In-situ gas observation in thermal-driven degradation of LiFePO<sub>4</sub> battery

<p>Lithium-ion batteries are gaining prominence as energy storage needs evolve to meet modern performance and sustainability demands. Lithium iron phosphate batteries, despite their high thermal stability, face safety risks from flammable gas emissions during thermal runaway. Determining the pathways of gas evolution reactions is essential for understanding the thermal runaway mechanism. This study systematically investigates characteristic gas generation pathways through in situ analysis coupled with structural characterization of the LiFePO<sub>4</sub> cathode, proposing six key gas generation reactions involved in the thermal degradation of LiFePO<sub>4</sub> batteries. The internal reaction mechanisms are inherently dependent on environmental conditions, and the product distribution is essentially a probabilistic process. The in-situ analysis shows that ethylene and carbon dioxide are the primary gases produced during thermal runaway, mainly resulting from chemical reactions involving electrolyte decomposition. Diethyl carbonate undergoes concurrent evaporation and thermal degradation, while ethylene carbonate preferentially reacts with active electrode materials. Although cathode structural transformations occur during heating, no direct oxygen evolution was detected in our experimental conditions. The primary thermal runaway drivers are identified as anode-electrolyte reactions that synergistically release heat and gases during 200-300°C. Furthermore, correlation analysis was performed to investigate the source of hydrogen, indicating that a significant amount of hydrogen in cell-level tests was generated by reactions involving metallic lithium and trace water in the reductive environment. These insights advance both fundamental understanding of battery degradation chemistry and practical design of next-generation LiFePO<sub>4</sub> pack systems with intrinsic thermal safety.</p>