quantitative analysis of gitt measurements of li-s...

Quantitative analysis of GITT measurements of Li-S batteries

James Dibden, Nina Meddings, Nuria Garcia-Araez, and John R. Owen

1

Acknowledgements to Oxis and EPSRC for EP/M50662X/1 - CASE studentship, EP/P019099/1- MESS project co-funded by Innovate UK.

2

time

Constant current pulse + relaxation step

-cu

rren

t

IRu

IRu

constant potential

after relaxation

ERELAX

time

-vo

lta

ge EPULSE

Cathode. thickness L

Li+ e-

cLi

depth

l 0 L

The Galvanostatic Intermittent Titration Technique - as applied to Li-ION

Li e Li Host

3

The Galvanostatic Intermittent Titration Technique - as applied to Li-S

time

Constant current pulse + relaxation step

- cu

rren

t

IRu

IRu

constant potential

after relaxation

ERELAX

time

- v

olt

ag

e

EPULSE

Li

e-

c(LiSn)

depth

0 L

Li+

Li+ S

Sn-

OHARA

nS ne S

GITT in Li-ion battery materials

4

Weppner and Huggins. J. Electrochem. Soc, 1977, 124, 1569-1578.

Chemical diffusion coefficient Equilibrium

voltage profile

224

PULSE

RELAX

E

ELD =

Thermodynamically enhanced diffusion

5 Weppner and Huggins. J. Electrochem. Soc, 1977, 124, 1569-1578.

Enhancement factor:

Enhancement of the chemical diffusion coefficient

ln

ln

Li

Li

y Fd a dE

d c RT d

GITT in Li-S cells ― complications

• Redox reactions in the liquid state

• Multiple polysulfide species (not fully identified)

• Polysulfide shuttle can cause self discharge

6

GITT in Li-S cells ― our approach

• Polysulfide shuttling avoided using a lithium selective membrane (Ohara)

• Equations derived for (complicated) solution redox reactions

Plan

• Use GITT in Li-S cells to obtain quantitative information of:

– Mass transport rate (diffusion coefficient)

– Reaction rate (relaxation rate)

– Composition-dependent activity coefficients

7

Aim

• Validate our approach with a model redox system

• Apply it to cells containing dissolved sulfur

• Apply it to cells containing dissolved polysulfides

• Apply it to Li-S cells

Validation

• Chemical diffusion coefficient determined by:

– Cyclic voltammetry

– Square-wave voltammetry

– Chronopotentiometry

– GITT

8

EtV2+ =

Model redox system:

Cell design

9

Results: Summary

10

0.01–2 mM EtV2+ in 0.1 M LiTFSI in Pyr14TFSI

Glassy carbon working electrode

Method D / cm2 s-1

Cyclic voltammetry 6.3 x 10-8

Chronopotentiometry 7.7 x 10-8

Square wave voltammetry 7.5 x 10-8

Cyclic voltammetry

11

System is electrochemically reversible (~ 63 mV)

DEtV2+ = 6.3 x 10-8 cm2 s-1

2 mM EtV2+ in 0.1 M LiTFSI in Pyr14TFSI

Chronopotentiometry

12


DEtV2+ = 7.7 x 10-8 cm2 s-1

Transition time:

Square wave voltammetry

13

0.01–2 mM EtV2+ in 0.1 M LiTFSI in Pyr14TFSI

DEtV2+ = 7.5 x 10-8 cm2 s-1

2 mM

GITT

14


Next: repeat the experiments with an smaller and thinner separator to decrease the electrolyte volume and thus increase cbulk.

30%

0.002

(0.6 mM)

mM 0 mV

surface

bulk RELAX

c

c E

224

PULSE

RELAX

E

ELD =

Cell design

15

Results: Summary

16

Method D / cm2 s-1

Cyclic voltammetry 2.2 x 10-6

Chronopotentiometry 2.4 x 10-6

GITT ≈2 x 10-6

5 mM EtV2+ in 1 M LiTFSI in DOL

Cyclic voltammetry

17


System is electrochemically reversible (~ 60 mV)

DEtV2+ = 2.2 x 10-6 cm2 s-1

Chronopotentiometry

18

DEtV2+ = 2.4 x 10-6 cm2 s-1

Glassy carbon C-coated Al foil

Transition time:


GITT (1)

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224

PULSE

RELAX

E

ELD =

Unrealistic variation of the chemical diffusion coefficient

GITT (2)

20

Equilibrium voltage profile in agreement with Nernst equation

Evolution of voltage change induced by pulses is unexpected


224

PULSE

RELAX

E

ELD =

EtV2+ + e- →EtV+

20 ln EtV

EtV

cRTE E

F c

GITT analysis

21

Fick’s first law: 0

0x

I cnFD

A x

Evolution of surface concentrations with time:

2 2 2

0 02x initial initial

EtV EtV EtV

I tc c c

AnF D

Assumption: EPULSE is proportional to c(x=0)

0 /bulk

I nFc

AL

Since

224

PULSE

RELAX

E

ELD =

2EtV e EtV

0 020x

EtV

I tc c x

AnF D

( 0)PULSEE k c x

and the proportionality constant is RELAX bulkE k c

It is concluded that: 2

PULSE RELAX

L tE E

D

And for t=:

GITT analysis

22

Fick’s first law: 0

0x

I cnFD

A x

Evolution of surface concentrations with time:

2 2

0 02x initial

EtV EtV

I tc c

AnF D

2EtV e EtV

0 02x

EtV

I tc

AnF D

Assumption: EPULSE is calculated with the Nernst equation

0 /bulk

I nFc

AL

2

0

0

0ln

x

EtVPULSE x

EtV

cRTE E

nF c

and E0 is obtained from the equilibrium voltage profile (2.44V)

taking into account:

GITT (3)

23


Pulse 1 Pulse 5 Pulse 15

The evolution of the voltage change induced by the pulse is in agreement with the Nernst equation

Conclusions

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• Theoretical framework to analyze GITT results of Li-S cells

• Validation of the evaluation of the diffusion coefficient by:

– Cyclic voltammetry

– Square wave voltammetry

– Chronopotentiometry

– GITT

• Next: GITT as diagnostic tool of Li-S cells

quantitative analysis of gitt measurements of li-s...

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