Kosuke Nakamoto, Ayuko Kitajou*, Masato Ito* and Shigeto Okada* (IGSES, Kyushu University, *IMCE, Kyushu University)

Effect of Concentrated Electrolyte on High Voltage Aqueous Sodium-ion Battery　

Oct 6. (Thu) A01-0134

Introduction

This study Advantage

/disadvantage

　　　 Aqueous sodium-ion Non-inflammability, Cost, Power! Energy density

Post LIB Aqueous lithium-ion Sodium-ion

Electrolyte Aqueous Organic Solid

Commercial Nickel metal hydride Lithium-ion Sodium sulfur

Commercialized secondary batteries and post lithium-ion batteries

Components Lithium-ion Aqueous sodium-ion

Electrolyte solvent Organic WaterElectrolyte salt LiPF6, LiTFSI Na2SO4, NaClO4

Separator Polypropylene porous Nonwoven fabric

Anode current collector Cu Fe

Cathode active material Co, Ni Fe, Mn

Electrode slurry thickness ~ 100 µm ~ 20,000 µm

Primary requirement to the large scale energy storage system is the cost (Wh/$), rather than specific energy density (Wh/kg).

Hybrid capacitor (Aquion Energy)

Operation voltage ~ 4 V ~ 2 V

Electrode materials for aqueous lithium-ion battery

Very recent aqueous lithium-ion battery with highly concentrated electrolyterealized high voltage operation exceeding 1.23 V theoretical stability window.

-3

-2

-1

0

1

2 5

4

3

2

1

0

E (V

) vs.

Na/

Na+

E (V

) vs. Li/Li +E

(V) v

s. N

HE

E

(V) vs. A

g/AgC

l

E = 1.23 – 0.059pH O2↑

H2↑ E = – 0.059pH

Theoretical stability window of water

0 7 14 pH

LiNi0.5Mn1.5O4

Li4Ti5O12

LiTi2(PO4)3

LiCoO2

TiO2

LiMn2O4

4

3

2

1

0 -3

-2

-1

0

1

Extended practical stability window of aqueous lithium-ion electrolyte

Mo6S8

Polyimide

LiFePO4

VO2

LiV3O8

LiMn2O4 LiNi0.5Mn1.5O4

Mo6S8

Cathode Anode ElectrolyteVoltag

e /V

Discharge capacity /mAh g-1 Ref.

LiMn2O4 VO2 5 mol/l LiNO3 aq. 1.5 50 (electrodes) 1

LiNi0.81Co0.19O2 LiV3O8 1 mol/l Li2SO4 aq. 0.9 20 (electrodes) 2

LiMn2O4 LiTi2(PO4)3 1 mol/l Li2SO4 aq. 1.5 40 (electrodes) 3

LiFePO4 LiTi2(PO4)3 1 mol/l Li2SO4 aq. 0.9 55 (electrodes) 4

LiCoO2 Polyimide 5 mol/l LiNO3 aq. 1.1 71 (electrodes) 5

LiMn2O4 Mo6S8 21 mol/kg LiTFSI aq. 2.0 47 (electrodes) 6

LiMn2O4 TiO2

21 mol/kg LiTFSI + 7 mol/kg LiOTf aq.

2.1 48 (electrodes) 7

LiCoO2 Li4Ti5O12

20 mol/kg LiTFSI + 8 mol/kg LiBETI aq.

2.4 55 (electrodes)8

LiNi0.5Mn1.5O4 3.0 30 (electrodes)

Estimated cost of recent aqueous lithium-ion chemistries is still high.

Aqueous lithium-ion batteries

[1] W. Li, et al., Science, 264 (1994) 1115. [2] J. Köhler, et al., Electrochim. Acta, 46 (2000) 59.[3] J.Y. Luo, et al., Adv. Funct. Mater., 17 (2007) 3877. [4] J. Luo, et al., Nat. Chem., 2 (2010) 76 [5] H. Qin, et al., J. Power Sources, 249 (2014) 367. [6] L. Suo, et al., Science, 350 (2015) 938.[7] L. Suo, et al., Angew. Chemie., 85287 (2016) 7136. [8] Y. Yamada, et al., Nat. Energy, 1 (2016) 16129.

Cathode Anode ElectrolyteVoltage

/V Discharge capacity

/mAh g-1 Ref.

