Acceleration mechanism of the thermal runaway propagation in an enclosed LIB cluster and the fire disadvantage of liquid immersion

Thermal runaway propagation (TRP) in lithium-ion battery (LIB) modules presents a major safety concern, particularly in confined environment where heat and ejecta cannot dissipate effectively. This study systematically investigates the heat transfer mechanisms driving TRP under different conditions: open vs. confined spaces, inert vs. oxidative atmospheres, and full-immersion cooling scenarios. TRP in open space often terminates due to rapid heat dissipation, while confinement accelerates propagation significantly through sustained thermal feedback from scorching gases. The propagation rate of the rear rows is up to 9–11 times faster than that of front rows. Decoupling analysis revealed that heat transferred from the surrounding gases contributes more than 90% of the total heat received by a cell before TR, far outweighing radiative effects. Solid ejecta enhanced localized heating but played a secondary role compared to hot gases. Full-immersion with #5 Industrial White Oil has critical dual effects: the oil delays the TR of the triggered cell by about 1600 s, but once ignited it can lead to a much more catastrophic fire with peak HRR increasing from 16.76 kW to 366.62 kW. This study underscores the crucial role of gas-phase heat transfer, and the heat transfer pathways are quantified, which provide valuable advice to mitigate the TRP of an enclosed LIB cluster.