“Thermal Characterization Study of Lithium-Ion Cells” By ... · “Thermal Characterization...

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National Aeronautics and Space Administration

www.nasa.gov

“Thermal Characterization Study of Lithium-Ion Cells”

By Doris Britton, Tom Miller and Bill Bennett

Abstract

The primary challenge in designing a full scale lithium-ion (Li-ion) battery system is safety

under both normal operating as well as abusive conditions. The normal conditions involve

expected charge/discharge cycles and it is known that heat evolves in batteries during

those cycles. This is a major concern in the design for high power applications and

careful thermal management is necessary to alleviate this concern. An emerging thermal

measurement technology, such as the electrochemical calorimetric of batteries, will aid in

the development of advanced, safe battery system. To support this technology, several

“commercial-off-the-shelf” (COTS) Li-ion cells with different chemistries and designs are

being evaluated for different cycling regimes at a given operating temperature. The

Accelerated Rate Calorimeter (ARC)-Arbin cycler setup is used to measure the

temperature, voltage, and current of the cells at different charge/discharge rates. Initial

results demonstrated good cell cyclability. During the cycle testing, the cell exhibited an

endothermic cooling in the initial part of the charge cycle. The discharge portion of the

cycle is exothermic during the entire discharge period. The presence of an endothermic

reaction indicates a significant entropy effect during the beginning of charge cycle.

Further studies will be performed to understand the thermal characteristics of the Li-ion

cells at the different operating conditions. The effects on the thermal response on cell

aging and states-of-charge will also be identified.

https://ntrs.nasa.gov/search.jsp?R=20070032054 2020-02-16T07:17:31+00:00Z

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Thermal Characterization Study of Lithium-Ion Cells

by

Doris L. Britton & Thomas B. Miller

NASA Glenn Research Center

Cleveland, OH

William R. Bennett

ASRC Aerospace Corp.

Cleveland, OH

10th Electrochemical Power Sources SymposiumAugust 20-23, 2007

Williamsburg, VA

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Characterization of Lithium-ion Cells

Objective & Approach

• Objectives:

– Apply calorimetry techniques to assess Li ion safe operatingconditions.

– Quantify self-heating rate and temperature limits

– Investigate new cathode and electrolyte interactions

– Ascertain failure modes.

– Understanding of life-limiting mechanisms of Li-ion cells duringvarious electrochemical conditions.

• Approaches:

– Reproduce published results of commercial off-the-shelf (COTS)cells and components (cathode and electrolyte)

– Generate data for the next generation materials (high specificenergy cathodes, next generation electrolytes, and nanocompositeanodes).

– Observe cell and component response to thermal and overchargeabuse conditions.

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Thermal Analysis Techniques

• Accelerating Rate Calorimeter (ARC) -Thermal Hazards Technology

(UK)

– Adiabatic temperature conditions

– Measures heat, gas and Onset of Thermal Runaway (OTR)

• Battery Cycler - Arbin Instruments (Texas)

- Performs battery/cell cycling and measures cell voltage and

temperature

The blast enclosure and control rack of the ARC and the Arbin cycler

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Part I – Li ion Components

• Components

– Cathode

• Transition metal (Li) – Ni, Mn, Co

• Phosphate cathodes

– Electrolyte

• Standard - 1M LiPF6 in 1:1 EC:DMC

• Baseline – 1 M LiPF6 in 1:1:1:3 EC:DEC:DMC:EMC

• Low Temperature

• Non-flammable

• Ester-based

– Anode

• New generation carbon or graphite

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Thermal Stability of Li ion components

• Materials studied so far:

– Electrolyte:

• 1M LiPF6 in 1:1 EC:DMC

• Instrument Used:

– ARC = adiabatic calorimeter

– Test sample holder material =

Titanium, 0.8 mm wall diam, 8 g

– Thermocouple = to measure

variation in temperature.

Annealed Ti bomb

New Ti bomb

Sample bomb suspended from the top of the ARC.

