electrochemical performance) of type B

and the commercial LFP.

Figure 7.8: Electrochemical data at rate 0.1C of LiFePO4 prepared by chemical

lithiation of FePO4 in synthetic brines; i.e. type A and B. The results obtained

with the initial LiFePO4 (Tantung) are also included in the graph for a

comparison. (This figure was published in March 201413).

The results of the electrochemical cycling of LiFePO4, obtained from

commercial, synthetic brine type A and B, at different rates and cycles are

shown in Figure 7.9.

At a slow cycling rate, all electrodes showed no significant changes in the

charge delivered. However, the LiFePO4 obtained from brine type B delivered

smaller charges at fast cycling rates. The second 0.1C and 1C runs shows no

major significant difference in the specific charge than the first run.

-20 0 20 40 60 80 100 120 140 1603.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

4.6

Po

ten

tial

/ V v

s. L

i/Li+

Specific Charge/ mAh g-1

Brine type A Brine type B LiFePO

4


32-/ FePO

4

Reagents

139

0 10 20 30 40 500

20

40

60

80

100

120

140

160

180

Sp

ecif

ic c

har

ge

/mA

h g

-1

Cycle number

Figure 7.9: Electrochemical cycling of LiFePO4 at different cycling rates, as

indicated (0.1C to C).

: Electrodes prepared with the initial LiFePO4 (Tatung)

: The synthetic brine type A : The synthetic brine type B Closed symbols: charge; open symbols: discharge. (This figure was published in March 201413).

The summary of Chapter 7 is given below:

These results confirm that the complete conversion of LiFePO4

can be

achieved using Na2S

2O

3 under synthetic brine conditions.

The lithium uptake can be as high as 46 milligram per gram of FePO4

and lithium selectivity can be as high as 5600, depending on lithium ion

to metal ion ratio in solution.

The electrochemical performance of both brine type samples were very

satisfactory, compared to the commercial LiFePO4.

C/10

C

2C

4C

6C 8C

10C

C/10

C


32-/ FePO

4

Reagents

140

References 7.3

(1) Chitrakar, R.; Kanoh, H.; Miyai, Y.; Ooi, K. Recovery of Lithium from Seawater Using Manganese Oxide Adsorbent (H

1.6Mn

1.6O

4) Derived from

Li1.6

Mn1.6


(2) Zhang, Q.-H.; Li, S.-P.; Sun, S.-Y.; Yin, X.-S.; Yu, J.-G. Lithium selective adsorption on 1-D MnO


2009, 20, 432-437. (3) Chitrakar, R.; Makita, Y.; Ooi, K.; Sonoda, A. Selective Uptake of Lithium Ion from Brine by H

1.33Mn

1.67O

4 and H

1.6Mn

1.6O


41, 1647-1649. (4) Ooi, K.; Miyai, Y.; Katoh, S.; Maeda, H.; Abe, M. Topotactic lithium(1+) insertion to .lambda.-manganese dioxide in the aqueous phase. Langmuir 1989, 5, 150-157. (5) Ooi, K.; Miyai, Y.; Sakakihara, J. Mechanism of lithium(1+) insertion in spinel-type manganese oxide. Redox and ion-exchange reactions. Langmuir 1991, 7, 1167-1171. (6) Pasta, M.; Battistel, A.; La Mantia, F. Batteries for lithium recovery from brines. Energy & Environmental Science 2012, 5, 9487-9491. (7) Atkins, P. W.; De Paula, J.: Mettallic, Ionic, and Covalent solids. In The elements of physical chemistry; 4th ed.; Oxford University Press: Oxford, 2005; pp 376-400. (8) Yu, Q.; Sasaki, K.; Hirajima, T. Bio-templated synthesis of lithium manganese oxide microtubes and their application in Li+ recovery. Journal of Hazardous Materials 2013, 262, 38-47. (9) Wang, L.; Meng, C. G.; Ma, W. Study on Li+ uptake by lithium ion-sieve via the pH technique. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2009, 334, 34-39. (10) Tian, L.; Ma, W.; Han, M. Adsorption behavior of Li+ onto nano-lithium ion sieve from hybrid magnesium/lithium manganese oxide. Chemical Engineering Journal 2010, 156, 134-140. (11) Wang, L.; Meng, C. G.; Han, M.; Ma, W. Lithium uptake in fixed-pH solution by ion sieves. 2008, 325, 31-40. (12) Risacher, F.; Fritz, B. Quaternary geochemical evolution of the salars of Uyuni and Coipasa, Central Altiplano, Bolivia. Chemical Geology 1991, 90, 211-231. (13) Intaranont, N.; Garcia-Araez, N.; Hector, A. L.; Milton, J. A.; Owen, J. R. Selective lithium extraction from brines by chemical reaction with battery materials. Journal of Materials Chemistry A 2014, 2, 6374-6377.

