Electrode materials for Na-ion batteries:a new route for ...€¦ · Electrode materials for Na-ion batteries:a new route for low-cost energy storage. ... =-3.04V versus standard

1

Electrode materials for Na-ion batteries:a new route for low-cost

energy storage.

Massaccesi Valentina

Instituto Superior Técnico, Universidade Técnica de Lisboa

Abstract. With the imminent exhaustion of fossil fuel resources and increasing environmental

problems, a variety of renewable and clean energy sources, such as the wind and sun, are growing

rapidly . The use of these discontinuous energy source requires a large-scale energy storage system

(ESS) to shift electrical energy from peak to off-peak periods, with the aim to realize smart grid

management. Among various energy storage technologies, room-temperature stationary sodium-ion

batteries have attracted great attention particularly in large scale electric energy storage applications

for renewable energy and smart grid because of the huge abundant sodium resources and low cost1.

The research work presented in this thesis deals with the investigation of electrochemical properties

of electrode materials for this tipe of batteries, in particular NaxCoO2 as cathodic compound. In the

first part of this thesis, several synthetic routes have been studied. The active materials obtained

have been investigated by XRD and ICP-MS analysis to evaluate the correlation between

stechiometry and crystal structure. A morphological characterization was conduced using SEM. In

the second part of this thesis, matherials have been tested electrochemically by GCPL, CV and

PEIS. Finally an optimization of the system have been conduced evaluating the use of different

elecrolytes and binders.

Key words: Na-Ion Batteries, NaxCoO2 Cathode Material, X-Ray Powder Diffraction,

Electrochemical Characterization, Electrochemical Impedance Spectroscopy.

Introduction

Energy storage has become a growing global

concern over the past decade as a result of

increased energy demand, combined with

drastic increases in the price of refined fossil

fuels and the environmental consequences of

their use. This has increased the call for

environmentally responsible alternative

sources for both energy generation and

storage. Although wind and solar generated

electricity is becoming increasingly popular in

several industrialized countries, these sources

provide intermittent energy; thus energy

storage systems are required for load-

levelling.

Lithium-ion batteries, the most common type

of secondary cells found in almost all portable

electronic devices, are a possible solution to

these larger global concerns1. Lithium based

electrochemistry offers several appealing

attributes: lithium is the lightest metallic

element and has a very low redox potential

(E°Li+

/Li=-3.04V versus standard hydrogen

electrode), which enables cells with high

voltage and high energy density. Furthermore,

Li+ has a small ionic radius which is

beneficial for diffusion in solids. Coupled

with its long cycle life and rate capability,

these properties have enabled Li-ion

technology to capture the portable electronics

market. The demand for lithium-ion batteries

as a major power source in portable electronic

2

devices and vehicles is rapidly increasing.

With the likelihood of enormous demands on

available global lithium resources, concerns

over lithium supply, but mostly its cost, have

arisen2. Even if extensive battery recycling

programs were established, it is possible that

recycling could not prevent this resource

depletion in time. While the debate over the

feasibility and environmental impact of

lithium carbonate production continues,

sodium-based compounds are under

consideration as options for large scale energy

storage coupled to renewable energy sources,

for example.

With sodium’s high abundance, low cost and

very suitable redox potential (E°Na+

/Na=-2.71

V versus standard hydrogen electrode, only

0.3 V above that of lithium), rechargeable

electrochemical cells based on sodium

represent the most promising device for

energy storage applications3. All

characteristic of this alkali holds to make this

element strategic in innovative research of

energy storage systems4. The use of Na

instead of Li in rocking chair batteries could

mitigate the feasible shortage of lithium in an

economic way, due to the unlimited sodium

sources, the ease to recover it and its lower

price.

Figure 1:Main characteristics of Na and Li materials

Moreover, for positive electrode materials

sodium intercalation chemistry is very similar

to Li, thus making it possible to use very

similar compounds for both kinds of systems.

Furthermore, if a rechargeable sodium-ion

battery with good performance characteristics

could be developed, it could have the

advantage of using electrolyte systems of

lower decomposition potential due to the

higher half-reaction potential for sodium

relative to lithium. This low voltage operation

would make Na-ion cells cheaper, because

water-based electrolytes could be used instead

of organic ones. It must be pointed out that

electrochemical Na-ion cells will always fall

short of meeting energy densities compared to

Li-ion batteries. First, because equivalent

weight of Na is higher than Li, and second

because the size of the alkali metal is bigger.