Thermocouple

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Thermal Stability of Li-ion

Procedure

• Measured sample loaded to the Ti bomb

• Thermocouple attached to the bomb to measure variation intemperature

• Controller is programmed to increase the calorimeter'stemperature via a predetermined profile.

• Sample temperature increases due to convection andconduction

• If the sample undergoes chemical reactions that generate heat,the sample temperature will rise

• If the self-heating is greater than the threshold level, the ARCproceeds into the exotherm mode

• Self-heating follows until the rate falls below the detection limitor until the end point temperature is reached.

• Adiabatic self-heating rate of the sample is measured as afunction of time and temperature.

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Temperature & Pressure Profiles

(1 M LiPF6 in 1:1 EC:DMC)

0

50

100

150

200

250

300

350

400

450

0 500 1000 1500 2000 2500

Time, min

Tem

peratu

re, °C

0

20

40

60

80

Pre

ss

ure

, b

ar

Temperatur

Pressure

-Sample weight = 2.7 g

-Temp = 40 to 400°C

-Temp step = 5°C; wait time = 15 min

-Sensitivity threshold rate= 0.020 °C/min

Temperature/Pressure-Time Profile

0.01

0.1

1

10

150 200 250 300 350 400

Temperature, °C

Self

Heat

Rate

(S

HR

),

°C/m

in

1

10

100

Pre

ss

ure

R

ate

, b

ar/

min

SHR

Pressure

Self-heat & Pressure rate profile

StartingT = 40°C

Onset T = 179°C

End T = 400°C

Onset T = 179°C at 0.069°C/min

T = 266.5°C with max. SHR = 6.95°C/min

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Part II - Lithium-ion cells

• Li ion cells

– COTS 18650 cell

– Experimental cells

– Prototype cells with advanced components

– Other cells

• Electrochemical and thermal profile of Li-ion cells

– Different charge/discharge cycles

• Thermal profile and runaway of Li ion cells

– Different depths-of-discharge (DOD) = 20, 40, & 60%

– Different states-of-charge (SOC) = 0, 25, 50, 75, & 100%

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Effect of charging and discharging on thermal stability

of Panasonic 18650 Li ion cell

ARC conditions

- Isothermal mode

- Start T = 35°C (kept at that temperature untilcycling is terminated)

- Temperature rate sensitivity = 0.1 °C/min

Cycling conditions

- Charge @ C/5 rate to 4.2V

- Taper charge C/50 current or 8 hours

- Discharge @ C/2 to 3.0V

- Rest for 2 hours

30

32

34

36

38

40

42

44

46

48

50

0 500 1000 1500 2000

Time, min

Te

mp

era

ture

, °C

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Vo

lta

ge

, V

Cell Voltage

Cell temperature

Charge TaperCharge

Discharge

Rest

Endothermiccooling @33.26°C

42°C

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Temperature during first charge/discharge cycle

30

32

34

36

38

40

42

44

0 50 100 150 200 250 300

Time, min

Te

mp

era

ture

, °C

Charge

Discharge

T = 42.06°C

T = 34.8°C

T = 33.26°C

16.2 min

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Temperature profiles of 18650 cell as a function of DOD

(LEO cycling regime)

30

35

40

45

50

55

60

0 200 400 600 800 1000 1200 1400

Te

mp

era

ture

, °C

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Vo

ltag

e, V

VOLTAGE

TEMPERATURE

Operating Temperature = 35°C

- Net cooling effect during charge

33°C at beginning of charge

-Highly exothermic effect duringdischarge (LEO regime)

20% DOD = 36°C

40% DOD = 40°C

60% DOD = 46°C

20%DOD

40%DOD

60%DOD

30

35

40

45

50

55

60

0 200 400 600 800 1000 1200 1400

Time, min

Te

mp

era

ture

, °C

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Vo

lta

ge

, V

VOLTAGE

TEMPERATURE

30

35

40

45

50

55

60

Te

mp

era

ture

, °C

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Vo

ltag

e, VVOLTAGE

TEMPERATURE

Significant temperature riseat increasing DOD.

