Revealing the self-ignition mechanism of lithium iron phosphate battery modules: the coupling effect of battery inconsistency and BMS failure

Lithium iron phosphate (LFP) batteries are widely used in energy storage stations (ESS) and electric vehicles owing to their intrinsic safety and long cycle life. While flame formation during thermal runaway (TR) is rarely observed at the single-cell level, module-level fires have been increasingly reported in operational ESS installations. In this study, we experimentally reproduced spontaneous ignition in LFP modules under conditions of BMS failure and state of charge (SOC) mismatch. Our results show that, although a single LFP cell does not self-ignite during TR, module-level thermal runaway propagation (TRP) can concentrate heat and accumulate electrolytes, thereby creating conditions favorable for ignition. Two primary ignition mechanisms were identified: (1) frictional sparks arising from safety valve ruptures, and (2) arc triggered by pooled electrolytes that cause external short circuits. Furthermore, TRP accelerates heat accumulation and mechanical expansion, forming a positive feedback loop that intensifies fire hazards. Notably, the TRP time interval between successive internal rolls was reduced by 85.5 % (from 241 s to 35 s) once ignition occurred, while the module expansion force increased by 136.3 % compared with the pre-TR state (from 167.4 kgf to 395.6 kgf). These findings challenge the conventional single-cell safety paradigm and highlight the urgent need for revised module-level safety strategies in the design of electrochemical ESS.