λ-MnO2 Active Carbon 1 mol/l Na2SO4 aq. 1.2 50 (electrolyte) 9

NaVPO4F Polyimide 5 mol/l NaNO3 aq. 1.1 40 (electrodes) 5

Na3V2O(PO4)2F NaTi2(PO4)3 *10 mol/l NaClO4 aq. 1.4 40 (cathode) 10

Na4Mn9O18 NaTi2(PO4)3 1 mol/l Na2SO4 aq. 1.0 100 (anode) 11

Na2FeP2O7 NaTi2(PO4)3 4 mol/l NaClO4 aq. 0.9 48 (cathode) 12

Na2Ni[Fe(CN)6] NaTi2(PO4)3 1 mol/l Na2SO4 aq. 1.3 100 (anode) 13

Na2Cu[Fe(CN)6] NaTi2(PO4)3 1 mol/l Na2SO4 aq. 1.4 102 (anode) 14

NaCr[Mn(CN)6] Na2Mn[Mn(CN)6] *10 mol/l NaClO4 aq. 1.0 28 (electrodes) 15

Na2Co[Fe(CN)6] NaTi2(PO4)3 1 mol/l Na2SO4 aq. 1.6 120 (cathode) 16

NaFe[Fe(CN)6] (Active Carbon) 1 mol/l Na2SO4 aq. (> 1.5) 60 (cathode) 17

We focus on rocking-chair aqueous sodium-ion batteries (not capacitors). Active materials should be low cost & yield high voltage output to maximize the cost performance index.

Aqueous sodium-ion batteries

[9] J.F. Whitacre, et al., J. Power Sources, 213 (2012) 255. [10] P.R. Kumar, et al., Mater. Chem. A, 3 (2015) 6271. [11] W. Wu, et al., J. Electrochem. Soc., 162 (2015) A803. [12] K. Nakamoto, et al., J. Power Sources, 327 (2016) 327. [13] X. Wu, et al., Electrochem. Commun., 31 (2013) 145. [14] X. Wu, et al., ChemSusChem, 7 (2014) 407. [15] M. Pasta, et al., Nat. Commun., 5 (2014) 3007. [16] X. Wu, et al., ChemNanoMat., 1 (2015) 188. [17] X. Wu, et al., Nano Energy, 13 (2015) 117.

*10 M NaClO4 aq. ≒ 17 m NaClO4 aq.

M Ni Cu Co Fe

Initial C/D capacity /mAh g-1

74/65 71/59 142/128 102/122

E/V vs. Ag/AgCl 0.5 0.6 0.9

0.4 1.0 0.2

Electrolyte 1 mol/l Na2SO4 aq. 1 mol/l Na2SO4 aq. 1 mol/l Na2SO4 aq. 1 mol/l Na2SO4 aq.

Upper redox Inactive Inactive [Fe(CN)6]4-/3- Fe2+/3+

Lower redox [Fe(CN)6]4-/3- [Fe(CN)6]4-/3- Co2+/3+ [Fe(CN)6]4-/3-

Weak point

Low capacity Expensive

Low capacity Expensive Expensive Low initial capacity

Air-stability

Sodium metal hexacyanoferrates Na2M[Fe(CN)6], M = Ni, Cu, Fe, Co, Mn

0.5

1.0

0.0E[V

] vs.

Ag/

AgC

l After Wu [13] After Wu [14] After Wu [16] After Wu [17]

Na2Mn[Fe(CN)6] is low cost and was reported high voltage operation in non-aqueous electrolyte but has never been realized in aqueous electrolyte.

Capacity [mAh/g] 1501500 150 150

O2↑

Capacity [mAh/g] Capacity [mAh/g] Capacity [mAh/g]

Sodium metal hexacyanoferrates Na2M[Fe(CN)6], M = Ni, Cu, Fe, Co, Mn

M Mn (in Non-aq.) Co (in Aq.) Fe (in Aq.)

Morph.

Property Round particle with defects

Cubic without defects

Cubic without defects

After Wu [16] After Wu [17]

Na2Mn[Fe(CN)6] is attractive because of 2 redox-active sites. However, the round particles with defects may dissolve and cannot suppress water decomposition in diluted electrolyte.

E [V

] vs.

Na/

Na+

After Song [18]

3.5

4.0

3.0

→Other methods should be considered as suppressing dissolution and water decomposition.

After Song [18] After Wu [16] After Wu [17]

Capacity [mAh/g]Capacity [mAh/g]

0.5

1.0

0.0

0.5

1.0

0.0

E [V

] vs. Ag/A

gCl

O2↑

E [V

] vs.

Ag/

AgC

l

1500 50 100 150 0 50 100 0 50 100

Capacity [mAh/g]

Approx. saturated concentration [mol/kg]

Cation Weak points Ref.

