M. Rohde, C. Ziebert, H.J. Seifert...M. Rohde, C. Ziebert, H.J. Seifert Safety studies on Li-ion...

KIT – The Research University in the Helmholtz Association www.kit.edu

Institute for Applied Materials – Applied Materials Physics (IAM-AWP)

M. Rohde, C. Ziebert, H.J. Seifert

Safety studies on Li-ion cells with combined calorimetric and electrochemical methods

JRC-Workshop, 8.-9. 3. 2018, Petten, Netherlands KIT, IAM-AWP2

Thermal characterization and safety studies

Objective: Validated heat flow and temperature data for thermal management systems

www.techatplay.com

http://insideevs.com

→ Overheating

→ Overcharge

→ Overdischarge

→ Short Circuit

→ Accident

Possible Safety Impacts

Increasing scale

Thermal characterization

Accelerating Rate Calorimetry


Operating conditions: Lithium-ion batteries

Open circuit operation orstorage temperature < 80 °C

Operating temperature 30 – 40 °C

Charge / discharge between 0 – 60 °C

DT (Cell) < 10 K

DT (Pack: between cells) < 3 - 5 K


Heat generation in an electrochemical cell

S. Hallaj, H. Maleki, J.S. Hong, J.R. Selman, J. Power Sources 83 (1999) 1-8

∆𝐺 = −𝑛𝐹𝐸0

Gibbs Free Energy

∆𝑆 = 𝑛𝐹𝑑𝐸0

𝑑𝑇

Entropy change

𝑊𝑒𝑙 = 𝑛𝐹𝐸

Electric work

n number of electrons, Faraday constant F = 96485.3365 C/mol, E0 open circuit voltage (OCV), E voltage under load

𝑄 = ∆𝐺 + 𝑇∆𝑆 + 𝑊𝑒𝑙

ሶ𝑸𝒈 = −𝑰 𝑬𝟎 − 𝑬 − 𝑰𝑻𝒅𝑬𝟎𝒅𝑻

Parts of heat generation rate

1. “Reversible” heat rate caused bychemical reactions in the cell

ሶ𝑸𝒓𝒆𝒗 =𝒅

𝒅𝒕𝑻 ∙ ∆𝑺 = 𝑰𝑻

𝒅𝑬𝟎𝒅𝑻

ሶ𝑸𝒊𝒓𝒓𝒆𝒗 =𝒅

𝒅𝒕∆𝑮 +𝑾𝒆𝒍 = 𝑰(𝑬 − 𝑬𝟎)2. “Irreversible” heat rate caused by

Ohmic resistance and polarisation


EV-ARC: Ø: 25 cmh: 50 cm

ES-ARC: Ø: 10 cmh: 10 cm

Battery Calorimeter (ARC: Accelerating Rate Calorimeter)

EV+ ARC: Ø: 40 cmh: 44 cm

ARC combined with internal or external cycler

= Battery Calorimeter

+


𝑅𝑡ℎ defined

𝑇𝐶 constant

𝑇𝑆 𝑡 = 𝑇𝑆0 + 𝛼 ∙ 𝑡

𝑻𝑪

𝑻𝑺

𝑅𝑡ℎ very high

𝑇𝐶 = 𝑇𝐶 𝑡= 𝑇𝐶0 + 𝛼 ∙ 𝑡

𝑻𝑪

𝑻𝑺

Battery calorimetry: Accelerating Rate Calorimeter (ARC)

Isoperibolic Adiabatic


Conversion of thermal data (temperature, temperature rate) to heat (Joule) and power (Watt) with the aim

of understanding of heat release to determine heat removal requirements for thermal management.

To be measured:

Heat generation of the cell during charging and discharging

Key data for thermal management and safety

Methods for the determination of total generated heat

Cell effective specific heat capacity

Heat transfer coefficient

Reversible heat rate

Irreversible heat rate


Heat generation rate ሶ𝑄𝑔 Heat dissipation rate ሶ𝑄𝑑

Temperaturechange with time

Energy balance for lumped heat transfer system with convective and radiative heat loss

𝑚𝑐𝑝𝑑𝑇

𝑑𝑡= ሶ𝑄𝑔 − ሶ𝑄𝑑

ሶ𝑄𝑑=0

Adiabatic conditions:

ሶ𝑄𝑔 = 𝑚𝑐𝑝𝑑𝑇

𝑑𝑡

Determination of the generated heat

ሶ𝑄𝑑 = 𝐴ℎ ∙ 𝑇𝑆 − 𝑇𝐶 + 𝜀𝜎𝐴 ∙ (𝑇𝑆4 − 𝑇𝐶

4)

Isoperibolic conditions:


Measurement of the effective specific heat capacity cp

e.g. at 30 °C 𝑐𝑝 = 1.095 ൗ𝐽𝑔 ∙ 𝐾

Sandwich setup for pouch cells

Control of the current applied to the heater mat to ensure a constant heating rate

𝑐𝑝 =∆𝑄

𝑚 ∙ ∆𝑇𝑎𝑑=𝑈 ∙ 𝐼 𝑑𝑡

𝑚 ∙ ∆𝑇𝑎𝑑(4)

m: Mass of the cell

∆𝑇𝑎𝑑: Temperature difference underadiabatic conditions


Isoperibolic Measurements

Negative temperature coefficient

Discharge parameter:- method: constant current (CC)- Umin = 3,0V- I = 5A → C/8-rate