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Thermal Runaway of Li ion cell

• Thermal testing of cell to determine the self heating

thermal runaway of the cells under different

conditions.

• ARC conditions:

– Temperature = 35 to 160°C

– Temperature rate sensitivity = 0.020 °C/min

– Temperature step = 5.0°C

• Different conditions:

– 0%, 50%, 75%, 100% SOC

– Different aging conditions (temperature & days in OC)

• Arbin cycler monitors cell temperature & voltage

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Temperature & Voltage versus Time(0% SOC)

0

20

40

60

80

100

120

140

160

180

0 500 1000 1500 2000 2500 3000 3500

Time, min

Te

mp

era

ture

, °C

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

4

Vo

lta

ge

, V

Voltage

Temperature

Onset T = 106.5°C

- ARC T = between 35 and 160 C- Onset T = 106.5°C voltage started to fall- Second exotherm after 2652 min, 0.822 V at 127.8°C- After 3257 min, 0.0 V at 149°C indicating internal cell shorting.- No runaway before 160°C

127.8°C149°C

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Temperature Profiles of Panasonic 18650 cell

(50% SOC)

20

40

60

80

100

120

140

160

180

0 200 400 600 800 1000 1200

Time, min

Te

mp

era

ture

, °C

2

3

4

5

6

7

8

9

10

11

Vo

ltag

e, VTemperature

Voltage

Temperature vs. Time

Self-Heat Rate vs. Temperature

120

122

124

126

128

130

132

134

800 850 900 950 1000

Time, min

Te

mp

era

ture

, °C

3.75

3.76

3.77

3.78

3.79

3.8V

olt

ag

e, V

Voltage

Temperature

Arbin malfunction

0.01

0.1

1

120 130 140 150 160

Temperature,°C

Se

lf-h

ea

t ra

te,

°C/m

in

Onset T = 126°C, 0.02°C/min

T-131.49°C, 0.065°C/min

153.86°C, 0.263°C/min

135.5°C, 0.058°C/min

Expanded view

Thermal runaway

- Onset T= 126°C Voltage started to fall

-Several peaks corresponding to different reactionsinvolving anode, cathode and electrolyte

-Arbin malfunction, voltage went up to 10V

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Before and after venting

• Before After

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Heat Generation

Objective:

•calculate internal heat generation for lithium-ion cells

•contrast and compare two different cathode chemistries

•LiCoO2

•LiFePO4

•use results to identify operating conditions which canlead to excessive internal temperature increase

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Heat Generation

T

EnFS =

nF

STIq

rev=

T

EITq

rev=

( )EEIqeqirrev

=

Thermal contributions:

•irreversible heat due to polarization(always positive)

•reversible heat due to entropy change(may be positive or negative)

•entropy, S, can be calculated from cellequilibrium potential/temperature data

•total internal heat generation

irrevrevtotalqqq +=

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Heat Generation

Equilibrium potential measurement:

•fully charge cell under “standard”conditions (C/20, 23°C)

•adjust temperature to desired valueand equilibrate for 1 day

•use incremental discharge followedby 4-6 hour rest

•use voltage after rest as equilibriumpotential

Layered cathode cell at 23 °C

2

2.5

3

3.5

4

4.5

0 2 4 6 8 10

Test_Time(d)

Ce

ll P

ote

nti

al

(V)

-0.25

-0.15

-0.05

0.05

0.15

0.25

0.35

0.45

Cu

rren

t(A

)

rest time affects accuracy. how long?