Thus, Na-based cells will have difficulties

competing with Li based cells in terms of

energy density. However, they can be

considered for use in applications where the

weight and footprint requirement is less

drastic, such as storage of off-peak and

essentially fluctuating renewable energies,

such as wind and solar farms. In spite of these

considerations, there exists growing interest

on Na-ion technology5.

The research work presented in this thesis

deals with the investigation of

electrochemical properties of electrode

materials for this sodium-ion batteries, in

particular NaxCoO2 as cathode.

In the first part of this thesis, several synthetic

routes have been studied. The active materials

obtained have been investigated by X-ray and

Inductively Coupled Plasma Mass

Spectrometry (ICP-MS) analysis to evaluate

the correlation between stoichiometry and

crystal structure. A morphological

characterization was carried out using

Scanning Electron Microscope (SEM). In the

second part of this thesis, materials have been

tested electrochemically by Galvanostatic

Cycling with Potential Limitation (GCPL),

Cyclic Voltammetry (CV) and

Electrochemical Impedance Spectroscopy

(EIS). Finally an optimization of the system

3

has been done evaluating the use of different

electrolytes and binders.

Experimental Section

Synthetic techniques:

NaxCoO2 has been synthetized by solid state

method, ball milling accompanied by post firing

and sol-gel method.

The first is the most common method in

which stoichiometric mixture of starting

materials is ground together and the resultant

mixture is heat–treated in furnace. In the case

of NaxCoO2, appropriate amount of starting

materials, Na2Co3 and CoO4, are thoroughly

mixed in the ratio of 1:1. Subsequently, the

mixture is ground to ensure complete

reaction. After drying, the powder is calcined

in a preheated furnace at 800° C to form the

precursor. Initially the reaction is carried out

for 12 hours. The product was again subjected

to solid state reaction for 12 hours, under

oxygen flow, after intermediate grinding .

The purity of the material depends on the

choice of the ratio of starting materials,

calcination temperature and time.

The Ball Mill and post firing method involve

the use of a particular grinder characterized

by a hollow cylindrical bowl rotating about its

axis, partially filled with balls (grinding

media). This is an alternative way to use the

classical solid state synthetic method.

Reagents are mixed together in a different

way, using a ball mill. It is used wherever the

highest degree of fineness is required and it

works on the principle of impact and attrition,

size reduction is done by impact as the balls

drop from near the top of the bowl. In this

case sodium cobalt oxide is synthesized from

Na2CO3 and CoCO3 powders in a 1:2 mol

ratio through the alternative approach, which

employs ball milling and subsequent firing.

For this, the mixture is subjected to

mechanical milling in an agate bowl with

agate balls at 200 rpm for one hour. Then the

powder is fired in a preheated furnace at

T=800°C for 12 hours in air6.

The sol-gel method can overcome some

disadvantages of conventional solid state

method thanks to its low processing

temperature, high homogeneity, possibility of

controlling size and morphology of the

particles7. Molecular precursors are converted

to nanometre-sized particles, to form a

colloidal suspension, or sol. Usually,

stoichiometric amounts of sodium acetate (

CH3COONa ) and Cobalt(II) acetate

tetrahydrate ( Co(CH3COO)2(H2O)4 ) are

dissolved in an appropriate quantity of

distilled water at room temperature. The

solution is stirred at T=50° C. Then calculated

amount of citric acid is added as a

complexing agent in the polymeric matrix, in

order to form the sol. The amount of the citric

acid and acetates is maintained at 3:1 molar

ratio. The temperature of the solution is raised

to T=100°C for about 5 hours and continued

stirring till the solution turned into high-

viscous pink gel. Subsequently, ethylene

glycol is added to the solution as gelling

agent. This solution is further heated at T=80°

C in order to get a precursor. The product

results to be crystalline and purple. It is finely

ground and calcined (at T=250° C for 10 h

and at 700° for 10 h) to obtain the final

product. Finally, the black colored calcined

product is ground, dried under vacuum and

collected.

Chemical, structural and morphological

characterization techniques:

The chemical, structural and morphological

characterization of synthesized powders has

been carried out by using several techniques,

such as X-Ray Powder Diffraction (XRD),

Scanning Electron Microscopy (SEM) and

4

Iductively Coupled Plasma Mass

Spectrometry (ICP-MS).

