Quantity effect of thermal runaway in sodium-ion battery modules: Synchronization failure and Hazard escalation driven by thermal anisotropy

This study systematically investigates how the number of sodium-ion cells influences thermal runaway (TR) behavior in sodium-ion battery (SIB) modules, with emphasis on how gas-generation characteristics, TR propagation, and explosion hazard evolve with cell count and spatial arrangement. Results indicate that the effect of cell number is manifested primarily through its impact on the TR propagation mode: as the number of cells increases—particularly under two-row layouts—the pathway shifts from sequential triggering to synchronous triggering. Under sequential TR, the jet-induced plume merging phenomenon between adjacent cells was identified for the first time. The module exhibits a temperature rise beyond 100 °C in rear-surface relative to a single cell; the peak temperature increases from 502 °C to 660 °C, and the rear-surface maximum temperature rise rate (dT/dt)max accelerates from 4.5 °C/s to 9–10 °C/s. These trends indicate a pronounced runaway-stacking effect after cell aggregation: local temperatures surge, in some regions exceeding the melting point of the aluminum casing, thereby breaching the enclosure, compromising cell structural integrity, and escalating the hazard. By contrast, under synchronous TR, the peak temperature (Tmax), (dT/dt)max, and gas-concentration peaks surge: Tmax exceeding 810 °C, (dT/dt)max rises to 21 °C/s, and both the peak concentration and generation rate of gas increase by >400%. This behavior originates from a large mismatch between intra-row and inter-row heat-transfer rates, such that intra-row TR propagates roughly an order of magnitude faster than inter-row transport, yielding sequential TR within a row but near-simultaneous TR across rows in the SIB module. Critically, synchronous TR culminated in an explosion, escalating the event severity. The cause was a rapid rise in vent-gas concentration during simultaneous TR, yielding a confined-space mixture of 16.1%, which exceeds the calculated lower explosive limit (LEL) of 4.94%; concurrently, copious hot sparks generated by synchronous TR provided ignition and explosion. Accordingly, module-level TR protection for SIBs must be strengthened—particularly ventilation and deflagration-mitigation measures—to preclude explosive outcomes.