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Overcharge Protection for PHEV Batteries

Guoying Chen and Thomas J. Richardson Lawrence Berkeley National Laboratory

May 17, 2012

Project ID: ES037

Overview

Timeline

• Start date: March 2009

• End date: ongoing

• Percent complete: ongoing

Budget

• Total project funding - FY09 $190K - FY10 $190K - FY11 $240K

Partners

• ANL, BNL, INL, and SNL

• Berkeley program lead: Venkat Srinivasan

Barriers Addressed

• Cycle life

• Abuse tolerance for PHEV Li-ion batteries

Objectives\Milestones

• Develop a reliable, inexpensive overcharge-protection mechanism. • Use electroactive polymers for internal, self-actuating protection. • Minimize cost, maximize rate capability and cycle life of

overcharge protection in high-energy Li-ion batteries for PHEV applications.

Objectives

Milestones

• Investigate overcharge protection performance of polymer-fiber incorporated composite separators (Jun. 2012).

• Evaluate the property and performance of new high-voltage electroactive polymer candidates (Jul. 2012).

• Report overcharge protection for pouch cells and other large-scale battery cells (Sep. 2012).

• Attend review meetings and present research results.

The Need for Overcharge Protection

http://www.rokemneedlearts.com

Causes of overcharge • Cell imbalance in the battery pack • Charging exceeding electrode capacity • Over-voltage excursions • Low-temperature operation under high

internal resistance

Consequences of overcharge • Cathode degradation, metal ion

dissolution, O2 evolution • Electrolyte breakdown, CO2 evolution • Li deposition on anode, H2 evolution • Overheating, breakdown of anode SEI

layer and thermal runaway • Current collector corrosion • Explosion, fire, toxics released • Accelerated capacity/power fade,

shortened battery life

http://stores.headway-headquarters.com Series-connected LiFePO4 battery pack

0 0.02

0.04

0.07

0.11 0.14 0.19 0.22 0.25

0.09

0.06

0.03

SOC Conductivity (S/cm)

1 x 10-1

1 x 10-2

3 x 10-3

1 x 10-4

1 x 10-5

1 x 10-6

3 x 10-7

1 x 10-8

7 x 10-9

3 x 10-9

2 x 10-9

1 x 10-9 (neutral)

(oxidized)

Anio

n do

ping

leve

l

• Highly reversible redox reactions – capable of reversible, long-term protection. • Rapid changes in electronic conductivity upon the redox reaction – cell voltage

regulates the resistivity of the polymer shunt.

Our Approach – Electroactive Polymers

-0.4

-0.2

0.0

0.2

0.4

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5Potential vs. Li+/Li (V)

Cur

rent

Den

sity

(mA

/cm

2 )

Potential vs. Li+/Li (V)

S SS

R R

R

x

+ PF6-

(Polaron)

S SS

R R

R

x

++ 2PF6-

(Bipolaron)

S

R

n

20 cycles

• Potential drop occurs at the small region close to the negative electrode – Provides soft-shorting – Good for heat transfer out of the cell

• Polymer is capable of carrying a large amount of current. • System simulation validated by experimental observation.

Our Approach – Protection Mechanism

0.1

1

10

100

Current density (mA/cm2)

SOC 0 0.25

+

+

+

+

-

-

-

-

Our Approach – Advantages

Do

Dr

+ _ -e-

A° A°

A+ A+

+e-

Redox shuttle method • Diffusion limited - low rate capability and poor low temperature performance • Interference with the cell chemistry • Solubility and volatility issues

External electronics

• Expensive • Added weight and volume • One bad cell kills the whole string

Anode

Cathode

Current Collector

Current Collector

Electroactive Polymer Composite

Sandwich

+ -

Ano

de

Cat

hode

Cur

rent

Col

lect

or

S

epar

ator

Cur

rent

Col

lect

or

Cur

rent

Col

lect

or

Electroactive Polymer Composite A

node

Cur

rent

Col

lect

or

Cur

rent

Col

lect

or

External

Versatility of the polymer approach – cell configuration

Anode

Cathode

Current Collector

Separator

Current Collector Current Collector

Ele

ctro

activ

e Po

lym

er

Com

posi

te

Parallel

Tabs

Technical Accomplishments

• PFOP was found to have an extended stability window in lithium battery electrolytes. Its ability to provide single-polymer overcharge protection was demonstrated. • Modification on electroactive polymer composites led to 20x increase in sustainable current density and excellent long-term overcharge protections for several cell chemistries, including Gen2 and Gen3. • Electroactive polymer-fibers and their composite mats were prepared by an electrospinning technique. The behavior of the fibers as charge carriers in Li-ion batteries was characterized in an in situ optical cell.

