Investigation on the thermal runaway evolution Mechanism, gas generation behavior, and multi-medium suppression performance of lithium-ion batteries under immersion conditions

With the rapid advancement of lithium-ion batteries, the risks of fire and explosion triggered by thermal-runaway-induced off-gassing have become increasingly prominent, particularly under overcharge conditions. Compared with conventional fire-extinguishing methods, immersion suppression offers potential in cooling, thermal propagation inhibition, and gas control, but its mechanism remains unclear. Therefore, this study examined thermal runaway evolution in 52 Ah LiFePO₄ batteries under heating, overcharge, and immersion-overcharge conditions, explored the immersion suppression mechanism, and to evaluate the suppression performance differences among five immersion media (deionized water, FW-1800, water–ethylene glycol mixture, dimethyl silicone oil, and KOSTCHILL). Results show overcharge yields the highest gas (2.0591 mol) and heating the highest temperature (457 °C). Water immersion reduces gas to 1.6087 mol, peak temperature to 98 °C, and extends the thermal runaway window by 83.3 %. In terms of gas generation, immersion effectively suppresses high-temperature-dependent gases like H₂ (1.185 → 0.832 mol) and CO (0.085 → 0.075 mol). Different immersion media show distinct suppression behaviors. Water-based systems perform better in temperature and gas suppression, suitable for single cells; FW-1800 performs best (T_max = 88 °C, V_total = 1.1904 mol). Oil-based systems offer superior insulation (0 μS/cm), suitable for high-voltage modules; KOSTCHILL performs best (T_max = 160.6 °C, V_total = 1.657 mol). This study provides theoretical and data support for developing immersion thermal safety technologies and optimizing immersion media for energy storage systems.