Figure 2: SEM images of Sample 1(Ball-Milling and

post firing) powder at different magnifications (32100

X on the left and 5060 X on the right)

Figure 3: SEM images of Sample 2 (solid state

reaction) powder at different magnifications (32840 X

on the left and 5000 X on the right).

Figure 4: SEM images of Sample 3 (sol-gel method)

powder at different magnifications (33070 X on the left

and 5000 X on the right).

The morphology of the powders was probed

by Scanning electrode microscopy (SEM).

Images were obtained with JEOL Model

JSM-5400 equipped with a Shimadzu 800HS

EDX detector. From these images, it is

possible to observe that the Sample 2 shows

the worst distribution of the particles size, it

presents irregular shaped agglomerates.

Probably, this phenomenon depends on the

fact that the powders were grounded only by a

mortar and hence with an inadequate energy.

On the contrary, Sample 3 present the best

distribution, it is characterized by uniformly

distributed particles. Probably, it is a

consequence of the synthetic method. In fact

sol-gel synthesis permit to “dissolve” the

compound in a liquid in order to bring it back

as a solid in a controlled manner.

The chemical characterization of synthesized

powders was carried out by the Inductively

Coupled Plasma Mass Spectrometry analysis,

using an Agilent 7500 series spectrometer,

with high frequency 3MHz quadrupole. This

analysis was conducted with the aim to

5

understand the stoichiometry of metal oxides

obtained with different synthetic routes.

Metal oxides synthetized result to be:

Sample 1 Na 0.83 CoO 2;

Sample 2 Na 0.65 CoO 2;

Sample 3 Na 0.28 CoO 2.

The structural characterization of synthesized

powders was carried out by the X-ray

diffraction technique (XRD) using a Philips

X-ray diffractometer with Cu Kα radiation.

The diffraction patterns were obtained

between 10° and 70°. In the Figure 2 we can

observe the X-Ray diffraction patterns of the

synthetized powders.

Figure 2:Comparison of XRD pattern obtained by the

three different synthesis.

Comparing these data with those of “JCPDS-

International Centre for Diffraction Data”

database, chemical impurities and phase were

identified.

Figure 6:XRD pattern of Sample 1.

In the Figure 6 we can identify the presence

of Na2CO3 reagent impurities. The pattern

correspond to the Na0.71CoO2 gamma phase

(mix phase γ + α’).

Figure 7: XRD pattern of Sample 2.

In the Figure we can identify the same

reagent impurities and diffraction pattern. On

the contrary, in this case there are different

synthetic conditions (800°C, 0.65:1=Na:Co),

that lead to pure γ phase.

Figure 8: XRD pattern of Sample 3.

In the Figure we can identify the presence of

CO3O4 impurities. The pattern correspond to

the Na0.60CoO2 beta phase.

6

Electrochemical characterization techniques

The electrochemical measurement are

performed employing T-shaped

polypropylene type cells (Figure 9) equipped

with stainless steel current collectors. Disk of

high-purity sodium foil are used as counter

and reference electrodes. A glass fiber

(Whatman GF/A) with a diameter of 14 mm

is used.

Figure 3:Schematic representation of a T-cell.

The cells are assembled into dry-box. All the

electrochemical characterizations are

performed using a Galvanostatic/potentiostat

VMP2/Z by Bio-Logic.

The electrodes based on this three type of

NaxCoO2 were prepared by using different

binders: PVdF, Na-CMC and PAA. At the

same time two different electrolyte were

tested: NaPF6 and NaClO4. Each binder

presents a different preparation technique.

Electrodes processing procedure:

Three different layer, for each powder

synthetized, were manufactured using

Polyvinylidene fluoride. These layers have

been prepared by casting a slurry of NaxCoO2

(active material), Super C65 (conductive

carbon) and PVdF (binder) in NM2P

(solvent), whose composition is shown in

Table 1.

Table 1: Percentage composition of Co01, Co02 and

Co03 layers.