-0.4

-0.2

0.0

0.2

0.4

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5Cur

rent

Den

sity

(mA/

cm2 )

Potential vs. Li+/Li (V)

1.5<V<4.3V

-0.4

-0.2

0.0

0.2

0.4

1.5 2.0 2.5 3.0 3.5 4.0 4.5

Cur

rent

Den

sity

(mA/

cm2 )

2<V<4.3V

1 M LiPF6/EC+PC, 5 mV/s

Poly(3-phenylthiophene) (P3PT)

S nS n

Polymer Stability Window

Ano

de

Cat

hode

e - e -

High V composite Low V composite

Ano

de

Cat

hode

e - e -

• A bilayer arrangement was previously adapted to protect the high-voltage polymer from degradation at the anode potential.

Single-layer

Bilayer

cycle number

• PFOP has the highest onset oxidation voltage (4.25V) among the investigated electroactive polymers. • Improved low-voltage stability at anode.

Extended Redox Window in PFOP

0.0 1.0 2.0 3.0 4.0 5.0-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

Cu

rren

t (m

A)

Voltage (V)

5 mV/s, 10 cycles

Cycle number

Poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-phenylene)] (PFOP)

0 10 20 30 40 5080

120

160

200

240

Spec

ific

capa

city

(mAh

/g)

Cycling number

charge discharge

Li

LiFePO4

Current Collector

Current Collector

PFOP/Celgard

Single-polymer Protection Achieved

• Cell cycled at 0.5C and 40% overcharge. • Improved low-voltage stability allows for stable single-polymer protection. • Improved discharge capacity upon cycling may be a result of enhanced conduction in the electrode.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

2.5

3.0

3.5

4.0

4.5

5.0

Volta

ge/V

Time/h

10 20 30 36 38 40 41 43 50

PDFP-LFP-OC1h 0.5C

Composites Modified for Better Polymer Distribution

Polypropylene membrane (Celgard) 25µm, 55% porosity

Glass fiber membrane (Whatman) 85µm, ~75% porosity

Membrane substrate

10wt% P3BT

12wt% P3BT

P3BT + membrane composite

P3BT S n

• Large porosity and open pore structure in the glass fibers led to more uniform polymer distribution in their composites.

Improved Sustainable Current Density in Modified Polymer-composites

Li Polymer composite

• Up to 20x improvement in sustainable current density.

3.0

3.5

4.0

4.5

5.0

5.5

6.0

0 5 10 15 20 25 30 35Time (min)

Cell

Pote

ntia

l (V

)

0.05 0.1 0.20.5

1 mA/cm2

3.0

3.5

4.0

4.5

5.0

5.5

6.0

0 5 10 15 20 25 30 35Time (min)

Cell

Pote

ntia

l (V

)

0.05 0.1 0.20.5

1 mA/cm2

30 40 503.0

3.5

4.0

4.5

5.0

5.5

6.0

2

0.05Ce

ll pot

entia

l (V)

Time (min)

0.1 0.2 0.5 15

10

20 mA/cm2

P3BT/Celgard composite P3BT/glass fiber composite

580 585 590 595 600 605 610 6152.5

3.0

3.5

4.0

4.5

5.0

Volta

ge (V

)

Time (h)

158th

• Cycled at 0.5C rate and 50% overcharge.

• Upper cell voltage limit at 4.4V, maintained for more than 470 overcharge cycles.

1740 1745 1750 1755 1760 1765 1770 17752.5

3.0

3.5

4.0

4.5

5.0

Volta

ge (V

)

Time (h)

469th

Long-term Overcharge Protection Achieved – LiFePO4 Cell

Li Current Collector

Current Collector

P3BT composite

PFO composite

Cathode

Poly(9,9-dioctylfluorene) (PFO)

n

0 100 200 300 400 5000

50

100

150

200

250

Charge DischargeSp

ecifi

c ca

paci

ty (m

Ah/g

)

Cycle number

• Increased upper cell voltage limit at higher rates.

• Protection achieved even at 5C charging rate.

• 95% capacity retention after the first 350 overcharge cycles at 0.5C.

340 350 360 370 380 390 4000

50

100

150

200

250

Spec

ific

capa

city

(mAh

/g)

Cycle number

0.088mA (0.5C)

0.088mA (0.5C)

ChargeDischarge

0.35(2C)

0.175 (1C)

0.525 (3C)

0.876 (5C)

1438 1440 1442 14442.5

3.0

3.5

4.0

4.5

5.0

5.5

Time (h)

Volta

ge (V

)

5C

3C2C

Long-term Overcharge Protection Achieved – LiFePO4 Cell

Long-term Overcharge Protection Achieved – Spinel Li1.05Mn1.95O4 Cell

• Cycled at 0.2C rate and 60% overcharge. • Upper cell voltage limit at 4.45V. • Discharge capacity maintained for more than 120 overcharge cycles.