Li+ Na+

Anion

Cl- 18 6 Anodic oxidation & gas evolution - OH- 5 32 Prussian blue decomposition in alkali 19 NO3

- 13 10 Ti based NASICON corrosion 11 SO4

2- 3 2 Low solubility - N(SO2CF3)2

- 21 9 High cost TFSI- 6

SO2CF3- 22 9 High cost OTf- 7

N(SO2C2F5)2- ND ND High cost BETI- 8

ClO4- 6 17 Explosive -

Highly concentrated NaClO4 aqueous electrolyte will suppress dissolution or side reaction and support high voltage operation.

Electrolyte selection for aqueous sodium-ion battery

[6] L. Suo, et al., Science, 350 (2015) 938. [7] L. Suo, et al., Angew. Chemie., 85287 (2016) 7136. [8] Y. Yamada, et al., Nat. Energy, 1 (2016) 16129. [11] W. Wu, et al., J. Electrochem. Soc., 162 (2015) A803. [19] R. Koncki, et al., Anal. Chem., 70 (1998) 2544.　

17

Cathode Electrolyte Anode Na2Mn[Fe(CN)6]

(NMHCF) 17 mol/kg NaClO4 aq. NaTi2(PO4)3 (NTP)

Experiment

Synthesis of NaxMn[Fe(CN)6]y・zH2O

Stir (in H2O + EtOH) @ RT

Na4[Fe(CN)6] aq.

Green blue NaxMn[Fe(CN)6]y・zH2O

Filter & Wash　(H2O + EtOH)

NaCl aq.

Light green precipitation

MnCl2 aq.

Vacuum dry @100 ℃ (over night)

[18] J. Song, et al., J. Am. Chem. Soc., 137 (2015) 2658.

Conventional co-precipitation method [18]

Green blue NaxMn[Fe(CN)6]y・zH2O

Morphological & structural properties of NMHCF

(100

)

(110

)

(200

)

(210

) (2

11)

(220

)

(310

) (3

00)

Na2MnFe(CN)6 Pm-3m Cubic

ICSD #75-4637

2θ/degree

Inte

nsity

/a. u

.

200 nm

NMHCF powder was identified as cubic with Pm-3m diffraction pattern consistent with Na2Mn[Fe(CN)6]. Approx. 200 nm sized round particles not nano-cubes were observed.

XRD SEM

Na Mn Fe H2O 1.24 1 0.81 1.28

By ICP-AES & TGA

As-prepared NMHCF

605040302010

[20] Y. Morimoto, et al., Energies, 8 (2015) 9486.

[20]

Na1.24Mn[Fe(CN)6]0.81·1.28H2O

(AB : Acetylene black, PTFE ： Polytetrafluoroethylene)

WE Ti mesh

CE Ti mesh

WE pellet (~ 2 mg)

CE pellet (~ 3 mg)

Ion-type cell Na2Mn[Fe(CN)6] + NaTi2(PO4)3 ⇄ Mn[Fe(CN)6] + Na3Ti2(PO4)3

Electrochemical cell

Beaker-type cell

RE

Na2MnFe(CN)6//NaTi2(PO4)3

Working electrode (WE)

Electrolyte (EL)

Reference electrode (RE)

Counter electrode (CE)

Na2Mn[Fe(CN)6]：AB：PTFE ＝70：25：5 (wt%)

1 or 17 mol/kg

NaClO4 aq.Silver-silver chloride　

(Ag/AgCl) in sat. KCl aq.NaTi2(PO4)3：AB：PTFE

＝70：25：5 (wt%)

EL

Prussian blue analogues [21] Na2Mn[Fe(CN)6] NMHCF Sodium manganese hexacyanoferrate

NASICON-type NaTi2(PO4)3 NTP

Sodium titanium phosphate

[21] T. Tojo, et al., Electrochem. Acta, 207 (2016) 22.

Result & discussion

1 & 17 mol/kg NaClO4 aqueous electrolyte had 1.9 V & 2.7 V practical stability windows, respectively. The windows were larger than 1.23 V theoretical stability window of water.

Cyclic voltammetry on Ti current collector & active materials

43214321

Voltage/V vs. Ag/AgCl

Voltage/V vs. Na/Na+

Cur

rent

/mA

0.5

-0.5

0.0

Cur

rent

den

sity

/A g

-1

0.5

-0.5

0.0

-2 -1 0 1 2

-2

-1

0

1

2

-2

-1

0

1

2

-2 -1 0 1 2

NMHCF

NTP

NMHCF

NTP

1 mol/kg NaClO4 aq.

