External pressure effects on thermal runaway in prismatic LiFePO4 batteries: Mechanistic insights for safer battery systems in electric vehicles

External pressure significantly influences the thermal runaway (TR) behavior of lithium-ion batteries (LIBs). However, the underlying mechanisms by which external pressure affects exothermic reactions, heat transfer, and gas generation during TR remain to be fully clarified. In this study, the mechanistic effects of external pressure on TR in prismatic lithium iron phosphate (LFP) batteries were systematically investigated through thermal analysis, time-resolved gas chromatography, and postmortem material characterization. Results indicate that external pressures of 0.1 and 0.2 MPa enhance interfacial contact within the battery, thereby increasing internal thermal conductivity. This improvement results in a more uniform temperature distribution, which raises the TR initiation temperature and shifts the initial TR location inward from the battery edge toward the center. However, external pressure accelerates thermal runaway propagation (TRP), with propagation speed at 0.2 MPa increasing by approximately 65 % compared to 0 MPa. Moreover, gas evolution analysis reveals a substantial reduction in total gas yield with increasing external pressure, exhibiting decreases of about 28 % at 0.1 MPa and 53 % at 0.2 MPa relative to 0 MPa. This reduction is primarily attributed to earlier safety venting and prolonged electrolyte evaporation periods. Postmortem characterization highlights intensified exothermic reactions under elevated external pressure, reflecting deeper electrode material degradation. These findings highlight the risk-mitigation effect of external pressure, thereby lowering explosion risk despite the acceleration of TRP, and inform the design and modeling of safer battery systems under realistic mechanical constraints.