A solution of PVdF 5% in NMP (0.95 ml)

.has been prepared into a vial. NaxCoO2 and

SC65 have been mixed finely in a mortar to

obtain an homogeneous powder and then they

have been poured into the same vial which

contains the solution of binder. Then, 0.25

mL of NM2P have been added in order to

obtain a suspension liquid enough. The slurry

has been stirred overnight with a magnetic

anchor. The mixture has been stratified on an

Al foil, scratched by employing sandpaper,

through Doctor Blade technique setting a

thickness of 200 µm. The obtained layers has

been dried at 50°C, under hood, in order to

remove completely the solvent. Several

circular electrodes with diameter of 9mm,

have been cut before (Co01B, Co02B and

Co03B) and after (Co01A, Co02A and

Co03A) the use of a roll press.


synthetized, were manufactured using Sodium

Carboxymethyl Cellulose. These layers haves

been prepared by casting a slurry of NaxCoO2


carbon) and Na-CMC (binder) in ultrapure

H2O (solvent), whose composition is shown

in Table 2.


Co13 layers.

A solution of Na-CMC 5% in ultrapure H2O

(0.95 ml) has been prepared into a vial.

NaxCoO2 and SC65 have been mixed finely

in a mortar to obtain an homogeneous powder

and then they have been poured the same vial

which contains the solution of binder. Then,

7

0.55 mL of H2O ultrapure have been added in

order to obtain a suspension liquid enough.

The slurry has been stirred for five hours with

a magnetic anchor. The mixture has been

stratified on an Al foil, previously scratched

by employing sandpaper, through Doctor

Blade technique setting a thickness of 200

µm. The obtained layers has been dried at

room temperature, in order to remove

completely the solvent. Several circular

electrodes with diameter of 9mm, have been

cut before (Co11B, Co12B and Co13B) and

after (Co11A, Co12A and Co13A) the use of

a roll press.


synthetized, were manufactured using

Polyacrylic Acid. These layers have been

prepared by casting a slurry of NaxCoO2


carbon) and PAA (binder) in ethanol

(solvent), whose composition is shown in

Table 3.


Co23 layers.

A solution of PAA 5% in Ethanol (0.4 ml)

.has been prepared into a vial. NaxCoO2 and

SC65 have been mixed finely in a mortar to

obtain an homogeneous powder and then they

have been poured the same vial which

contains the solution of binder. Then, 0.4 ml

of ethanol have been added in order to obtain

a suspension enough liquid. The slurry has

been stirred for five hours with a magnetic

stirrer. The mixture has been stratified on an

Al foil, scratched by employing sandpaper,

through Doctor Blade technique setting a

thickness of 200 µm. The obtained layers has

been dried at 60°C for two hours, in order to

remove completely the solvent. Several

circular electrodes with diameter of 9mm,

have been cut before (Co21B, Co22B and

Co23B) and after (Co21A, Co22A and

Co23A) the use of a roll press.

The capacity of each electrodes have been

computed considering a specific theoretical

capacity of:

Sample 1 (Na0.83CoO2) 235 mAh/g

Sample 2 (Na0.65CoO2)253 mAh/g

Sample 3 (Na0.28CoO2)275 mAh/g

All obtained electrodes have been dried

overnight at 120 °C, under vacuum, and then

put into dry-box. Electrochemical experiments:

A preliminary test was conducted for Co01

(PVdF) layer, in order to characterize the

material, with galvanostatic charge/discharge

cycles at different C-rates: 5 cycle at C/10, 5

cycles at C/5, 5 cycles at C/2, 5 cycle 1C and

5 cycles at 2C. Charge/discharge cycles have

been carried out within the potential window

2-3.8 V, comparing pressed and non-pressed

electrodes in NaPF6 electrolyte. All the

potentials are given vs. Na+/Na.

Figure 10: Comparison of specific capacity vs cycle

number between Co01A (pressed) and Co01B (non-

pressed).

0 10 20

0

20

40

60

80

100

Sp

ecific

Ca

pa

city(m

Ah

/g)

Cycle Number

Charge

Discharge

C/10C/5

C/2

1C

2C

80:10:10=Na0.82

CoO2:PVdF:SC65

UNCOMPRESSED

EC:DMC=1:1 NaPF6 1M

0 10 20

0

20

40

60

80

100

80:10:10=Na0.82

CoO2:PVdF:SC65

COMPRESSED

EC:DMC=1:1 NaPF6 1M

Sp

ecific

Ca

pa

city (

mA

h/g

)

Cycle Number

Charge

Discharge

C/10

C/5

C/2

1C

2C

8

The cells exhibit an Open Circuit Voltage of

2.87 V. As we can see in the galvanostatic

curves Na deintercalation/ intercalation,

cycled between 2 and 3.8 V, undergoes

complicated series of successive phase

transitions. First sodium deintercalation curve

shows four voltage plateaus and for the initial

discharge profiles eight plateaus are shown in

Figure 11. For this reason, a differential

analysis of galvanostatic cycles have been

done, in order to have a better understanding

of mechanism occurring during cycling.