Chapter 8: Alternative Reducing and Oxidising Agents

141

Alternative Reducing and Chapter 8:

Oxidising Agents

Introduction 8.1

As revealed in Chapter 1 and 5, the potential range for looking for useful

reducing agents for FePO4 lithiation is between 3.04 and 3.45 V vs. Li/Li+. These

numbers correspond to the onset potential of NaFePO4 and LiFePO

4 formation,

respectively.1-5 Sodium thiosulphate (Na

2S

2O

3) has a potential of 3.13 V vs. Li/Li+

6 (Equation 8.1). It was successfully examined as the useful reducing agent,

although identifying an alternative reducing agent is also useful because of

environmental concerns and price of the reagent.

Equation 8.1 2 2

0.08 . , 0.17 . , 3.13 /

The above potentials are based on thermodynamic potentials whereas in

practice kinetic considerations could extend the range to reducing reagents

having potentials lower than 3.04 V vs. Li/Li+. Also, the conversion of E0 values

between aqueous and lithium scales is not very accurate. Therefore, possible

reducing agents could include sodium nitrite (NaNO2)6, formaldehyde (CH

2O)6,

formic acid (HCOOH)6, and sodium sulphite (Na2SO

3)6, as shown in Equation 8.2-

Equation 8.5.

Equation 8.2 2 2

14, 0.01 . , 0.24 . , 3.06 . /

7, 0.42 . , 0.18 . , 3.47 . /

Equation 8.3 4 4

0, 0.07 . , 0.32 . , 2.98 /

7, 0.48 . , 0.73 . ,2.57 . /


142

Equation 8.4 2 2

0, 0.20 . , 0.45 . , 2.85 /

7, 0.61 . , 0.86 . ,2.44 . /

Equation 8.5 2 2

14, 0.93 . , 1.18 . , 2.12 /

7, 0.52 . , 0.76 . , 2.53 . /

The following experiments were undertaken to test the four reducing agents by

cyclic voltammetry to determine their ability to provide electrons at a rate that

would be useful for lithiation of FePO4. The experiments were first carried out

with thiosulphate as a control experiment to confirm the validity of the test

procedures.

Experimental details 8.2

A standard three-electrode cell was used in all experiments, using a Pt mesh

counter and a saturated calomel electrode (SCE) reference electrode. Four types

of CV experiments were conducted in order to study Na2S

2O

3 as the reducing

agent:

1) Measuring the CV of a Pt electrode, assumed to be inert, in 1.5 M Li2SO

4.

2) Measuring the CV of a Pt electrode in a mixed solution of 1.5 M Li2SO

4

and 1.5 M Na2S

2O

3.

3) Measuring the CV of a Pt electrode coated with LiFePO4 (LFP) in a

solution of 1.5 M Li2SO

4.

4) Measuring the CV of a LFP coated Pt electrode in a mixed solution of 1.5

M Li2SO

4 and 1.5 M Na

2S

2O

3.