17 mol/kg NaClO4 aq.

1 mol/kg NaClO4 aq.

17 mol/kg NaClO4 aq.

Theoretical 1.23 V pH = 7

Practical 1.9 V

Practical 2.7 V

O2↑

H2↑

H2↑

O2↑ O2

↑

H2↑

Theoretical 1.23 V pH = 6

Na1.24Mn[Fe(CN)6]0.81·1.28H2O & NaTi2(PO4)3 half cells

17 mol/kg electrolyte suppressed both of O2/H2 evolution and supported the reversible operation. In contrast, 1 mol/kg electrolyte does not allow cycling.

Volta

ge/V

vs.

Ag/

AgC

l

Specific capacity/mAh g-1-anode

NMHCF

Voltage/V vs. N

a/Na

+

Specific capacity/mAh g-1-anode

Specific capacity/mAh g-1-cathode Specific capacity/mAh g-1-cathode

NTP NTP

NMHCF 4

3

2

1

1.3 V cut1.2 V cut

1 mol/kg NaClO4 aq. 2.0 mA cm-2

17 mol/kg NaClO4 aq. 2.0 mA cm-2

150100500

150100500

1st 2nd

-2

-1

0

1

24003002001000

1st 2nd

150100500

40302010

Ex-situ XRD patterns of NMHCF cathode in charge/discharge process

403020102θ/degree

1.5 1.0 0.5 0.0

300

250

200

150

100

50

0

-50

Voltage/V vs. Ag/AgCl

Cap

acity

/mA

h g-

1

2θ/degree 1.5 1.0 0.5 0.0

400

300

200

100

0

Cap

acity

/mA

h g-

1

Voltage/V vs. Ag/AgCl

1 mol/kg NaClO4 aq. NMHCF

XRD intensities of NMHCF in 1 mol/kg electrolyte were weakened at higher voltage range, and some small peaks were observed again at 0.2 V indicating some deposition.

Inte

nsity

/a. u

.

Inte

nsity

/a. u

.

0.2 V

0.7 V

1.3 V

1.2 V

0.9 V

OCV

0.2 V

0.7 V

1.3 V

0.9 V

OCV

0.2 V

0.7 V

1.3 V

1.2 V

0.9 V

OCV

0.2 V

0.7 V

1.3 V

0.9 V

OCV

Deposition

17 mol/kg NaClO4 aq. NMHCF

NMHCF cathode deterioration in 1 mol/kg NaClO4 (color, pH, metal ion ICP)

1.51.0

0.50.0

300

250

200

150

100500

-50

Volta

ge/V

vs.

Ag/

AgC

l

Capacity/mAh g-1

17 mol/kg NaClO4 aq.

1.51.0

0.50.0

400

300

200

1000

Capacity/mAh g-1Vo

ltage

/V v

s. A

g/A

gCl

1 mol/kg NaClO4 aq.

Voltage/V Prep. Ini. 0.9 1.3 0.7 0.2

pH 6 5 5 0.5 0.8 0.8

Prep. Ini. 0.9 1.2 1.3 0.7 0.2

7 6 4 2 2 2 2

Fe/mol% 0.0 0.0 0.0 0.0 0.0 0.0

Mn/mol% 0.0 0.0 0.0 0.0 0.0 0.0

Ti/mol% 0.0 0.0 0.0 0.0 0.0 0.0

0.0 4.0 6.8 28 27 26 15

0.0 7.3 8.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0

In 1 mol/kg electrolyte, NMHCF dissolved as [Fe(CN)6]4- at lower, [Fe(CN)6]3- at higher voltage, and MnO precipitating accompanied with Mn2+ dissolution on the cathode and OH- generated on NTP.

[Fe(CN)6]4-

dissolution

[Fe(CN)6]3-

dissolution

MnO precipitation

[Fe(CN)6]α- deposition

H3O+ extraction Partially O2↑

[Fe(CN)6]4-

dissolution [Fe(CN)6]3- dissolution

MnO precipitation No dissolution or no precipitation

Strong acidic Mild acidic

deposition

Deterioration process in 1 mol/kg NaClO4 aq.

Volta

ge/V

vs.

Ag/

AgC

l

Specific capacity/mAh g-1-anode

NMHCF

Voltage/V vs. N

a/Na

+

Specific capacity/mAh g-1-anode

Specific capacity/mAh g-1-cathode Specific capacity/mAh g-1-cathode

NTP NTP

NMHCF 4

3

2

1

1.3 V cut1.2 V cut

1 mol/kg NaClO4 aq. 2.0 mA cm-2

17 mol/kg NaClO4 aq. 2.0 mA cm-2

150100500

150100500

1st 2nd

-2

-1

0

1

24003002001000

1st 2nd

150100500

Deterioration process in 1 mol/kg NaClO4 aq.