Figure11: Galvanostatic curves of Co01B at different

rate.

The cells exhibit an Open Circuit Voltage of

2.87 V. As we can see in the galvanostatic

curves Na deintercalation/ intercalation,

cycled between 2 and 3.8 V, undergoes

complicated series of successive phase

transitions. First sodium deintercalation curve

shows four voltage plateaus and for the initial

discharge profiles eight plateaus are shown in

Figure 11. For this reason, a differential

analysis of galvanostatic cycles have been

done, in order to have a better understanding

of mechanism occurring during cycling.

Figure12: dQ/dE vs E curves of Co01A.

At this point an optimization of this type of

active material was conducted with three type

of binders, comparing pressing conditions,

differents potential windows and two different

electrolyte. In the following figures are depict

the rate capabilities of Co01 (PVdF/Sample1)

layers in different electrochemical

environments.


number between Co01A (pressed), Co01B (non-

pressed), using NaPF6 and NaClO4.

From the Figure 13 we can observe that, with

the same electrolyte, Co01 layer present better

cycling performance, when the electrodes are

pressed. In fact the pressed electrode shows a

specific capacity of 89.9 mA/h (charge C/10)

and 87 mA/h (discharge C/10), with a

capacity retention of 105.5% at C/5 rate,

characterized by an excellent reversibility.

Paying attention to capacity retention, the

high value could be due to wettability of the

electrode and to the change of active material

structure during charge/discharge cycles. A

probable capacity loss is due to the presence

of Na2CO3 impurities. We can find

comparable capacity value in literature

although the specific theoretical capacity is

much higher (235 mAh/g). We must

emphasize the fact that this type of battery is

not able to cycle at 1C rate with acceptable

capacity values. In the third experiment,

reported in Figure 13, an higher potential

window was used, observing an increase in

specific capacity value, accompanied by a

loss of reversibility.

In the Figure 14, rate capabilities of Co11

(Na-CMC/Sample 1) non pressed layers are

2,0 2,2 2,4 2,6 2,8 3,0 3,2 3,4 3,6 3,8-300

-200

-100

0

100

200

300

400

500

600

dQ

/dE

(m

Ah

/gV

)

E (V vs Na+/Na)

1st

cycle C/10

2nd

cycle C/10

3rd

cycle C/10

9

depicted. Charge/discharge cycles have been

carried out within the potential window 2-

3.8V, comparing NaPF6 and NaClO4

electrolytes. The experiment was conducted

with two different type of protocol. The

Co01B/NaPF6/Na cell has been cycled at five

different rates: 5 cycle C/10, 5 cycle C/5, 5

cycle C/2, 5 cycle 1C and 5 cycle 2C. In the

other side, the Co01B/NaClO4/Na cell has

been cycled at 5 cycle at C/10, 5 cycles at

C/5, 5 cycles at C/2, 5 cycles at 1C, 5 cycles

at 2C and then again 5 cycles at C/10, 5

cycles at C/5, 5 cycles at 1C and finally again

5 cycles at C/5. All the potentials are given

vs. Na+/Na. From both experiment it emerges

that this type of active material does not work

with Na-CMC as binder. The cells have too

low capacity, this type of binder probably

provide stronger interaction than the other,

providing insulating properties to the layers.


number of Co11B/NaPF6/Na and Co11B/NaClO4/Na

cells.

In the Figure 15 the rate capabilities of

Co21A (PAA/Sample1) layers in different

electrochemical environments are depicted.


number of Co21A/NaPF6/Na and Co21A/NaClO4/Na

cells.

