64 May/June 2024 | E-Mobility Engineering Peter Donaldson speaks to experts who are working on cell design and chemistry to avoid thermal runaway First line of defence Protection from thermal runaway in an electric vehicle (EV) battery requires a holistic solution that affects every aspect of the pack’s design and operation. The consensus seems to be that protection options are limited to prevention and mitigation rather than stopping a runaway once it has started. Prevention begins with cell design and chemistry, while stopping propagation from the initiating cell to its neighbours involves pack and module design. Many aspects of design, including chemical, structural and electrical/electronic have an impact, and measures external and internal to the cell can be applied, including rapid cooling, fuses, built-in shutdown mechanisms and venting. We quizzed leading experts in these areas. Starting with the cell Cell engineering and electrochemical development are vital to reducing the probability of thermal events. Any measure that reduces the likelihood of dendrite formation would be a good starting point; for example, special coatings on electrodes, electrolyte formulation, etc. Stringent quality control on assembled cells is also critical to minimise any latent manufacturing defects that could lead to internal shorts and eventually a runaway. Cell chemistries behave differently in thermal runaway. Lithium iron phosphate (LFP), for example, has maximum temperatures of about 600 C, significantly lower than nickel manganese cobalt oxide (NMC) with over 1000 C, an expert says. “The particle ejection of an LFP cell is also not as strong as with an NMC cell. This leads to different impacts on the environment in the battery (in terms of propagation), but also in terms of the risk to the occupants.” Normally, the lower energy density chemistries, such as LFP and low nickel content NMC cells, present significantly lower risk, even if one of the cells catches fire, as the adjacent cells are less likely to fail. The fundamental properties of different cell formats and sizes also have to be considered, another expert says. For example, cylindrical and prismatic cells have a vent due to their design, which means a runaway can be better prevented with the support of a current interruption device. Smaller cells have a lower capacity and therefore store less energy. In this respect, the influence of a thermal runaway of a smaller cell is less than that of a larger one. A car fire resulting from a thermal runaway is tackled with Coldcut Systems’ Cobra high-pressure, water mist-injection system (Image courtesy of Coldcut Systems)
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