THERMAL PROPAGATION IN LITHIUM-ION BATTERIES

Transcription

1 THERMAL PROPAGATION IN LITHIUM-ION BATTERIES Fredrik Larsson, PhD March 2018 Research Institutes of Sweden SAFETY AND TRANSPORT ELECTRONICS

2 Gasoline very dangerous We have learnt how to make it safe Li-ion batteries are still new We are in the learning process 2

3 Li-ion battery incidents will happen No intrinsically safe commercial useful cells Flammable electrolyte Incidents with cell failures will happen Safety incidents can be reduced by, e.g. Yet, Battery design, cell and system High quality cell High quality BMS The BMS can not protect from all abuse cases The BMS and its sensors can fail External factors will worsen failure incidents 3

4 Large Li-ion battery systems 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) 4

5 Propagation characteristic 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 5

6 Propagation initiation and mitigation is possible at several levels 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 6

7 How much heat/energy can be released? 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 7

8 Heat release rate (HRR) Fire test with external propane burner 5x7 Ah LFP pouch Outbursts Fire calorimetry: Oxygen consumption method, corrected for CO 2 Heat release rate (HRR) Integration of HRR = Total heat release (THR) Burner HRR subtracted 8

9 Combustion energy ~ 5-20 x electrical energy 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) = kj/wh Fire of 100 kwh pack ~ liter gasoline 9

10 Toxic gas release Very limited publications Huge numbers of complex gases can be released toxic and with unknown toxicity Solvents and decomposition products, e.g. CO, CO 2, H 2, CH 4, Fluoride gases Unknown compositions present Confined spaces are extra problematic; tunnels, underground car parks, 10

11 Toxic fluoride gases Hydrogen fluoride (HF) most in focus, still few quantitative measurement published Other fluoride gases - not much attention Source: Li-salt, LiPF 6 Binder (e.g. PVdF), additives in electrode and/or electrolyte LiPF 6 + H 2 O LiF + POF 3 + 2HF HF well-known toxicity POF 3 no toxicity data available 11

12 Hydrogen fluoride (HF) Total HF amount released: 20 to 200 mg/wh External fire tests 7 commercial Li-ion cells 12

13 Amounts of fluoride Two independent and parallel measurement techniques FTIR Gas-washing bottles 13

14 Time-resolved HF production rates vs state of charge Fire test 14

15 Fire is not needed for HF to be released Fluoride gas release in external heating abuse (oven) About 60 minutes heating time to thermal runaway Nominal 6.8 Ah carbon/lco cell HF and POF 3 present both with and without fire 3 separate vents occurred 5/11 tested cells resulted in a gas explosion 15 F. Larsson et al., Gas explosions and thermal runaways during external heating abuse of commercial lithium-ion graphite-licoo 2 cells at different levels of ageing, Journal of Power Sources, 373, (2018).

16 Toxicity of hydrogen fluoride 1.7 mg/m 3 Allowed exposure level at work in Sweden 25 mg/m 3 IDLH = Immediately Dangerous to Life or Health (30 min) 139 mg/m 3 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 m 3 of air with the IDLH-value a volume of a large shopping center 16

17 HF release in confined scenarios 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 m 3 ) Concentration: 2 20 kg HF / 7500 m mg/m 3 = ppm May be challenge for the fire brigade HF gas may penetrate ordinary suits Toxic skin contact HF gas sensor needed for detection 17

18 Degree of combustion 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 18

19 Firefighting of Li-ion batteries 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 19

20 The battery fire problem Fire good or bad? Not good for a small consumer battery. Also a fire source for other ignitable materials etc The risk for fire can be reduced using e.g. new electrode materials and additives such as flame retardants Some containing more Fluorine!!! But without fire there is a potential for more toxic gas What is worse? 20

21 Another aspect of gas release Gas Explosion VIDEO Videos available at: 21 F. Larsson et al., Gas explosions and thermal runaways during external heating abuse of commercial lithium-ion graphite-licoo 2 cells at different levels of ageing, Journal of Power Sources, 373, (2018).

22 Battery explosion types Cell case explosion Gas explosion Delayed ignition of released gases mixed with air in a confined/semiconfined space Can be much more severe 22

23 Gas release (venting) 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. 23

24 Complex area holistic perspective needed Example with LFP cathode 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? 24

25 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 25

26 Flame retardants and other additives to reduce flame/ignition 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 26

27 Numerical simulations Thermal propagation of cell-to-cell fire propagation 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 27 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), A (2016).

28 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 28 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), A (2016).

29 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 29 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), A (2016).

30 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:

31 THANKS! Fredrik Larsson Research Institutes of Sweden SAFETY AND TRANSPORT ELECTRONICS