Charge/discharge cycles have been carried out

within two different potential window (2-4V

and 2-4.2 V), comparing NaPF6 and NaClO4

electrolytes. The cells have been cycled with

the same protocol, at 5 cycle at C/10, 5 cycles

at C/5, 5 cycles at C/2, 5 cycles at 1C, 5

cycles at 2C and then again 5 cycles at C/10,

5 cycles at C/5, 5 cycles at 1C and finally

again 5 cycles at C/5. All the potentials are

given vs. Na+/Na. The best electrochemical

performance has been obtained for the

Co21A/NaPF6/Na cell using a 2-4V potential

window, although it present a low

reversibility. The electrode shows a specific

capacity of 128.9 mA/h (charge C/10) and

103 mA/h (discharge C/10), with a capacity

retention of 112.5% at C/5 rate. This type of

result is probably due to the electrode loading.

In fact this electrode present 1.80 mg as

weight, compared to 3.17 mg and 2.98 mg of

the others two.

Electrodes, obtained from Sample 2 powder,

have been subjected to test with different

binder, electrolyte and pressing.


number of Co02A/B layers with NaPF6 and NaClO4.

Galvanostatic charge/discharge cycles,

differential analysis of galvanostatic cycles,

cyclic voltammetry and Electrochemical

Impedance Spectroscopy have been

conducted. In the Figure 16 the rate

capabilities of Co02B (PVdF-Sample2) layers

in different electrochemical environment are

depicted. Charge/discharge cycles have been

carried out within the potential window 2-3.8

10

V, comparing NaPF6 and NaClO4 electrolytes,

for pressed and non-pressed electrodes. The

experiment was conducted with two different

type of protocol. The non-pressed electrodes

has been cycled at six different rates: 5 cycle

C/10, 5 cycle C/5, 5 cycle C/2, 5 cycle 1C, 5

cycle 2C and finally again C/5 for 30 cycles.

On the other side, the pressed electrodes has






vs. Na+/Na. The best electrochemical

behaviour is shown by the unpressed

electrode, using NaClO4 as electrolyte, that

presents a specific capacity of 98 mA/h

(charge C/10) and 94 mA/h (discharge C/10),

with a capacity retention of 101% at C/5 rate.

In the Figure 17 the rate capabilities of Co12

(Na-CMC-Sample 2) layers in different


Figure 17: Specific capacity vs cycle number of

Co12B/NaPF6/Na.

The experiment was conducted with two

different type of protocol. The first cell has

been cycled at five different rates: 5 cycle

C/10, 5 cycle C/5, 5 cycle C/2, 5 cycle 1C and

5 cycle 2C. In the other side, the other cell has






vs. Na+/Na. Also in this case, from both

experiment it emerges that this type of active

material does not work with Na-CMC as

binder. The cells have too low capacity.

In the Figure 18 the rate capabilities of Co22

(PAA-Sample2) layers in different



number of Co22A with NaPF6 and NaClO4.

Charge/discharge cycles have been carried out

within 2-4V potential window, comparing

NaPF6 and NaClO4 electrolytes. The cells

have been cycled with the same protocol, at 5

cycle at C/10, 5 cycles at C/5, 5 cycles at C/2,

5 cycles at 1C, 5 cycles at 2C and then again

5 cycles at C/10, 5 cycles at C/5, 5 cycles at

1C and finally again cycled at C/5. All the

potentials are given vs. Na+/Na. The best

electrochemical performance has been

acquired for the Co22A/NaPF6/Na cell,

although it presents a low reversibility. The

electrode shows a specific capacity of 123

mA/h (charge C/10) and 104 mA/h (discharge

C/10), with a capacity retention of 96% at C/5

rate.

Electrodes, obtained from Sample 3 powder,

have been subjected to test only with NaPF6

electrolyte, using PVdF as binder.

Galvanostatic charge/discharge cycles have

been conducted, obtaining very low specific

capacity values. In the Figure 19 the rate

capabilities of Co03 (PVdF-Sample 3) layer

are depicted. The cell has been cycled at five

different rates: 5 cycle C/10, 5 cycle C/5, 5

cycle C/2, 5 cycle 1C and 5 cycle 2C. All the

potentials are given vs. Na+/Na. Only one

11

experiment has been conducted with this

sample because it shows too low capacity

values. It could be due to the different phase

presented by this type of stoichiometry, and

by the low amount of sodium in the starting

active material of the electrode.

Figure 19: Specific capacity vs cycle number of

Co03B/NaPF6/Na cell.