143

For the other alternative reagents, four types of CV experiment were measured

using a mixture of solutions as follows:

1) 1.5 M Li2SO

4 and 1.5 M NaNO

2

2) 1.5 M Li2SO

4 and 1.5 M formaldehyde

3) 1.5 M Li2SO

4 and 1.5 M formic acid

4) 1.5 M Li2SO

4 and 1.5 M Na

2SO

3

An electrolyte solution was made, for example, a solution of 1.5 M Li2SO

4 and

1.5 M Na2S

2O

3 by adding 12.37 g of Li

2SO

4 (0.1125 mol) (Sigma-Aldrich, ≥98.5

%) and 27.91 g of Na2S

2O

3 (0.1125 mol) (Na

2S

2O

3·5H

2O, Timstar Laboratory

Suppliers Ltd.) to 75 ml of de-ionized water (DI water) and mixed well. For the

other solutions, the solution was set up in a similar way. The same weight of

Li2SO

4 was used with the addition of a reducing agent, as shown in Table 8.1,

in 75 ml of DI water.

The electrode preparation and cell setup were mentioned in the previous

chapter, i.e. Chapter 2. The cell was purged with argon for ~1 hour. These

experiments were observed for any reduction or oxidation between the

potential limits of -1.2 to 1.2 V vs. SCE with a scan rate of 10 mV s-1, towards

positive potential, for 20 cycles.

Table 8.1: Composition of each solution

Solution Electrolyte Electrode

Reagent Amount Active

material Weight/ mg

1* Li2SO

4 12.37 g Pt N/A

2* Li2SO

4+Na

2S

2O

3 12.37 g+27.91 g Pt N/A

3* Li2SO

4 12.37 g LFP 12.32

4* Li2SO

4+Na

2S

2O

3 12.37 g+27.91 g LFP 10.62

1 Li2SO

4+NaNO

2 12.37 g+7.76 g LFP 9.46

2 Li2SO

4+CH

2O 12.37 g+4.14 ml LFP 21.44

3 Li2SO

4+HCOOH 12.37 g+4.24 ml LFP 23.85

4 Li2SO

4+Na

2SO

3 12.37 g+14.18 g LFP 27.72

* Control experiment


144

Results and discussion 8.3

All the CV experiments were started at the open circuit potential, indicated by

the green arrow, as shown in Figure 8.1. The voltammogram were measured at

10 mV s-1 scan rate in a potential window of 1.2 V to -1.2 V vs. SCE.

Figure 8.1: Cyclic voltammogram of a Pt working electrode in: a) 1.5 M Li

2SO

4 (Solution 1*; black)

b) 1.5 M Li2SO

4 and 1.5 M Na

2S

2O

3 (Solution 2*; blue)

The CV profiles were recorded at the rate of 10 mV s-1 with the potential range of 1.2 and -1.2 V vs. SCE.

Figure 8.1 shows the CV profiles of a Pt electrode in 1.5 Li2SO

4 (black CV with

the onset reduction potential of -0.80 V) and in a mixed solution of 1.5 M

Li2SO

4 and 1.5 M Na

2S

2O

3 (blue CV with the onset oxidation and reduction

potential of 0.23 V and -0.32 V, respectively). The oxidation current in the

presence of Na2S

2O

3 is ascribed to the oxidation of S

2O

32- to tetrathionate (S

4O

62-).

The reduction current is ascribed to the reduction of S4O

62- back to S

2O

32-, as

shown in Equation 8.1. The reduction peak on the CV (black curve in Figure

8.1) indicates hydrogen evolution7, as shown in Equation 8.6.

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

1.8 2.3 2.8 3.3 3.8 4.3 4.8

-40

-20

0

20

40

60

80

Pt electrode in 1.5 M Li2SO

4


4+1.5 M Na

2S

2O

3

Potential/ V vs. Li/ Li+

Cu

rren

t/ m

A

Potential/ V vs. SCE

2S2O

3

2- S4O

6

2-+2 e-

S4O

6

2-+ 2e- 2S2O

3

2-

2H2O++2e- H

2+2OH-

SO4

2- S2O

8

2-+ +2e-


145


0.00 . , 0.25 . , 3.05 /

7, 0.41 . , 0.66 . , 2.64 . /

Figure 8.2 shows a comparison of three CV profiles, i.e. the LFP electrode in

1.5 M Li2SO

4 (black CV), the Pt working electrode in a mixed solution of 1.5 M

Li2SO

4 + 1.5 M Na

2S

2O

3 (blue CV), and the LFP electrode in a mixed solution of

1.5 M Li2SO

4 + 1.5 M Na

2S

2O

3 (red CV). The black CV profile indicates an

oxidation reaction of LFP to FePO4 on the positive scan. The backward reaction

shows a reduction of FePO4 to LFP. The blue CV profile in Figure 8.2 is the

same as in Figure 8.1. Last, the red CV profile indicates an oxidation of LiFePO4

