Investigation on gas generation and corresponding explosion characteristics of lithium-ion batteries during thermal runaway at different charge states

This study investigates the gas generation characteristics and explosion limits of the gas generated by 18650type LiNi1/3Co1/3Mn1/3O2 (NCM) cells during thermal runaway (TR) at different states of charge (SOCs). An accelerating rate calorimeter is employed to initialize TR, together with an airtight jar for gas measurement. Based on the detected gas composition for NCM cells, the corresponding explosion limits are computed and further compared with those of LiFePO4 (LFP) and LiNi0.80Co0.15Al0.05O2 (NCA) cells. The results reveal that the gas generation rates are slightly higher for NCM cells with lower SOCs prior to the violent TR processes compared to those for high-SOC cells, primarily due to the elevated reaction temperature. Moreover, during violent TR, multiple-peak features are observed for the gas generation rate curves, especially at high-SOC conditions. On the other hand, typical Z-shaped explosion limit curves are observed for all generated mixtures. As the cathode material varies from NCM to LFP and NCA, the explosion limits shift from high-temperature regions to low-temperature regions. In contrast to common beliefs that the generation gas at high SOCs is more explosive, non-monotonic responses of the explosion limit curves are found with respect to cell SOC. The significance of minor components of the generated gas, including ethylene and ethane, in reducing the explosivity of the mixture, is highlighted through sensitivity analyses.