Initializing of thermal runaway for lithium-ion cells

Workshop JRC, March 8./9. 2018Petten

H. Döring, M. WörzZentrum für Sonnenenergie- und Wasserstoff-Forschung Ulm

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uncontrolled heat generation

cell burning

smoke formation

gas emission

extension of the fire

external heat

external short circuit

deformation/shock/vibration

internal short circuit

dendrite formation

separator-failure

cell balance, converting

(Li-dendrite)

depth discharge

(Cu-dendrite)

separator melting

manufacturing defect

separator

heat generation

overcharge

pressure build-up

instability anode

cathode

reaction with

electrolyte

gas formation CO, H2,

CO2, O2……

cell opening

thermal runaway

The Safety Hazard of Li-Ion Batteries

l

Final catastrophic event thermal runaway:

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Definition: Thermal runaway reaction occurs when the thermal stability limit of the chemistry within a cell is exceeded, and the cell releases its inner energy rapidly. Thermal runaway could be detected when the measured voltage of the cell drops and dT/dt ≥[4°C/s] of the measured temperature and exceeds the maximum operating temperature defined by the manufacturer.There are a variety of causes that can trigger the thermal runaway.

Short circuit with fast temperature increase but without cell opening, without venting, without fire thermal runaway??

Economic and Social CouncilEVSTF09-02-TF5-01, 08.06.2016 ThermalPropagationtestprocedure-TF5

Variety of methods proposed for initializing thermal runaway:

• Overcharge (electrolyte decomposition, dendrite formation, phase stability of materials)

• Crush (crash, drop test) (multi layer strike between electrodes, electrodes and

construction elements)

• Thermal exposition (multi layer strike mainly via separator failing, activation self heating

process, balance heat generation/heat injection and heat dissipation)

• High voltage/current exposition (local overheating, high voltage breaking through)

• External short circuit (question: internal and external energy release, usually current not

high enough to cause thermal runaway for single cell, self heating of cell during short

circuit already thermal runaway? Definition of thermal runaway?)

• Simulation internal short circuit (single layer strike)

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Different methods proposed for simulation of internal

short circuit:

• Nail or needle penetration (multi layer strike, extent depends on nail and depth, special

screw penetration)

• Bullet firing through test (multi layer strike)

• Blunt rod test (damage of separator mechanically, single or multilayer strike)

• Incorporation of particles and pressing (single layer strike )

• Incorporation of metals with low melting points (e.g. Wood’s metal, heat exposure for

melting, single layer strike )

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Charge rate: C/10, C/3, 1C, 5CVoltage limit: 5V, 1.2*Vcharge, 2*Vcharge , 20V, open limitStart: 0%....100% SOC, T: RT….60°CTermination: voltage limit, SOC limit, event

Problem: some cells are equipped with passive protection devices CID, OSDNecessary to block it if possible to get to TR

Overcharge for initialization of the thermal runaway

Overcharge possible initialization method for TR, however additional charge of energy (electric, thermal, chemical) and manipulation of protection device might be necessary, high voltage for stable separators necessary

100% SOC, RT, tool: Cylinder Ø 150 mm50% deformation

Crush for single cells pouch vs. prismatic

Crush- not specific to thermal runaway initiation, complex impact, strong interaction with construction parts, useful to qualify robustness cell-module-pack-car , not promised for initialization TR for investigation propagation

100% SOC, RT, Heating rate: steps with rest of e.g. 30 min, continuous 1…2 °K/minSEI layer breakdown, electrolyte degradation, venting by evaporated electrolyte, reaction anode-electrolyte, cathode electrolyte, gas formation, shut down separator, melting separator, internal short circuit, massive energy release, fire

Problems heating rate to slow: • slow thermal discharge of the cell, energy release of a longer time • Neighboring cells get preheated as well, promoting propagation effect

Thermal exposition for initialization of the thermal runaway

Thermal exposition possible initialization method for TR, however additional charge of energy (thermal) and manipulation for installation the heating device necessary, fast heating prevent excessive heating of neighboring cells, good heat transfer to the cell required

100% SOC, RT…55°C, 0.3 mΩ < Rloop < 100 mΩComparison: SC 1s and 5s configuration, Rloop 1.6 mΩ, Rcell: 0.7 mΩ distribution of the heat generation according to the resistance of the elements

