The trade-off characteristic between battery thermal runaway and combustion

Lithium iron phosphate (LFP) lithium-ion batteries are widely believed to be more thermally safe than nickel-rich layered LiNixCoyMnzO2 (NCM) batteries because LFP cathodes are more stable. However, LFP batteries have been reported to cause serious combustion accidents. The relationship between internal thermal runaway and external combustion in LFP and NCM batteries remain unclear. Herein, we found that there is a trade-off between thermal runaway within the battery and external combustion. Cathode oxidizability is linearly correlated with the intensity of thermochemical reactions within battery components. The nickel-rich LiNi0.9Co0.05Mn0.05O2|Graphite (NCM955|Gr) battery exhibits the most intense exothermic reactions and energy release due to the strong oxidation of its cathode, leading to the highest thermal runaway temperature of 862.4°C within the battery. The LiFePO4|Graphite (LFP|Gr) battery with the weak cathode oxidizability shows the lowest temperature of 297.4°C within the battery, but with the highest laminar flame speed of 55cm s−1 outside the battery. The reason for this is that the LFP cathode rarely oxidizes the electrolyte, instead, the electrolyte reacts with the anode primarily, thereby generating substantial reductive gases and accumulating the explosion risks. This work provides the theoretical basis and data support for future battery selection and design.