Observing that non-conclusive results have

been obtained with the differential analysis,

cyclic voltammetries have been done. Cyclic

voltagram curves of Na/NaPF6/Co01B and

Na/NaPF6/Co02B cells at the scan rate of

20μV/s are shown in Figure 20 and 21.

Figure 20:Cyclic voltagram for Co01B/NaPF6/Na cell

between 2-3.8 V.

Figure 21 Cyclic voltagram for Co02B/NaPF6/Na cell

between 2-4.2 V.

During the first cycle four cathodic peaks

were found at 2.9V, 2.95V, 3.3V and 3.7V. In

the second cycle, there are eight distinct

cathodic peaks at 2.35 V, 2.43 V, 2.55 V, 2.6

V, 2.7 V , 2.9 V, 3.25 V and 3.7 V while we

can observe eight corresponding anodic

peaks. The strange shape of voltagram shown

in Figure 21, in the 4-4.2V region, could be

due to an overpotential. From further studies

about structure change conducted by J.J.Ding

et Al. Errore. Il segnalibro non è definito.

, we can

understand that sodium

intercalation/deintercalation into/from

lamellar structure should be responsible for

the changes of c-lattice parameter. The c-

lattice parameter become larger with

decreasing sodium content. It is believe that

the expansion in the c-axis direction should be

attributed to increase in electrostatic repulsion

from the negatively charged oxygen

interactions of [CoO2] layers with the removal

of sodium. It can be estimated that Na

intercalation and deintercalation leads to 4%

contraction and expansion along c- axis. The

variation of the interatomic distance is closely

related to the change in the valence state of

the transition metal. In this direction we can

ipotize a phase change during

charge/discharge cycles of cell.

To better understand the mechanism of the

processes during cycling, changing the

potential as a linear function of time, every 20

mV impedance spectra have been acquired in

the frequency region range from 101kHz to

4.9 mHz. This type of analysis has been

conducted with PVdF layers of sample 1 and

2, using NaPF6 1M in EC:DMC=1:1 as

electrolyte. For every cell the same protocol

has been used: OCV (10000s), Linear Sweep

0 5 10 15 20 25

0

10

20

30

40

50

Unpressed

80:10:10=Na0.28

CoO2:PVdF:SC65

EC:DMC=1:1 NaPF6 1M

Sp

ecific

Ca

pa

city

Cycle Number

Charge

Discharge

C/10

C/5

C/2,1C,2C

12

Voltammetry, Staircase Potentio


(2V4V) and Staircase Potentio


(4V2V);

Figure 22 and 23 shows Nyquist plots of

Co01 and Co02 layers at T=25°C. The aim of

the experiment is to know how the variations

of the electronic resistance are related to the

quantity of sodium in the lattice.

The preliminary electrochemical impedance

spectroscopy study of these samples clearly

showed a strong dependence of the shape of

their EIS response spectra upon the working

electrode potential, suggesting the existence

of a potential region in which the electronic

conductivity of the material is the limiting

factor in controlling the electrochemical

process. In the figures, it appears that the

evolution of the spectra may be described as

associated with all the physical phenomena

that typically characterize a charge transfer at

passivated cathode materials , that is:

(i) a high-frequency region (>1 kHz)

characteristic of a SEI passivating layer;

(ii) an intermediate-frequency region

(between 10 Hz and 1 kHz) characteristic of a

charge transfer process;

(iii) a low-frequency region associated

with the electronic properties of the material;

(iv) the very low frequency region of the

ionic diffusion.

The data obtained from these experiment

show different tendencies. In the high

frequency region we can recognize the

increase of resistance related to the formation

of passivation layer and the gradual

decomposition of electrolyte. In the

intermediate frequency region, there is a

semicircle related to the charge transfer

between electrode and electrolyte resistance.

Finally there is a large arc that could be

associated to two different factors. In fact, the

high electronic resistance covers the diffusive

region, preventing us to recognize it.

Figure 22: Nyquist plots of Co01A oxidation and

reduction.

13

Figure 23: Nyquist plots of Co02A oxidation and

reduction.