1700 1720 1740 17602.5

3.0

3.5

4.0

4.5

5.0

Time (h)

Volta

ge (V

)

120th

0 20 40 60 80 100 1200

20

40

60

80

100

120

140

160

ChargeDischarge

Spec

ific

capa

city

(mAh

/g)

Cycling number

1190 1200 1210 12202.83.03.23.43.63.84.04.24.44.64.8

Volta

ge (V

)

Time (h)

200th

Long-term Overcharge Protection Achieved – LiNi1/3Co1/3Mn1/3O2 Cell

• Cycled at 0.5C and 50% overcharge. • Upper cell voltage limit increased

from 4.35 to 4.4V during the first 200 overcharge cycles, which results in a slight increase in discharge capacity.

0 50 100 150 20050

100

150

200

250

300

Charge DischargeSp

ecifi

c ca

paci

ty (m

Ah/g

)

Cycling number

670 680 690 700 710 720

2.83.03.23.43.63.84.04.24.44.64.8

Volta

ge (V

)

Time (h)

125th

Electroactive-fibers Synthesized by Electrospinning

• Modification in electrospinning successfully made to produce polymer fibers from non-aqueous solution in a easily scalable manner.

PFO(16%)-PMMA(84%)

Versatility of Electrospinning

PFO(36%)-PMMA(64%)

P3BT(75%)-PEO(25%)

• The technique can be used to prepare a range of electroactive fibers and fiber-composites.

• Porous structure results from solvent evaporation may be beneficial for electrolyte absorption and wettability.

Uniform Polymer Distribution in Fiber-composites

P3BT(22%)-PMMA(78%)

2.5 µm

C-K S-K

P3BT S n

CH2C

CH3

OH3CO

n

PMMA

(-CH2CH2O-)n PEO

P3BT(75%)-PEO(25%)

Uniform Polymer Distribution in Fiber-composites

P3BT S n

2.5 µm

S-K O-K • Polymers are well

mixed at individual fiber level.

• Electrospinning Improves utilization and efficiency of the electroactive polymer, and reduces the cost of overcharge protection.

I=40mA/cm2, V=4.25V

Pt

PFO-PEO fiber film Li (-) (+)

Current collector Open end

Current Passage in the Fiber-composite Film

• Color changes from yellow to black upon oxidation.

• Distinct boundary between the oxidized and the neutral PFO suggests sufficient inter-connection between fibers for charge carriers to propagate across.

Electroactive-fiber-composite Membranes Made by Electrospinning

PFO (25%) - PEO(75%)

• Dense electroactive-fiber membranes can be made in various thickness. • A cost-effective way to produce lithium-ion battery separators capable of

voltage-regulated shunting.

Collaborations

• Robert Kostecki (LBNL) – Raman and FTIR Spectroscopy

• Yuegang Zhang (Molecular Foundry) – Electrospinning techniques

• John Kerr (LBNL) – TGA and DSC, AFM

• Vince Battaglia, Marca Doeff, Gao Liu (LBNL) – Electrode fabrication

• Quy Ta, Brian Nguyen (American Dye Source, Inc.) – Electroactive polymer synthesis

Future Work

• Evaluate rate capability and cycle life of the cells protected by electrospun electroactive-fiber-separators.

• Explore alternative polymer placement in the cells that may lead to improved protection and lowered cost.

• Continue to explore other high-voltage electroactive polymers that are suitable for overcharge protection for PHEV batteries. Optimize the morphology of their composites for maximum protection.

• Investigate overcharge protection for the cells with other high-voltage cathodes, particularly the Li and Mn rich Li1+xM1-xO2-type cathodes.

• Collaborate with industry and other labs to “scaling-up” the approach.

Summary

• An electroactive polymer with extended stability window was discovered, which was found to be capable of single-polymer overcharge protection for lithium-ion battery cells.

• The distribution of polymer in the composite is critical for long-term overcharge protection. Protection for hundreds of cycles can be achieved by using the glass fiber composites with better distribution.

• Electroactive-fiber-composite membranes with uniform polymer distribution were made by electrospinning. This type of membranes are expected to provide improved protection with higher polymer utilization and efficiency, and they can be flexible in their placement in the cells.

• Electrospinning can be a cost-effective way to achieve overcharge protection using the electroactive polymer approach.