Research on the accelerated thermal runaway mechanism and fire-explosion risk mode of NCM523 lithium-ion batteries induced by rapid penetration under separator melting condition

The separator melting (or critical thermal load) is a key point in the thermal runaway (TR) process of lithium-ion battery (LIB). In this state, rapid penetration tends to accelerate the TR process, making the TR mechanism and evolution dynamics more complex, and significantly increasing the difficulty of early warning, prevention and control. In order to reveal the accelerated thermal runaway mechanism and fire-explosion risk of LIB induced by rapid penetration under separator melting condition (set as the initial thermal load of 100℃), an 18650 LIB coupling stimulation TR experimental platform was independently built. Taking NCM523 batteries with different state of charge (SOC) as the research object, the TR behavior: smoke diffusion, spark injection and flame propagation characteristics induced by rapid penetration under critical thermal load were experimentally studied. The results show that the temperature change of the batteries surface during the TR process can be divided into three typical stages: slow temperature rise, rapid temperature rise, and slow temperature drop. The maximum temperature of the batteries surface increase with the increase of SOC, from 122.4℃ at 0 % SOC to 691.2℃ at 100 % SOC. The temperature rise rate of the batteries also increases with the increase of SOC, with the maximum increase of 91.2℃/s. This phenomenon is attributed to the increase of power and the feedback heating behavior of the emitted flue gas/flame. According to the temperature rise rate of the batteries, the TR state induced by rapid penetration NCM523 batteries at 100℃ were divided into mild, moderate and severe TR. With the increase of SOC, the mass loss of the batteries after TR increased by 10.775 g, and the mass loss rate increased by 24.17 %. NCM523 batteries showed complex eruption behavior, with the increase of SOC, the eruption behavior of severe TR batteries experienced smoke diffusion mode, transverse spark jet mode, longitudinal spark jet mode and flame jet mode, and the spark jet area also increased significantly, with the maximum area increased by 41.44 %. The complex and changeable eruption dynamics make the TR of NCM523 battery presents the dual risks of smoke explosion of lithium iron phosphate battery and jet fire of high nickel LIBs. The research results provide important theoretical guidance and practical value for enriching the TR failure mechanism of NCM523 lithium-ion battery under extreme condition, carrying out fire and explosion risk assessment, and proposing fire and explosion risk prevention and control measures.