Polymer Electrolytes for High Energy Density Lithium Batteries

Ashoutosh Panday Scott Mullin

Nitash Balsara

Proposed Battery

anode (Li metal)

Li Li+ + e ­

e -

Li salt in

a hard solid electrolyte

Key advantages: •Non-flammable electrolyte

•Theoretical energy density of 300 Wh/kg.

•No dead weight and no reduction in potential as Li is depleted.

Work

cathode (active particles and carbon bound by polymer)

Li+ + e - + FePO4 LiFePO4

Fatal flaw in PEO

Repeated cycling led to the roughening of the Li surface and eventually to catastrophic dendrite growth.

PEO PEO PEO

Li

cathode

t=0 intermediate times dendrite short

high surface area Li

Solution: Make electrolyte hard and mechanically stopdendrites. How hard is hard?Modulus=109 Pa (Monroe, Newman, 2005)

Nanostructured Electrolyte

Channels ~10 nm

For ion conduction

Li

cath

od

e

Hard matrix

For mechanical support

Dendrite (1 µµµµm)

Decouple the mechanical and electrical properties of the electrolyte

Morphology-Conductivity Relation

Effective use of nanoscale objects

H2 H2

CH

C C

x

O

s-Bu C H yH2

First systematic study of the effect of molecular

weight of copolymer on conductivity.

Singh et al., Macromolecules, 2006

Conductivity versus MPEO C

ondu

ctivity [

S/c

m]

-4 8 10

-4 7 10

-4 6 10

-4 5 10

-4 4 10

-4 3 10

-4 2 10

-4 1 10

00 10

r=Li/EO=0.02

90C 100C 110C 120C

Normalize σ by

σmax =σpure PEOφPEO

4 4 4 4 52 10 4 10 6 10 8 10 1 10

M (g/mol)PEO

Normalized conductivity

0 2 104

4 104

6 104

8 104

1 105

90C 100C 110C 120C

2/3

r=0.02

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

values approaching

the theoretical

upper limit (0.67)

σmax =σpure PEOφPEO

N

σ/σ

σ/

σσσ /σ/σ

max

M [g/mol]PEO

Y Y

Trend holds for other r values

Effect of salt concentration on electrolyte conductivity

0.0E+00

2.0E-04

4.0E-04

6.0E-04

8.0E-04

1.0E-03

1.2E-03

1.4E-03

0 20000 40000 60000 80000 100000 120000

MW of EO Block

Co

nd

S/c

m

120

110

100

90

80

r=Li/EO=0.085

Improved mechanical properties

Mechanical properties 101010109999

G', G

" [Pa]

G',G

"[Pa

GG]

',G"[P

a]',G

"[Pa]

90 oC101010107777

SEO(36-25): r = 0.0SEO(36-25): r = 0.0SS 2EO(36-25): r = 0.02EO(36-25): r = 0.02101010105555SEO(36-25): r = 0.0SEO(36-25): r = 0.0SS 0EO(36-25): r = 0.00EO(36-25): r = 0.00PEO- 20KPEO- 20PP KEO- 20KEO- 20K

101010103333

101010101111

10101010----11111111 10111000 111100000000

ωωωω [rad/s][ s[[rad/ ]rad/s]rad/s]Nanostructured electrolytes have 1/3 the conductivity ofNanostructured electrolytes have 1/3 the conductivity of

PEO but larger shear modulus by several orders ofPEO but larger shear modulus by several orders of

magnitude.magnitude.