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Heat Generation

Equilibrium cell potential:

•for LiCoO2 cathode cell, cell potentialdecreases with increase in temperature

•for LiFePO4 cathode cell, cell potentialincreases with increase in temperature 3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

0% 20% 40% 60% 80% 100%

depth of discharge

eq

uil

ibri

um

po

ten

tia

l (V

)

0°C

23°C

40°C

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

0% 20% 40% 60% 80% 100%

depth of discharge

eq

uil

ibri

um

po

ten

tia

l (V

)

0°C

23°C

40°C

LiCoO2 cathode cell

LiFePO4 cathode

cell

Equilibrium potential is used to computepolarization

Change in equilibrium potential withtemperature is used to compute entropy

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Heat Generation

Compute derivative:

•plot potential versus temperature at agiven state-of-charge

•fit curve and compute slope (quadraticused here)

•repeat at other states-of-charge

LiCoO2 cathode cell at 32% DoD

y = 2.60E-06x2 - 2.67E-04x + 4.78E-03

-0.030

-0.020

-0.010

0.000

0.010

0.020

0.030

0 20 40 60

T (°C)

Eeq

rela

tive t

o 2

3°C

(V

)

32%

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Heat Generation

Computed derivatives:

•pronounced negative derivative forLiCoO2 cathode at DoD > 50%(heating effect on discharge)

•for LiFePO4 cathode, derivative isgenerally closer to zero, positive at<50% DoD

LiCoO2 cathode

LiFePO4 cathode

-0.002

-0.0015

-0.001

-0.0005

0

0.0005

0% 20% 40% 60% 80% 100%

DoD

dE

/dT

(V

/°C

)

linear

quadratic

-0.002

-0.0015

-0.001

-0.0005

0

0.0005

0% 20% 40% 60% 80% 100%

DoD

dE

/dT

(V

/°C

)

linear

quadratic

These results show that the reversible heat of the LiFePO4produces a slight effect on discharge.

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Heat Generation

Compare heat generation calculationwith measured cell temperature ofLiCoO2 cathode cell:

•computed heat generation correlateswell with temperature

•gradual heating begins immediatelyon discharge

LiCoO2 cathode cell at C/5, 23°C

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

0 2 4 6 8 10 12 14

time (h)

co

mp

ute

d h

ea

t (W

)

17

18

19

20

21

22

23

24

tem

pera

ture

(°C

)

-0.5

-0.3

-0.1

0.1

0.3

0.5

0.7

0.9

1.1

1.3

1.5

0 2 4 6 8 10 12 14

time (h)

cu

rren

t (A

)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

ce

ll V

olt

s

charge discharge

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-0.5

-0.3

-0.1

0.1

0.3

0.5

0.7

0.9

1.1

1.3

1.5

0 2 4 6 8 10 12 14

time (h)

cu

rren

t (A

)

0

0.5

1

1.5

2

2.5

3

3.5

4

ce

ll V

olt

s

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

0 2 4 6 8 10 12 14

time (h)

co

mp

ute

d h

ea

t (W

)

18

19

20

21

22

23

24

Heat Generation

Compare heat generation calculationswith measured cell temperature ofLiFePO4 cathode cell:

•computed heat generation correlateswell with temperature

•little temperature change withpronounced temperature rise at end ofdischarge

LiFePO4 cathode cell at C/5, 23°C

charge discharge

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Summary

• ARC is used to determine the temperature ranges where cell

starts self-heating and goes into thermal runaway

• Thermal stability studies as functions of DOD and SOC using

the ARC were initiated.

• ARC in combination with an Arbin battery cycler is currently

used to evaluate and study the performance and thermal

behavior of Li-ion cells.

• Significant temperature rise with increasing DOD

• Results will provide a fundamental database for cell safety from

a proper thermal management

• Heat generation correlates well with temperature

• Change in equilibrium potential with temperature is used to

calculate entropy.

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Future Research Work

• Continue thermal study of other Li-ion components

(anodes, cathodes & electrolytes).

• Continue thermal behavior study of Li-ion cells at

different conditions – SOC, aging, etc.

• Continue to understand the thermal stability and

heat generation from decomposition and

exothermic reaction of the materials within the cell.

• Initiate study of next generation cells and

components.

• Verify heat generation calculation at higher rate of

discharge/charge.

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