to FePO4, plus a conversion of FePO

4 to LiFePO

4 which was produced by S

2O

32- in

the solution.

Figure 8.2: Cyclic voltammogram of a) a Pt electrode coated with LiFePO

4 in

1.5 M Li2SO

4 (Solution 3*; black: onset oxidation potential = 0.26 V, Onset

reduction potential = 0.16 V), Pt electrode in 1.5 M Li

2SO

4 and 1.5 M Na

2S

2O

3

(Solution 2*; blue), and a Pt electrode coated with LiFePO

4 in 1.5 M Li

2SO

4 and

1.5 M Na2S

2O

3 aqueous electrolyte (Solution 4*; red: onset oxidation potential

= 0.39 V, onset reduction potential = 0.16 V). The CV profiles were recorded at the rate of 10 mV s-1 with the potential range of 1.2 and -1.2 V vs. SCE.

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

1.8 2.3 2.8 3.3 3.8 4.3 4.8

-50

0

50

100



Cu

rren

t/ m

A

LFP electrode in 1.5 M Li2SO

4


4+1.5 M Na

2S

2O

3


4+1.5 M Na

2S

2O

3

LiFePO4FePO

4+Li++e-

FePO4+Li++e- LiFePO

4

LiFePO4 FePO

4+Li++e-

2FePO4+2Li++2S

2O

3

22LiFePO4+S

4O

6

2-

2S2O

3

2- S4O

6

2-+2 e-

S4O

6

2-+ 2e- 2S2O

3

2-


146

Two features indicate that S2O

32- is the suitable reducing agent for FePO

4. The

increased oxidation current due to two reactions occurred, i.e. an oxidation of

LiFePO4 to FePO

4 and a conversion of FePO

4 to LiFePO

4 which was produced by

S2O

32-. Another feature is the decreased reduction current measured with the

LFP electrode in the presence of S2O

32-. This is due to the previous conversion

by S2O

32- that left a small amount of FePO

4 to reduce and form LiFePO

4.

Figure 8.3-Figure 8.6 demonstrate the CV profiles of a lithium electrode in the

other alternative reducing reagents, i.e. NaNO2, CH

2O, HCOOH and Na

2SO

3,

which are represented by the red graph. All of them were compared to the

profile of the LFP electrode in 1.5 M Li2SO

4 which is represented in black.


electrode in

a) 1.5 M Li2SO

4 (Solution 3*; black: onset oxidation potential = 0.26 V,

Onset reduction potential = 0.16 V) b) 1.5 M Li

2SO

4 and 1.5 M NaNO

2 (Solution 1; red: onset oxidation

potential = 0.26 V, Onset reduction potential = 0.14 V)

All the CV profiles of the alternative reagents show an increased oxidation

current, compared to the black profile, except for the CH2O one in Figure 8.4.

In the case of Na2SO

3 (Figure 8.6), a decreased reduction current was also

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

1.8 2.3 2.8 3.3 3.8 4.3 4.8

-50

0

50

100 LFP electrode in 1.5 M Li

2SO

4


4+1.5 M NaNO

2

Cu

rren

t/ m

A




147

observed, similar to the case of Na2S

2O

3 (Figure 8.2). This suggests that

probably Na2SO

3 will be an efficient reducing agent. All other reagents might

also work; however, further studies are required. The least promising reagent

is CH2O because the CV is almost identical to the black CV, i.e. measured

without CH2O.

