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Research Institutes of Sweden
THERMAL PROPAGATION IN LITHIUM-ION BATTERIES Fredrik Larsson, PhD
March 2018
SAFETY AND TRANSPORT ELECTRONICS
Gasoline – very dangerous We have learnt how to make it safe
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Li-ion batteries are still new We are in the learning process
No intrinsically safe commercial “useful” cells
Flammable electrolyte
Incidents with cell failures will happen
Safety incidents can be reduced by, e.g.
Battery design, cell and system
High quality cell
High quality BMS
Yet,
The BMS can not protect from all abuse cases
The BMS and its sensors can fail
External factors will worsen failure incidents
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Li-ion battery incidents will happen
Complex failure modes
Size scaling effects – not well studied
Gases may, at least for some situations, be the largest risk, fire secondary
Important factors
Battery size and type
Design and application implementation
Application type
Environment (e.g. people present, temperature, humidity, external conditions)
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Large Li-ion battery systems
Battery design significantly affects propagation
Firewall / thermal barriers, cell spacing, cell inter-material, e.g. cooling plates
Phase change materials
Physical separation of battery system in several parts
Adds weight, volume and costs – impact varies for different applications, e.g. electrified vehicles, vessels, stationary grid
Propagation is determined by the balance between heat generation and heat removal
The thermal management system of the battery:
Heating/cooling during normal use
Typically not designed to hinder propagation
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Propagation characteristic
• What scenarios to protect from?
• Important to delay/mitigate/stop
• For example
• Limit toxic gas amounts
• Limit heat and fire, explosion size
• Delay can be important – gives valuable time for detection, evacuation, fire fighting
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Propagation initiation and mitigation is possible at several levels
Heat release
Total value and release rates – both important for propagation
Heat value influenced by a multiple of factors
Cell type, test type and analysis methods
Value affected by e.g. access to air/oxygen, heating rate (e.g. slow ARC vs fast), ignition/no-ignition
Cell status – ageing/SOH, SOC
Battery system: Combustion involves - plastics, cables, electronics, etc…
DSC and ARC - only a part of the heat release
Combustion measurements are important, access to oxygen
External addition: e.g. external heating, fire, overcharge, mechanical crush energy
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How much heat/energy can be released?
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Heat release rate (HRR) Fire test with external propane burner – 5x7 Ah LFP pouch
Outbursts Burner HRR subtracted Fire calorimetry:
Oxygen consumption method, corrected for CO2
Heat release rate (HRR)
Integration of HRR = Total heat release (THR)
Combustion energy ~ 5-20 x electrical energy
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Values for fire tests (full combustion)
7 commercial Li-ion cells
Other test conditions and measurements can give other values
Literature values give ratios 0.5 - 2 , using other methods
Total heat release (THR) = 17-75 kJ/Wh
Fire of 100 kWh pack ~ 70-300 liter gasoline
https://www.nature.com/articles/s41598-017-09784-z
Very limited publications
Huge numbers of complex gases can be released – toxic and with unknown toxicity
Solvents and decomposition products, e.g. CO, CO2, H2, CH4, …
Fluoride gases
Unknown compositions present
Confined spaces are extra problematic; tunnels, underground car parks, …
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Toxic gas release
Hydrogen fluoride (HF) most in focus, still few quantitative measurement published
Other fluoride gases - not much attention
Source:
Li-salt, LiPF6
Binder (e.g. PVdF), additives in electrode and/or electrolyte
LiPF6 + H2O → LiF + POF3 + 2HF
HF well-known toxicity
POF3 no toxicity data available
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Toxic fluoride gases
Hydrogen fluoride (HF)
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Total HF amount released: 20 to 200 mg/Wh
External fire tests
7 commercial Li-ion cells
https://www.nature.com/articles/s41598-017-09784-z
Amounts of fluoride
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Two independent and parallel measurement techniques
FTIR
Gas-washing bottles
https://www.nature.com/articles/s41598-017-09784-z
Time-resolved HF production rates vs state of charge Fire test
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https://www.nature.com/articles/s41598-017-09784-z
Fire is not needed for HF to be released Fluoride gas release in external heating abuse (oven)
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About 60 minutes heating time to thermal runaway
Nominal 6.8 Ah carbon/LCO cell
HF and POF3 present both with and without fire
3 separate vents occurred
5/11 tested cells resulted in a gas explosion
F. Larsson et al., “Gas explosions and thermal runaways during external heating abuse of commercial lithium-ion graphite-LiCoO2 cells at different levels of ageing”, Journal of Power Sources, 373, 220–231 (2018).