External short circuit for initialization of the thermal runaway

Short circuit could be a possible initialization method for TR, however, current and heat production depends on the internal and external resistance, some cells have built in fuse like elements

Cell opened at the edges, slight venting

Cell opened, venting, thermal runaway, fire

Temperature

Voltage

Thermal runawayNo visible impact

Fire

Heat generation, discharge

Time Time Time

Internal short circuit is one of failure modes of Li-ion cells that is almost not externally controllable

They might have different sources:

• formation of Li or Cu dendrites

• mechanical failures like crack or pin holes in the separator

• penetration by particles

• mechanical impacts and others

Forced Internal Short Circuit Test in JIS 8714 Japan • Charge the cell to the upper voltage limit (e.g. 4.25V)

• Disassemble the cell and take out the winding core

• Insert a Nickel particle

• Re-assemble the cell

Difficult to prepare test sample, open for special manipulation

Positive activematerial coated area

Negative activematerial coated area

Separator

½ Width

Ni particle (0.2 mm height, 0.1 mm width, 1 mm long each side

Press

Separatornegative electrode

positive electrode

Source: JIS 8714

100% SOC, RTCrush tool: Cylinder Ø 150 mmcrush flat side, 50% deformation, rest 5 min and press to 150kN force.

Crush for cells with incorporated particle

Blunt rod test IIISC (Indentation Induced Internal Short Circuit )

Maximum load: 1.5 kNPress speed: 0.1 mm/s Voltage drop 100 mV

The CT-Scan of the tested cell shows the short was induced at outer layer

Tool proposed by UL

R = 45°, r = 0.9 mm

Source: UL 1642, Underwriters Laboratories, Inc.laurie.b.florence@us.ul.com

Tool collection @ ZSW

Results are more related to the mechanical properties of the separator, electrode structure, influenced by the case design

suitable for pouch, used for cylindrical cells (figure), seldom applied for prismatic hard case

Nail penetration / PunctureSANDIA/(SAEJ 2464): cell: Ø 3 mm, mild steel (conductive), perpendicular to electrode plates, through cell,

80mm/sec (<250 mm/s)module: Ø 20 mm, trough 3 cells or 100 mm

QCT 743: high-temperature resistant, steel spike of Ø 3 mm ~ Ø 8 mm through cell, at the rate of 10 mm/s ~ 40 mm/s (with the spike retained in the cell)

module: trough 3 cells

Additional influenced by: tip shape, surface of the nail, alloy composition, ……

Very strong impact• leaves space for variation of test conditions • limits reproducibility

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Nail penetration / PunctureDraft IEC DTR 62660-4 TR © IEC 2016 Nail: Type 1: Ø 3 mm ceramic nail, tip should be 45°

Type 2: Ø 1 mm ceramic nail with Ni tip with a nickel (Ni) tip of 0,35 mm in height, nail tip should be between 28° and 45°

constant velocity less than 0,1 mm/s. Displacement stopped when a voltage drop of at least 5 mV is detected.

Seems to be close to the conditions of internal short circuitFor the low intrusion depth it could be possible to

use a conventional steel nail or needle with similar effect

Problem: cell flexibility: once getting through the cell case, flexibility might cause multi layer penetration

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Nail penetration: Small variations in test parameters large difference in results

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Nail silver steel cell without plates hazard level 3 (cell damaged and opened)

Nail penetration: Small variations in test parameters large difference in results

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Nail ST37 cell without plates hazard level 6 (fire, rupture)

CT-image of nail penetrated cell, penetration in discharged status

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CT-image of nail penetrated cell, penetration in charged status, ceramic nail

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Cu dendrites in result of deep discharge Cu dissolution and deposition

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Deep discharge Cu-corrosion Cu deposition cathode short circuitDeep discharge Cu-corrosion recharge Cu deposition anode short circuit

Cell behavior during use after deep discharge

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Cycling after deep dischargeRecharge possible but virtual overcharge by-pass current via latent internal shorts

Fast charge after deep dischargeRecharge possible but at a certain time internal short circuit causes thermal runaway

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Estimation on internal short circuit by a Li dendrite

Statement TIAX:• Results show that a 100 Ω short circuit can be easily detected, with excellent signal-to-

noise ratio by TIAX• Simulation work at TIAX showed that internal short resistance less than 4 Ω was needed

to cause thermal runaway• Proprietary method

duration 100ms energy: 0.3 J

Alternative method for simulation internal short circuit • So fare used methods: generate a local short circuit situation