Beside this CV experiment of Na2SO

3, the actual experiment of FePO

4 lithiation

using Na2SO

3 as a reducing agent was performed and illustrated a successful

result of a fully converted FePO4, as shown in the appendix. Thus, SO

32- is

shown to be an alternative reducing agent, as expected from the CV profile.


electrode in

a) 1.5 M Li2SO

4 (Solution 3*; black: onset oxidation potential = 0.26 V,

Onset reduction potential = 0.16 V) b) 1.5 M Li

2SO

4 and 1.5 M formaldehyde

(Solution 2; red: onset oxidation

potential = 0.34 V, Onset reduction potential = 0.12 V)

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

1.8 2.3 2.8 3.3 3.8 4.3 4.8

-60

-40

-20

0

20

40

60


Cu

rren

t/ m

A



4


4+1.5 M CH

2O


148


electrode in a) 1.5 M Li

2SO

4 (Solution 3*; black: onset oxidation potential =

0.26 V, Onset reduction potential = 0.16 V) b) 1.5 M Li

2SO

4 and 1.5 M formic acid

(Solution 3; red: onset

oxidation potential = 0.25 V, Onset reduction potential = 0.12 V)


electrode in a) 1.5 M Li

2SO

4 (Solution 3*; black: onset oxidation potential =

0.26 V, Onset reduction potential = 0.16 V) b) 1.5 M Li

2SO

4 and 1.5 M Na

2SO

3 (Solution 4; red: onset

oxidation potential = 0.36 V, Onset reduction potential = 0.14 V)

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

1.8 2.3 2.8 3.3 3.8 4.3 4.8

-100

-50

0

50


4


4+1.5 M HCOOH


Cu

rren

t/ m

A


-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

1.8 2.3 2.8 3.3 3.8 4.3 4.8

-50

0

50

100 LFP electrode in 1.5 M Li

2SO

4


4+1.5 M Na

2SO

3


Cu

rren

t/ m

A


LiFePO4 FePO

4+Li++e-

2FePO4+2Li++SO

3

2-+2OH- 2LiFePO

4+ SO

4

2-+ H2O

FePO4+Li++e- LiFePO

4


149

The summary of Chapter 8 is given below:

The CH2O reagent found to be the least promising reducing agent.

NaNO2 and HCOOH might be the suitable reducing agents; however,

they further study is required.

Na2SO

3 was found to be the most suitable reducing agent for inserting

lithium ion into FePO4.

References 8.4

(1) Whiteside, A.; Fisher, C. A. J.; Parker, S. C.; Saiful Islam, M. Particle shapes and surface structures of olivine NaFePO4 in comparison to LiFePO4. Physical Chemistry Chemical Physics 2014, 16, 21788-21794. (2) Wu, B.; Ren, Y.; Li, N.: LiFePO

4 Cathode Material. In Electric Vehicles -

The Benefits and Barriers, 2011; pp 199-216. (3) Zaghib, K.; Trottier, J.; Hovington, P.; Brochu, F.; Guerfi, A.; Mauger, A.; Julien, C. M. Characterization of Na-based phosphate as electrode materials for electrochemical cells. Journal of Power Sources 2011, 196, 9612-9617. (4) Reale, P.; Panero, S.; Scrosati, B.; Garche, J.; Wohlfahrt-Mehrens, M.; Wachtler, M. A Safe, Low-Cost, and Sustainable Lithium-Ion Polymer Battery. Journal of The Electrochemical Society 2004, 151, A2138-A2142. (5) Dodd, J. L.; Yazami, R.; Fultz, B. Phase Diagram of Li

xFePO

4.

Electrochemical and Solid-State Letters 2006, 9, A151-A155. (6) Center, U. o. R. I. C. R.; Company, C. R.: Handbook of chemistry and physics 0363-3055; 70th ed.; CRC Press: Cleveland, Ohio, 1989. (7) Mi, C.; Zhang, X.; Li, H. Electrochemical behaviors of solid LiFePO

4

and Li0.99

Nb0.01

FePO4 in Li

2SO


Chemistry 2007, 602, 245-254.