High Overpotential EliminatedV

olt

ag

e(V

)

4 7.5

Initial Cycling Data5

2 Overpotential = 3V at 50 uA/cm2

2.5

0 0

-2.5

-2

Cu

rren

t( µµ µµ

A)

Cycling Data with spring loaded electrodes

Overpotential = 0.035V at 200 uA/cm2

30 -5

0.04 20

-4 -7.5

Liu

0 7.5 15

Time (h)

Compressive stressV

olt

ag

e(V

) 0.02 10

0 0

-10 -0.02

Battaglia Li -20

-0.04

Li -30

0 5 10 15 20

Force transducer Time (h)

Cu

rren

t( µµ µµ

A)

Interfacing SEO Electrolyte withCathode

Cathode : FePO4 (HQ)

Solvent: NMP Compressive stress

RT Swollen Dry electrolyte electrolyte

Vacuum

T = 120 C

SEO electrolytes are hard elastic solids at room temperature. In order to “fill” the pores, a

compressive stress is applied after the electrolyte has been softened by:

a.) swelling with a solvent that swells both PS and PEO phases

b.) heating the electrolyte above Tg

Liu, Battaglia, Karim

Li/SPE/FePO4 (HQ) Cell

30

Anode = Li metal 3.5 20

Cathode = FePO4 (HQ)

10

3.45

0

Cu

rren

t( µµ µµ

A)

r = 0.067T = 80°Ci = 200 uA/cm2

3.4 -10

-20 3.35

SEO electrolyte is stable against both of the electrodes. Low IR -30

0 10 20 30 40 50 drop is maintained when

Li/SEO/Li symmetric cell is replaced

Time (h) by Li/SEO/FePO4.

Vo

ltag

e(V

)

Reviewer: Begin working on full cells.

Improving Active Material UtilizationCathode casting

1. Use SEO electrolyte as the binder for the casting cathodes on Al foil

2. Remove the porosity by calendaring

3. Solvent cast SEO electrolyte on the cathode

1 2 3

Morphology effects

maximum for both cylinders and lamellae = 1.0

Morphology Factor Included T=100C, r=0.085

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 20 40 60 80 100 120

Molecular Weight of EO Block (kg/mol)

S/S

ma

x

(dim

en

sio

nle

ss

)

cylinders

N

Y N

Morphology factor is 1/3 for cylinders and 2/3 for lamellae, corresponding to effective diffusivities

MW effect on Li distribution

Gomez et al.

Li

thic

knes

s

EO

thic

knes

s

Reviewer: Explain conductivity results.

Using simulations

results and field theory for interpretation.

1.2

0.8

0.4

0 ~ 800 measurements 4 4 4 4 5

0 2 10 4 10 6 10 8 10 1 10MW (g/mol)

Reviewers Comments and Action

Comments:

• Work with simulation groups (Smith) to better understand transport mechanism in the block copolymer materials

• Measure Li transference number to determine if the unusual increase in

conductivity is due to higher Li transport or higher anion transport. Focus on explaining increase in σσσσ with increasing MW of PEO

• If successful, abuse tolerance aspects of Li batteries will be enhanced • Begin work on full cells using Li metal and appropriate cathode materials

• Newman’s theory does not support that dry polymers will prevent dendrites.

May be discontinued or redirected to Li ion conducting glass R&D to prevent dendrite formation

Actions and Future Plans:

• Preliminary experiments for Li/SPE/Li have been conducted performed. Extended experiments are under way.

• Transference number measurements are underway.

• Initial cathode casting experiments have worked.

• A new lab has been set up at LBL to perform extensive cycling experiments and easy cooperation with other BATT program members.

Technology Transfer

• A Venture-backed start-up called Seeo, Inc. was founded in May 2007, with two ex-students from my lab (Mohit Singh and Hany Eitouni).

• A 5500 square foot facility for making and testing batteries is fully operational with 8 employees.

• The proposed battery was chosen by LBNL as one of the entries for the RD 100 Award.

• Fundamental characterization of block copolymer electrolytes will be continued within the BATT program, in harmony with the comments of the referees and directions from BATT program management.

Nitash Balsara does have a financial interest in Seeo

Research Plans

• Continue synthesis and characterization of block copolymer electrolytes.

• Complete characterization if Li-Li symmetric cell cycling.

• Measure Li transference number and diffusion coefficient.

• Make measurements of battery charge/discharge cycles and see how they compare with predictions.