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dendrite

short circuit

current

heat generation

damage of the near surrounding

single layer

local damagedischarge

thermal runaway

laser impact

light beam

heat generation

damage of the near surrounding

single or multy layer

local damagedischarge

thermal runaway

Laser impact (single or multilayer strike)

• Alternative method: introduce localized heat source, not necessarily linked to a current flow

Laser – test - equipment

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Front end laser optic in protection box Laser exit and cell fixed at holder

• Pulsed Nd:YAG - solid state laser

• Laser class 4, 400W

• Wavelength 1064 nm

• glass fiber connection between laser source

and front optic

• beam diameter around 0.3 mm

• manufactured by LASAG industrial laser, CH

laser beam

lens

cell

Results 4 Ah NMC cell

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Commercial 4 Ah Li-ion cellLiC-NMCsize: 145 mm x 42 mm x 9 mm, mass: 103 g, Ri = 6.9 mΩ, pouch, thickness of the cell is comparable with cells of higher capacity,the smaller size was selected for the available CT device

frequency duration energyHz ms V J2 1.65 350 2

Laser parameters

Investigated at 0%, 50% and 100 % SOC

Results variation pulse energy

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• Higher number of pulses causes an increase in penetration depth with “saturation effect”• Higher pulse energy causes higher penetration at comparable number of pulses

•laser allows to burn slight conic holes in the cell•burned material evaporated•white dots are molten metal (Cu) •layers of the electrodes are clear visible, •separator is almost not visible •Hole diameter getting to a “saturation value”, question of laser divergence and focus

4Ah NMC, 50% SOC

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Pos 1: 1 pulse, just foil of the pouch pack is penetrated

Pos 2: 2 pulses, first electrode layers are penetrated

Results 4 Ah NMC cell

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The laser pulses cause an electric signal (U-drop)

Voltage drop can be a mathematically related to a short circuit current calculated from the voltage drop and the internal resistance of the cell.

Test results 4Ah NMC, 100% SOC

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100% SOC:

100 pulses about 10 electrode layers are penetrated

exfoliation of electrode layers at the bottom of the hole, it is assumed that

gases are formed and causes this exfoliation, influenced by SOC??

side penetration of the Cu-layers, white dots of molten Cu

Test results 4Ah NMC, 100% SOC

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After 100 pulses (2J, 1.65 ms) and a further delay of about 2 min for this cell a thermal runaway with venting and fire was observed

Results 4Ah LiC-NMC cell reference test nail and blunt, 100% SOC

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3 mm nail: Tmax: 256°C, mass loss 22.5 g (22%)

1 mm needle: Tmax: 252°C, mass loss 23 g (22%)

Blunt 10 mm: intrusion depth of 3.2 mm, Tmax: 91°C,

mass loss 4.2 g (4%)

Comparison Laser impact, nail and blunt test for 4 Ah NMC cell

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• Laser can realize very small impact

• Voltage drops are observed in different intensity related to pulse number

and intensity which might be comparable to internal short of a cell

• Laser with high intensity and high number of pulses causing permanent

moderate short circuit leading to a cell discharge

• Both nail tests (1 and 3 mm diameter) caused thermal runaway with

temperatures around 250°C

• Blunt rod test lead to a permanent internal short circuit and a energy

release comparable to an external short circuit.

Different methods proposed for initializing thermal runaway for

propagation investigation• Overcharge suitable, low manipulation level, problem inbuilt safety device (OSD, CID),

charge the cell with extra energy (thermal, chemical)

• Crush not so suitable, not specific, interaction with different construction parts

• Thermal exposition: suitable, medium manipulation, heat dissipation during heating to other cells as well, charge the cell with extra thermal energy

• High voltage/current exposition critical, difficult to control, no experience

• External short circuit usually not suitable for single cell as activation power might be limited

• Simulation internal short circuit rather difficult in pre-preparation

• Nail, screw: suitable, requires mechanical pre-preparation, no extra energy load

• Plunt: suitable for internal short circuit simulation, no advantage compared to nail

• Laser: suitable, but requires some pretest for adjustment parameters and set up

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Acknowledgement

• Team electrochemical accumulators, group abuse testing

• Daimler research center for financial support for the laser investigation

• German ministry of science and education within the SPEISI project

registered under 0325742C

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Thank you for your attention !!