150

Chapter 9: Conclusions and Future Work

151

Conclusions and Future Work Chapter 9:

Conclusions 9.1

It is true that lithium is abundant in nature. In terms of world economics,

lithium has not yet needed to be recycled. The demand of lithium is obviously

dramatic and has been raised due to the increased use of electronic devices

and vehicles.

Lithium sources are mainly brine in the form of lithium chloride (LiCl) which is

used to make lithium carbonate (Li2CO

3) for Li-ion battery manufacture. The

leading lithium mineral producers are in the continent of South America, i.e.

Argentina, Bolivia, and Chile, due to salt lake deposits. Tahil reported that

Li2CO

3 is chemically obtained from LiCl by a known process called lime soda

evaporation.1 It is the most efficient method employed so far. However, this

process takes almost 1.5 years to obtain LiCl by the evaporation of the salt

water. Therefore, the aim of this project is to search for any method that can

reduce the length of production in sequestering lithium in an environmentally

friendly way.

In this thesis, a chemical method of lithium sequestration from brine using

heterosite FePO4 was investigated and evaluated for effectiveness. The key

substances of the lithiation process are lithium sources/brine (Li2SO

4, LiCl), a

reducing agent (S2O

32-), and a frame work to trap lithium (heterosite FePO

4).

The heterosite FePO4 was chosen due to its structure.2 This structure can be

fitted with Li+ as a small cation, producing LiFePO4 with olivine structure. The

lithiation of FePO4 is achieved with a suitable reducing agent.


152

Heterosite FePO4 was formed by the delithiation of LiFePO

4 with the use of 0.1

M potassium persulphate (K2S

2O

8) as an oxidizing agent.3 It was revealed that a

higher concentration of K2S

2O

8 extracts Li+ from LiFePO

4 at a quicker rate.

Almost 99% completion was shown after 30 minutes in the higher molar ratio

of K2S

2O

8:LiFePO

4 (3:2 and 6:2) and after 1 h in the lower molar ratio (1:2). As

demonstrated in the experiments conducted in Chapter 4.1, a depletion of

FePO4 could occur while K

2S

2O

8 was still in the mixed solution. However, the

conductivity measurement demonstrated that the reaction completed in 10

minutes.

Once the method of FePO4 delithiation was confirmed, the next phase was to

search for a reducing agent to extract Li+ from brine and insert it into the

heterosite FePO4. Various reducing agents were studied such as lithium iodide

(LiI) at high concentration levels and with an activating agent (Zinc; Zn) to aid

the reactions. The results however showed no insertion to FePO4. Nevertheless,

a reducing agent, Na2S

2O

3, was examined, giving positive results to complete

the conversion of LiFePO4 but not to NaFePO

4 conversion. After the processes

of insertion and deinsertion of FePO4 were completed, the final LiFePO

4 was

tested in a battery, showing a satisfactory performance of LiFePO4.

Two models were developed to explain the kinetic study, i.e. a diffusion limited

model and surface-reaction limited model. The experimental data could be

explained with a surface-reaction model, where it is assumed that the

oxidation of S2O

32- concentration was the rate determining step. The results

from the collated data showed that this reaction was a first-order rate reaction

with respect to S2O

32- concentration (S

2O

32->Li+) and the reaction rate was found

to increase with the square root of the lithium concentration (S2O

32-<Li+).

The chemical lithiation of FePO4 into LiFePO

4 could be achieved in the presence

of an excess of Na+, Mg2+, and in an artificial brines experiment. For the fully

lithiated samples, the amount of lithium absorbed into heterosite FePO4 can be

as high as ~46 milligrams Li per gram of solid, whereas the insertion of other


153

cations; i.e. sodium, potassium, and magnesium, were smaller than ~ 4

milligrams per gram of solid. The heterosite FePO4 structure absorbs more

lithium than other types of absorbents, with the exception of some advanced

manganese oxide ion-sieves which absorb up to 38-40 mg g-1.4-7

An alternative reducing agent was found to aid lithiate the FePO4, i.e. sulphite

(SO32-). It works the same way as S

2O

32-. Both acted as a mediator for the redox

couple Fe3+/Fe2+, as shown clearly in the cyclic voltammograms (CV) in Chapter

8. In fact, a chemical lithiation of heterosite FePO4 by Na

2SO

3 experiment was

done to provide confirmation, resulting in a full conversion to LiFePO4

, as

shown in Appendix.