Toxicity of hydrogen fluoride
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1.7 mg/m3 Allowed exposure level at work in Sweden 25 mg/m3 IDLH = Immediately Dangerous to Life or Health (30 min)
139 mg/m3 The lethal 10-minute value (AEGL-3)
A fire where a 100 kWh Li-ion battery is consumed Emits 2 - 20 kg HF – a large amount ! Corresponding to a volume of 80 000 – 800 000 m3 of air with the IDLH-value – a volume of a large shopping center
HF release in confined scenarios
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Theoretical case (will not work in a real case)
Extrapolation for 100 kWh
Homogenously distributed HF, no losses
Underground car park: 50 x 50 m, 3 m height (7500 m3)
Concentration: 2 – 20 kg HF / 7500 m3
270 – 2700 mg/m3 = 320 – 3200 ppm
May be challenge for the fire brigade
HF gas may penetrate ordinary suits
Toxic skin contact
HF gas sensor needed for detection
No ignition gas and smoke
Instant ignition
Delayed ignition
Generally for fires: without flame/ignition – typically worse gas compositions
Degree of combustion influences smoke/gas composition
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Degree of combustion
Not well studied
Typically water is recommended and to use a lot of it
Water likely to be the best candidate
Access difficult
Needs to cool down the surface of the cell(s)
High tightness, e.g. IP67
Design for in/out flooding a solution?
Water mist – may capture and transform the toxic gas problem to a toxic liquid problem
The runoff water e.g. after firefighting may be highly toxic
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Firefighting of Li-ion batteries
The battery fire problem
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The risk for fire can be reduced using e.g. new electrode materials and additives such as flame retardants Some containing more Fluorine!!!
Fire – good or bad? Not good for a small consumer battery. Also a fire source for other ignitable materials etc
But without fire there is a potential for more toxic gas… What is worse?
Another aspect of gas release – Gas Explosion VIDEO
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F. Larsson et al., “Gas explosions and thermal runaways during external heating abuse of commercial lithium-ion graphite-LiCoO2 cells at different levels of ageing”, Journal of Power Sources, 373, 220–231 (2018).
Videos available at: https://www.sciencedirect.com/science/article/pii/S0378775317314398
Battery explosion types
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Cell case explosion
Gas explosion Delayed ignition of
released gases mixed with air in a confined/semi-confined space Can be much more severe
Can occur:
at less than 100 ºC
Before and without thermal runaway
Multiple vents may occur, some not visible by eye
If gases are mixed with air and confined – a gas explosion can occur in case of ignition
Ignition via: autoignition due to hot parts/electrical connections, sparks, external source, etc.
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Gas release (venting)
Complex area – holistic perspective needed Example with LFP cathode
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LFP cells typically generate less heat at thermal runaway and seldom ignite = The safest cathode? But without fire/combustion - LFP cells releases flammable gases = increased risk for gas explosion = Less safe cathode? Overall safety?
• Fire/flames are sometimes preferred – to reduce severe gas risks
• Battery size, application and its environment determines !
• Yet few studies and incident statistics about it
• We must better understand the mechanism of Li-ion battery risks
• Only then we:
• Can assess if the risks are small or large
• Can begin investigating counter-actions to handle/lower risks
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Additives can actually have negative effects and introduce new safety risks
Pollution and health effects
Their use should be minimized or removed
The holistic perspective (heat, gas, fire, explosion, application type, environmental type) needs assessment
Example: For some scenarios with large batteries – the use of flame retardants may be contra-productive
Enabling risks for gas explosions, which may be the worst case risk in such a scenario
Potentially more toxic gases
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Flame retardants and other additives to reduce flame/ignition
Symmetric fictive part of battery pack
Firewall between modules
1 mm Al-plate on one cell side
Cell, EiG 7 Ah carbon/LFP pouch
Heat data from fire test
Finite-element method (FEM) in COMSOL
Fire dynamics simulator (FDS)
Experimentally verified
Input data and model build-up important
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Numerical simulations Thermal propagation of cell-to-cell fire propagation
F. Larsson et al., “Thermal modelling of cell-to-cell fire propagation and cascading thermal runaway failure effects for lithium-ion battery cells and modules using fire walls”, Journal of The Electrochemical Society, 163 (14), A2854-2865 (2016).
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Cell-to-cell propagation – simulation results Propagation likely to occur without protection
Protection/mitigation by:
Cell cooling plate / cell spacing
Cooling – forced convection vs ideal heat sink
Firewalls between modules
F. Larsson et al., “Thermal modelling of cell-to-cell fire propagation and cascading thermal runaway failure effects for lithium-ion battery cells and modules using fire walls”, Journal of The Electrochemical Society, 163 (14), A2854-2865 (2016).
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Firewalls – simulation results
Thermal runaway in all ten cells in the module
Aluminum firewall
Different firewall thicknesses: 0-20 mm
Temperature on the other side of the firewall
F. Larsson et al., “Thermal modelling of cell-to-cell fire propagation and cascading thermal runaway failure effects for lithium-ion battery cells and modules using fire walls”, Journal of The Electrochemical Society, 163 (14), A2854-2865 (2016).
Publically available report
Construction guidelines from a fire and gas release perspective
Released October 2017
Free
Examples:
Gas filtration, detox
Ventilation strategy
Gas explosion mitigation
Link: https://www.diva-portal.org/smash/get/diva2:1146859/FULLTEXT01.pdf
Research Institutes of Sweden
THANKS! Fredrik Larsson
+46 10 516 5928
SAFETY AND TRANSPORT ELECTRONICS