Water decomposition 2H2O + 2e- → H2↑ + 2OH-

Cathode decomposition Na2-xMn[Fe(CN)6] + 2NaOH → Na4-x[Fe(CN)6] + MnO↓ + H2O

2H2O + 2e- → H2↑ + 2OH-

100

50

010080604020

100

50

010080604020

Electrolyte concentration & rate dependences on cyclability of NMHCF cathode

Better cycle performances of NMHCF cathode were obtained in more concentrated electrolytes and at larger current densities.

Cycle number

Dis

char

ge c

apac

ity/m

Ah

g-1

Fe2+/Fe3+ + Mn2+/Mn3+

Fe2+/Fe3+

Dis

char

ge c

apac

ity re

tent

ion/

%

Cycle number

Concentration dependence at const. 2.0 mA cm-2

Rate dependence in const.17 mol/kg electrolyte

17 mol/kg

14 mol/kg

10 mol/kg

7 mol/kg

1 mol/kg

5.0 mA cm-2

2.0 mA cm-2

Binding energy/eV Binding energy/eV730 720 710 700

NMHCF cathode operation (structural & metal ion valence changes)

1.5 1.0 0.5 0.0

300

250

200

150

100

50

0

-50

Voltage/V vs. Ag/AgCl

Cap

acity

/mA

h g-

1

XPS of Fe XPS of Mn XRDC/D profile of NMHCF in 17 mol/kg NaClO4 aq.

NMHCF cathode worked with Fe2+/Fe3+ redox, partial Mn2+/Mn3+ redox and Na ion extraction/insertion in highly concentrated 17 mol/kg NaClO4 aq.

181716660 650 6402θ/degree

Calc. valence

state

Fe2+

/Mn2+

Fe3+

/Mn2+

Fe3+

/Mn2.43+

Fe3+

/Mn2+

Fe2+

/Mn2+

Calc. Na

amount

1.24

0.42

0

0.42

1.24

monoclinic

monoclinic

cubic

cubic

tetragonal

100

50

020151050

1.00.50.02.5

2.0

1.5

1.0

0.5

0.0150100500

1.51.00.50.0

1st 2nd

x in Na1.24-xMn[Fe(CN)6]0.81·1.28H2O

Capacity/mAh g-1–cathode

Volta

ge/V

vs.

NaT

i 2(P

O4)

3

2.0 mA cm-2

0.5 ~ 2.0 V

100806040200

5040302010

Ret

entio

n/%

Cycle number

Current density/mA cm-2D

isch

arge

cap

acity

/mA

h g-

1 cat

hode

Cathode: 20 mg cm-2, 200 µm

Anode: 30 mg cm-2, 200 µm

High voltage aqueous sodium-ion battery of NMHCF/17 m NaClO4 aq./NTP

Na1.24Mn[Fe(CN)6]0.81/17 mol/kg NaClO4 aq./NaTi2(PO4)3 operates at 1.3, 1.5 & 1.8 V. The cell exhibited initial discharge capacity of 117 mAh g-1, good cycle & rate performances.

Current density/A g-1-cathode

0.5 ~ 2.0 V

Conclusions

Conclusion

Electrodes selection

Na2MnFe(CN)6 cathode & NaTi2(PO4)3 anode were selected because of high voltage combination and low cost of the materials.

Electrolyte selection

Low cost NaClO4 salt can realize highly concentrated aqueous electrolyte, which suppresses water decomposition.

Effect of concentrated electrolyte

Concentrated 17 mol/kg electrolyte suppressed the water decomposition and dissolution of NMHCF cathode compared to diluted 1 mol/kg electrolyte.

Factor of cathode deterioration in 1 mol/kg electrolyte

Prussian blue analogue cathode was decomposed by hydroxide ion occurred on the anode because of the small practical stability window of 1 mol/kg electrolyte.

High voltage aqueous sodium-ion battery

Na1.24Mn[Fe(CN)6]0.81/17 mol/kg NaClO4 aq./NaTi2(PO4)3 operates over 1.2 V. The cell delivered initial discharge capacity of 117 mAh g-1, good cycle & rate performances.

Thank you for your attention

Acknowledgement

This research was financially supported by ESICB, Elements Strategy Initiative for Catalysts and Batteries

Project, MEXT, Japan.