Future work 9.2

Further kinetic studies with respect to Li+ and S2O

32- concentrations would be

useful, for example, the concentrations could be reduced below 0.03 M of Li+

and 0.1 M of S2O

32- to achieve more accurate reaction rate data.

The lithiation reaction of FePO4 in synthetic brine solutions has been studied.

The next step is to try with actual brine including sea water to observe the

selectivity of lithium ion into the framework. Therefore, this method can be

attempted anywhere even in Thailand. This method cannot just be done in

brine, but also in all sorts of lithium solutions. It is thus suitable for recycling

lithium ion batteries.

As mentioned in Chapter 8, one of the alternative reducing agents is sulphite

(SO32-). The kinetics of SO

32- should be studied to compare with S

2O

32-. This will

lead to the superior choice of a suitable and economical reducing agent. A

conductivity measurement would also be interesting to pursue with both

reagents owing to the comparison with the oxidizing agent, i.e. K2S

2O

8.


154

For practical applications, the framework (FePO4 heterosite) will be used for

many cycles of lithiation and delithiation. According to Talebi-Esfandrani’s

research, platinum (Pt) was doped into LiFePO4 compound which gave an

outstanding performance on charge and discharge. Pt works as pillars for the

structure which create a strong and stable framework.8 Regardless of the price

of platinum, research into Pt doped LiFePO4 as a framework for lithium

recycling/recovering could be interesting. Thus, a study of suitable doped

material other than Pt is worth considering for further work.

In order to make this study more useful and economical in the lithium industry,

all the main raw material/reagents costs, i.e. LiFePO4/ K

2S

2O

8 /Na

2S

2O

3, and the

selling price of Li+ products in the market should be calculated. Another

question that should be considered is the amount of times the lithiation/

delithiation cycle could be done with the framework, i.e. heterosite FePO4,

before it loses its effectiveness. These results will give an estimated turnover

of this process.


155

References 9.3

(1) Tahil, W. The Trouble with Lithium. Implications of Future PHEV Production for Lithium Demand. Martainville: Meridian International Research 2007, 5-6. (2) Chen, J.; Vacchio, M. J.; Wang, S.; Chernova, N.; Zavalij, P. Y.; Whittingham, M. S. The hydrothermal synthesis and characterization of olivines and related compounds for electrochemical applications. 2008, 178, 1676-1693. (3) Ramana, C. V.; Mauger, A.; Gendron, F.; Julien, C. M.; Zaghib, K. Study of the Li-insertion/extraction process in LiFePO4/FePO4. Journal of Power Sources 2009, 187, 555-564. (4) Chitrakar, R.; Kanoh, H.; Miyai, Y.; Ooi, K. Recovery of Lithium from Seawater Using Manganese Oxide Adsorbent (H

1.6Mn

1.6O

4) Derived from

Li1.6

Mn1.6


(5) Chitrakar, R.; Makita, Y.; Ooi, K.; Sonoda, A. Selective Uptake of Lithium Ion from Brine by H

1.33Mn

1.67O

4 and H

1.6Mn

1.6O


41, 1647-1649. (6) Yu, Q.; Sasaki, K.; Hirajima, T. Bio-templated synthesis of lithium manganese oxide microtubes and their application in Li+ recovery. Journal of Hazardous Materials 2013, 262, 38-47. (7) Zhang, Q.-H.; Li, S.-P.; Sun, S.-Y.; Yin, X.-S.; Yu, J.-G. Lithium selective adsorption on 1-D MnO


2009, 20, 432-437. (8) Talebi-Esfandarani, M.; Savadogo, O. Synthesis and characterization of Pt-doped LiFePO4/C composites using the sol–gel method as the cathode material in lithium-ion batteries. Journal of Applied Electrochemistry 2014, 44, 555-562.


