Study on the propagation characteristics of electrothermal-coupled thermal runaway in lithium-ion battery modules

Thermal runaway propagation within lithium-ion battery modules endangers electric vehicles and energy storage systems. The state of charge (SOC) and charge/discharge rates are key operational parameters that considerably influence battery thermal stability. However, the combined electrothermal effects of these parameters on the propagation behavior of thermal runaway, especially under dynamic charging or discharging conditions, remain insufficiently understood. This study aimed to quantify how different SOC levels and charge/discharge rates affect the propagation characteristics of thermal runaway in ternary 18650 battery modules and to reveal the underlying electrothermal coupling mechanisms.

Commercial ternary 18650 lithium-ion battery cells (nominal capacity: 2.6 Ah) were assembled into modules comprising four cells connected in series with a fixed spacing. Thermal runaway was initiated by locally overheating the first cell using a heating film. The experiments were conducted under a controlled ambient temperature of 25±2 ℃. Four SOC levels (25%, 50%, 75%, and 100%) were tested under static conditions (no current). For the charge/discharge rate tests, modules at 100% SOC were subjected to galvanostatic charging or discharging at 1 C and 2 C during thermal runaway propagation, with a no-current condition serving as the baseline. Temperature evolution was recorded using K-type thermocouples attached to the center of the surface of each cell and to the busbars. The voltage of each cell was monitored simultaneously. The module mass before and after each experiment was measured using a high-precision balance. Propagation time was defined as the interval between the onset of the temperature increase in the triggering cell and that in the adjacent cell. The trigger temperature and maximum cell temperature were extracted from the temperature curves. Each experimental condition was repeated three times to ensure reproducibility.

The results showed that charging accelerated thermal runaway propagation, whereas discharging delayed it. At charging rates of 1 C and 2 C, the average propagation time decreased by 7% (from 45.2 s to 42.0 s) and 12% (to 35.3 s), respectively, compared with the no-current condition. Concurrently, the trigger temperature of the adjacent cell decreased by 5.8 ℃ (1 C) and 12.4 ℃ (2 C), whereas the maximum temperature of that cell increased by 12.2 ℃ and 24.5 ℃, respectively. In contrast, at discharging rates of 1 C and 2 C, the propagation time increased by 17.3% (to 53.0 s) and 19.6% (to 54.1 s), respectively, compared with the no-current condition. The trigger temperature increased by 9.1 ℃ (1 C) and 10.6 ℃ (2 C), whereas the maximum temperature decreased by 15.4 ℃ and 18.9 ℃, respectively. The mass loss measurements indicated that higher SOC levels and charging rates aggravated electrolyte venting and internal material ejection. At 100% SOC, the accelerating effect of charging was most pronounced, whereas at a low SOC (25%), thermal runaway propagation was considerably suppressed regardless of the current condition.

The observed differences originated from the chain-reaction propagation characteristics of thermal runaway and the opposing feedback polarities under electrothermal coupling mechanisms. During charging, Joule heating generated by the high current, combined with increased reversible capacity, enhanced internal side reactions (e.g., solid electrolyte interphase decomposition and anode–electrolyte reactions), forming a positive feedback loop that accelerated heat accumulation and thermal runaway propagation. During discharging, the capacity decreased as the cell delivered energy, reducing the available lithium inventory and mitigating the intensity of subsequent side reactions, thereby creating a negative feedback effect that slowed propagation. Higher charging rates (2 C vs. 1 C) amplified the Joule heating effect, further accelerating propagation. Higher discharging rates increased capacity reduction, but the propagation delay saturated above 1 C because of competing ohmic heating. These findings provide quantitative insights for designing safer battery thermal management systems and suggest that controlled discharging or low-SOC operation can serve as emergency mitigation strategies for delaying thermal runaway propagation. Future work should explore the influence of module configuration and cooling conditions.