Charge parameter:- method: constant current,

constant voltage (CCCV)- Umax = 4,1V- I = 5A → C/8-rate- Imin = 0.5A

Pouch cell 40 Ah NMC - GraphiteIdeal conditions→ Single cell


Discharge parameter:- method: constant current (CC)- Umin = 3,0V- I = 5A → C/8-rate

Charge parameter:- method: constant current,

constant voltage (CCCV)- Umax = 4,1V- I = 5A → C/8-rate- Imin = 0.5A

Adiabatic Measurements

Tst = 23°C (RT)

Adiabatic and Isoperibolic Measurements:

Pouch cell 40Ah NMC - GraphiteWorst Case Conditions→ Cell in a pack surrounded by other cells


Total cell energy = electrical energy + „chemical“ energy(normal operation) (abuse condition)

Stringfellow, R. et al. “Lithium-Ion Battery Safety Field-Failure Mechanisms.”, 218th ECS Meeting, Las Vegas, 2010.

Energy stored in a cell


Heat-Wait-Seek Method

Source: Thermal Hazard Technology (THT, 2014)

Example of a Heat-Wait-Seek step.

Thermal Runaway tests

Seek


0 1000 2000 3000 4000 5000

0

100

200

300

400

500

600

700

800 Samsung_NMC

Sony_LMO

A123_LFP

Te

mp

era

ture

in

°C

Time in min

Thermal Runaway: 18650 cells with different cathode materials

80<T<130°C: low rate reaction, 0.02 - 0.05 °C/min: decomposition of the SEI

100 200 300 400 500 600 700 800

1E-3

0.01

0.1

1

10

100

1000

10000

Samsung_NMC

Sony_LMO

A123_LFP

Te

mp

era

ture

Rate

in

°C

/min

Temperature in °C

130<T<200°C: medium rate reaction, 0.05 - 25 °C/min: anode and electrolyte

=> reduction of electrolyte at negative electrode

T > 200°C: high rate reaction, higher than 25 °C/min: reaction cathode and electrolyte=> rapid generation of oxygen


Study of ageing effects by thermal runaway tests

0 500 1000 1500 2000 2500

50

100

150

200

250

Tem

pera

ture

in

°C

Time in min

fresh cell

aged cell

80 100 120 140 160 180 200 220 240 260

0.01

0.1

1

10

100

1000

fresh cell

aged cell

Tem

pera

ture

Rate

in

°C

/min

Temperature in °C


Development of external pressure measurement methods

0 500 1000 1500 2000 2500 3000 3500

0

50

100

150

200

250

300

350

400

450

Temperature

External pressure

Time in min

Tem

pera

ture

in °

C

0

10

20

Ext

erna

l pre

ssur

e in

bar

Safety vent opened


Internal pressure measurement in 18650 cells

0 500 1000 1500 2000 2500

0

100

200

300

400

500

600

Temperature

Internal pressure

Time in minT

em

pera

ture

in

°C

0

2

4

6

8

10

12

14

Inte

rnal

pre

ssu

re in

bar

Pressure line (Ø 1.5 mm) inside core hole

TC at outer surface

Time delay between Pmax - Tmax

Safety vent opened


Nail penetration test in the ARC at a 4.5 Ah pouch cell

Nail penetration test on pouch cells in the ARC

-20 -10 0 10 20

50

100

150

200

250

300

350

400

Te

mp

era

ture

in

°C

Time in s

Source: LookKIT 1/2015


• Extended time for thermal runaway propagation: 9 min

• Improved thermal protection: temperature outside of battery box < 80 °C during thermal runaway

Protective Material evaluation in battery calorimeters:Red: heater mat for thermal runaway initiationGray: protective material for cell 4 and lid of

battery box

Prevention of thermal propagation

0 10 20 30 40 500

200

400

600

800

1000

Tem

pera

ture

in

°C

Time in min

Cell 1

Cell 2

Cell 3

Cell 4

Dt = 9 min

0 10 20 30 40 5020

30

40

50

60

70

80

Tem

pera

ture

on

to

p o

f b

att

ery

bo

x

Time in min


Summary: Possible measurements with a battery calorimeter

Normal operation

Isoperibolic measurement: constant environmental temperature

Adiabatic measurement: No heat exchange between cell and surrounding area

Measurement of temperature curve and temperature distribution during cycling (full cycles, or application-specific load profiles)

Determination of the generated heat, Separation of heat in reversible and irreversible parts

Abuse conditions

Thermal abuse

External short circuit, nail penetration test

Overcharge, deep discharge

Temperature measurement

External or internal pressure measurement

Gas collection


Acknowledgements

This work has been partially funded by the Federal Ministry for Education and Research (BMBF)within the framework “IKT 2020 Research for Innovations” under the grant 16N12515 and issupervised by the Project Management Agency VDI|VDE|IT.

Additional funding by the German Research Foundation priority programme SPP1473 WeNDeLIBis gratefully acknowledged.

Supervised by

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M. Rohde, C. Ziebert, H.J. Seifert...M. Rohde, C. Ziebert, H.J. Seifert Safety studies on Li-ion...

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