Stage-resolved direct current resistance evolution for thermal runaway warning in commercial LiFePO₄ cells under thermal abuse

Thermal runaway warning under abuse conditions is essential for the safe application of lithium-ion batteries in electric transportation and stationary energy storage systems. From a process-safety perspective, practical warning methods should not only be sensitive to internal degradation, but also provide sufficiently early information for intervention before severe failure develops. However, most existing warning methods still suffer from delayed response, high hardware complexity, or limited onboard applicability. In this work, the stage-resolved evolution of direct current resistance (DCR) is investigated as a thermally responsive indicator for thermal runaway warning in commercial 18650 LiFePO₄ cells under thermal abuse. By combining pulse-current-based DCR measurements with electrochemical impedance spectroscopy and post-mortem material characterization, the coupling among temperature rise, internal degradation, and resistance evolution is systematically clarified. The results show that DCR exhibits a characteristic three-stage evolution during thermal abuse, including an early Arrhenius-type decrease in the low-temperature region, an intermediate plateau/slight-rise region consistent with the competing effects of thermally enhanced ion transport and degradation-induced resistance increase, and a sharp high-temperature rise mainly associated with separator shutdown and structural deterioration. Based on these stage-specific electro-thermal characteristics, a three-level warning strategy is established for quiescent thermal abuse, while a separate warning strategy is developed for dynamic charge-discharge conditions based on the coupled DCR-temperature-SOC relationship. Under quiescent thermal-abuse conditions, the DCR transition features preceded safety-valve triggering by 312–988 s. Under dynamic conditions, the proposed strategy achieves a temperature prediction error below 5.61%. These results demonstrate that DCR can provide a low-cost and practically deployable route for thermal-safety warning and diagnosis in commercial 18650 LFP cells under thermal abuse.