Experimental study on the fire phenomenon and the internal structure damage of LiFePO4 and LiNixCoyMnzO2 battery during thermal runaway

LiFePO 4 (LFP) and LiNi x Co y Mn z O 2 (NCM) lithium-ion batteries are widely used in electric vehicles and energy storage applications. And both of the LFP and NCM batteries are easy to catch fire due to their thermal runaway. Currently, explanations for the coupling mechanism between fire phenomena and the internal structure damage of LFP or NCM batteries have not been developed. In this work, fire experiments are employed to investigate the heat release characteristics, fire gases emission patterns, and the internal structural changes of the LFP and NCM batteries with different state of charge(SOC). Experimental results show that both of the total heat release and peak heat release rate during the thermal runaway process of NCM and LFP batteries exhibit a good positive linear correlation with the SOC. With a SOC lower than 50%, during the thermal runaway process the oxidation reactions dominate, and the proportions of CO and carbon dioxide CO 2 in the fire gases are relatively higher. With a SOC higher than 50%, the thermal decomposition reactions dominate, and hydrocarbons and hydrogen make up a relatively large proportion of the fire gases. During their thermal runaway process, the high temperature and pressure generated by the reactions to produce fire gases lead to the destruction of the internal electrode structure of the battery. High SOC and the combustion catalytic effects of NCM materials lead to thermal decomposition reactions dominating and occur at a faster rate, which cause more severe damage to the internal structure of the battery. • LiFePO 4 (LFP) and LiNi x Co y Mn z O 2 (NCM) batteries’ total heat release and peak heat release rate have significant linear correlation with state of charge(SOC). • Clarified SOC’s regulation on fire gases during thermal runaway: SOC>50% dominates electrolyte thermal decomposition (more H 2 /hydrocarbons), SOC<50% favors oxidation (higher CO/CO 2 ) for both batteries. • Revealed differentiated internal structure damage: LFP forms gas channels (SOC<50%)/uniform gaps (SOC>50%), NCM’s Al foil melts/fractures at >500°C causing severe damage.