What Happens if a Cell goes into Thermal Runaway?

Author: Nrusimhan Seshadri, Balance Batteries

Thermal hazards of (Li-ion batteries) LiB’s are well known. The spontaneous exothermic release of energy from LiBs when subjected thermal, mechanical and/or electrical abuse is referred as thermal runaway. Though the chances of thermal runaway in modern EV’s are extremely low, the danger posed  by EV battery fires can’t be overlooked. In this series of articles, we discuss the challenges posed in reducing the impact of Li-ion battery fires and explore passive fire safety of Li-ion battery packs used in the automotive industry.

What exactly happens if a Cell goes into thermal runaway?

During an event of thermal runaway we observe two main features, one is the so-called ‘jet-fire’ and the other is ‘volcano particle ejecta’. At temperatures above 90°C, the decomposition of the solid electrolyte interface (SEI) inside the cell leads to build up of highly flammable Volatile Organic Compound (VOC) gases inside the cell. These gases are usually ejected through safety vents, when the cell’s internal pressure goes beyond a certain threshold. In cylindrical and prismatic cells the vents are on the top surface, whilst pouches are designed to vent at the location where the tabs pass through the pouch wall. This is the first stage in thermal runaway. Even after the opening of safety vents the cell is still filled with flammable liquid electrolyte and there are still a lot of chemicals (in the form of active materials) left inside the cell. Multiple mechanisms, for example; the internal short circuit (ISC) caused due the collapse of the polypropylene/polyethylene separators, the decomposition of cathodes, decomposition of the liquid electrolyte, etc are highly exothermic in nature and increase the cell’s temperature exponentially. Once the cell temperature is above 130°C, the exothermic reactions cannot be stopped and thermal runaway begins. Due to high temperatures and the sparks from debris of decomposing cell’s electrodes, the VOC’s ignite and a very distinctive jet flame is observed. The ballistic velocities of the gas ejecta carry the broken hot metal particles out of the cells and a volcano like fire with particle debris can be observed as shown in the figure below.

Figure 1 Volcano particle ejecta and evolution of a jet flame (Jung et al.,)

With EV packs optimised for high energy densities and closely packed, a single cell in thermal runaway could propagate heat across the pack leading to a cascading thermal runaway of multiple cells and modules if thermal barriers are not used.

Are there any fire safety regulations to which EV battery packs should comply with?

All EVs which are on European roads today (unless they are older than 2015) must have passed the regulatory requirements listed in ECE 324 UN R100. Specifically, annexe 9E mentions the test procedure to test the fire safety of energy storage devices used for vehicular applications. The test is often referred as the pan-fire/bonfire test by industry professionals and it is currently in its 3rd revised form.

Figure 2 Pan-fire test scenario as per ECE 324 UN R100 (Chen et al., 2020)

The test mandates the whole battery pack / vehicle be exposed to gasoline/LPG fire for a maximum of 130 seconds and there should be no explosion observed for a minimum of 3 hours after testing, in order to pass the test. The idea behind the test is to simulate the condition of a vehicle crash between and EV and a gasoline vehicle, resulting in fuel spillage which begins to burn. The Chinese regulation GB 38031-2020 is the newest and is more stringent standard than the European test. It specifies a 5-minute time window during which the passenger cabin remains isolated after a thermal event inside the battery pack. This time window is for the safe evacuation of the passengers from the vehicle. What this means is that there needs to be a very robust thermal propagation mitigation strategies considered while designing the battery pack. Standards in other markets have a similar protocol to the European test, while for many smaller markets there are no standards.    

In the next article, we will have a little Benchmarking of BEV battery packs to explore the strategies different OEM’s adopt to meet these stringent fire safety requirements.

About Balance Batteries – Founded in 2020, Balance Batteries consult on battery pack and module design and validation. Using a systems engineering approach we ensure your product requirements are met.

References

  1. Chen, H., Buston, J. E. H., Gill, J., Howard, D., Williams, R. C. E., Rao Vendra, C. M., Shelke, A., & Wen, J. X. (2020). An experimental study on thermal runaway characteristics of lithium-ion batteries with high specific energy and prediction of heat release rate. Journal of Power Sources, 472.
  2. Jung, H., Kim, K., Lee, K., & Kwon, H. (n.d.). A STUDY ON FIRE RESISTANCE TEST PROCEDURE FOR TRACTION BATTERY, Korea Automobile Testing & Research Institute (KATRI) Republic of Korea Paper Number 13-0353

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