156

Appendix

157

Appendix A

From Chapter 8, the actual lithiation experiment using sodium sulphite

(Na2SO

3) as a reducing agent was undertaken. The stoichiometry of the

chemical reaction is shown in Equation 7. A molar ratio of 1Li+: 2S2O

32-: 1FePO

4

was used, i.e. with an excess of reducing agent (S2O

3) to aid faster conversion.

A solution preparation and a sample collection procedure are described in

detail below.

Equation 7 2 2 2 2


Solutions of 0.3 M LiCl in de-ionized (DI) water were made. The solution was

made by adding 0.56 g of LiCl (Sigma-Aldrich, ≥ 99.9%) in 44.2 cm3 of DI water

and the solution was stirred until all the salt dissolved. Then, 3.34 g of Na2SO

3

(0.0273 mol; Sigma-Aldrich, ≥ 98%) was added, then the solution was stirred,

and heated to approximately 50°C to dissolve the Na2S

2O

3, then cooled to room

temperature before adding 2 g of FePO4 (0.0133 mol).


Once the solutions were dissolved and cooled to room temperature, the

insoluble FePO4 was added to form a suspension, as the timer was started. The

suspensions were stirred and samples were taken for filtration at 24 hours.

The filtration was done using a vacuum pump with grade 1 qualitative filter

paper diameter 110 mm. Each sample was then washed with DI water, and

dried at 80°C for approximately 12 h.

Samples were analysed using X-ray diffraction (XRD; the Bruker D2 Phaser).

Samples were scanned from 15 to 50 degrees of XRD pattern. The reference

XRD patterns of LiFePO4 and FePO

4 were obtained via the Inorganic Crystal

Structure Database (ICSD; RSC).

Appendix A

158

Result

Figure 7 shows a fully lithiated LiFePO4 as expected, according the cyclic

voltammogram experiment in Chapter 8.

Figure 7: The XRD fitting obtained from a sample treated with 0.3 M LiCl + 0.6 M Na

2SO

3 for 24 hours. The result shows pure olivine, i.e. LiFePO

4

Appendices

159

Appendices

List of paper

1. Intaranont, N.; Garcia-Araez, N.; Hector, A. L.; Milton, J. A.; Owen, J. R.

Selective lithium extraction from brines by chemical reaction with

battery materials. Journal of Materials Chemistry A 2014, 2, 6374-6377.

(in Biliography)

List of conferences

1. Attending- The 16th International Congress for Battery Recycling ICBR

2011 (September 21-23, 2011), Venice, Italy.

2. Attending- Resources That Don’t Cost the Earth (December 1-2, 2011),

Berlin, Germany.

3. Attending- The Advances in Li-Battery Research (April 10-11, 2013) at

University of Huddersfield, Huddersfield, United Kingdom.

4. Poster presentation- University of Southampton Chemistry Poster Day

(February 18, 2014), in Southampton, United Kingdom.

5. Poster presentation- The 2nd Workshop in the Advances of Li-Battery

Research (April 10-11, 2014) at University of Liverpool, Liverpool, United

Kingdom.

6. Oral presentation- Electrochemistry final year postgraduate

presentations at University of Southampton (May 28, 2014),

Southampton, United Kingdom

7. Oral presentation-The 2014 ECS and SMEQ Joint International Meeting

(October 5-10, 2014) in Cancun, Mexico.

In Preparation

An article of the second phase from the first paper is in preparation for

publication in the Journal of Energy and Environmental Science. The content is

focused on the kinetics of the chemical lithiation of FePO4 using Na

2S

2O

3 , by

varying the concentration of S2O

32- and Li+ independently (in Chapter 6).

Bibliography

161

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Reproduced by permission of The Royal Society of Chemistry:

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26/06/2015 Rightslink® by Copy

Title: Atomic-Scale Investigation of Logged in as:

Defects, Dopants, and Lithium Noramon Intaranont Transport in the LiFePO4

OlivineType Battery Material

Account #: 3000930549

Author: M. Saiful Islam, Daniel J. Driscoll,

Craig A. J. Fisher, et al

Publication: Chemistry of Materials Publisher: American Chemical Society Date: Oct 1, 2005

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