This phenomenon could be explained by

semi-conductor behaviour of the material, and

the change with E of the diameter of the low-

frequency arc may be ascribed to the complex

structural changes which occur in the

structure of the active material as Na is

(de)intercalated. For the ‘similar’ Li

intercalation cathodes, this behaviour has

been explained by a band electronic theory 8,

which stated that AxMX2 compounds are

divided in two categories in base on the RM-M

distance in the lattice. When RM-M is higher

than a critical value, the system assume an

insulating behaviour, or vice versa a

conductor behaviour. Increasing the RM-M,

wavefunction overlap decrease, producing

band separation and electron localization. In

the extreme case of Mott Insulators with

localized electrons, the conduction occurs

only through an hopping mechanisms of few

electrons that can jump when subjected to

thermic excitation or statistic deviation. We

can recognize this type of semi-conductor

behaviour in our Na-based samples, even if

the dependency of resistance on the changes

of potential and structure is hard to rationale

because of the complexity of phases involved

in NaxCoO2 (de)intercalation processes.

Further studies could be done by in situ X-ray

diffraction studies of the correlation between

the electronic resistance (i.e., charge

transport) and the structural properties (i.e.,

cell parameters) during the stages of sodium

deintercalation from NaCoO2, using an

electrochemical cell that permits in situ

measurements of ac-impedance dispersions

and X-ray diffraction spectra.

Conclusion and future developments

In this research work, a preliminary study of

NaxCoO2 , as cathodic material for sodium-

ion batteries , has been conducted. The

research was focused on the optimization of

the system, monitoring the effect of sodium

carboxymethyl cellulose (Na-CMC) and

Polyacrylic acid (PAA) as binder, and in the

same time the influence of different

electrolytes (NaPF6 and NaClO4).

From the data obtained, Na-CMC has shown

negative effect in all experiment. Testing

electrodes with galvanostatic

charge/discharge cycles at different C-rate,

CMC presented very low specific capacity, in

the range between 8-25 mAh/g. These values

result to be unacceptable for a battery device.

For what concern PAA as binder, it result to

be a green valid substitute of PVdF. In fact it

present good performances in particular

conditions (low values of mass loading). For

these reasons, further studies should be done

to test the effect of the layer thickness. An

optimization of electrode preparation could be

done, monitoring the capacity increase as a

function of decrease in mass loading.

On the other hand, no significant differences

were observed, using NaClO4 or NaPF6 as

the electrolyte salt. The electrochemical

performance, probably, depend on the

electrolyte solvent used. Further studies

should be done using EC:PC as electrolyte

solvent.

In regard on the active materials synthetized,

it was possible to observe a strong

dependence of charge/discharge behaviour on

the different stoichiometry obtained. This is

related to different phases assumed by the

layered oxide, influencing the

intercalation/deintercalation process and

strongly affecting the reversibility of

charge/discharge process. The Sample 1(mix

phase γ + α’) obtained by “Ball-Milling and

post firing”, result electrochemically active,

but a loss of specific capacity must be

emphasized. It is probably due by the

presence of impurities and two different

phases. Only one experiment has been

14

conducted with Sample 3 (β phase), obtained

by sol-gel method, because it shows too low

capacity values. It could be due to the not

suitable phase presented by this type of

stoichiometry, and to the low amount of

sodium in the starting active material. Further

studies could be done for the optimization of

the synthetic condition of sol gel method,

with the aim to obtain higher sodium content.

The Sample 2 (pure γ phase), obtained by

simple solid state reaction, result to be the

best choice. It present high stability,

reversibility and good specific capacities, so it

represents a good starting point for future

material and electrode optimization.

References

1. Tarascon JM, Armand M., Nature, 414

(2001) 359-367.

2. Yaksic A, Tilton JE., Resource Policy, 34

(2009);185.

3 Ellis B.L., Nazar L.F. , Science, 16 (2012)

168–169.

4 Palomares V., Casas-Cabanas M., Castillo-

Martınez E., Han M H, Rojo T., Energy

Environ. Sci., 6 (2013) 2312.

5 Palomares V., Serras P., Villaluenga I.,

Hueso K.B., Carretero-Gonzalez J., Rojo T.,

Energy Enviro, Sci.,5 (2012) 5884.

6 Amrtha Bhide, K. Hariharan, Solid State

Ionics,192(2011)369.

7 B.J. Hwang, R. Santhanam, D.G. Liu,

Journal of Power Sources,102(2001) 326.

8 J Molenda, A Stokłosa, T Ba̧k , Solid State

Ionics, 36-1/2, (1989)53-58.

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