Regime Diagram of Li-Ion Battery Thermal Runaway under Emergency Discharge beyond Exothermicity: Nonmonotonicity and Active Risk Control

Abstract Thermal runaway (TR) can be triggered in lithium-ion batteries under abuse conditions, leading to significant safety concerns for electric vehicles and energy storage systems. Emergency discharge has been considered as an effective strategy to mitigate TR by reducing the state of charge (SOC), whereas substantial ambiguities remain due to the wide discharge range beyond normal operation and the complex coupling between electrochemistry and TR. In this work, TR experiments are conducted in an Accelerating Rate Calorimeter (ARC) for commercial 18650 LiFePO4 cells, in a range of high discharge temperatures beyond exothermicity (90–110 °C) and discharge C-rates (0.05–18.75 C). Based on the experimental data, a regime diagram is established to isolate TR and No-TR regimes. As discharge temperature increases, the No-TR regime progressively narrows, indicating increasing TR risk. Meanwhile, for a given discharge temperature, increasing discharge C-rate leads to a nonmonotonic system response for TR occurrence, with TR occurrence in the low and high end of discharge rates, yet no-TR in between. Furthermore, the remaining SOC after discharge is nonmonotonic as discharge rate increases, with intermediate C rates achieving the lowest remaining SOC. Based on energy analysis, cells with similar remaining SOC exhibit nearly identical global kinetics regardless of discharge conditions. Statistical analysis reveals that the voltage-drop temperature collected from all the TR tests follows an approximately normal distribution, providing a risk-driven criterion for defining acceptable discharge windows. Measurement using a Simultaneous Thermal Analyzer (STA) has further demonstrated high consistency between the thermal stability of separator meltdown and the voltage drop temperature. These findings not only establish the regime diagram for TR under discharge, but also provide practical guidance for developing effective emergency discharge strategies for TR mitigation.