Experimental study on the thermal runaway acceleration mechanism and characteristics of NCM811 lithium-ion battery with critical thermal load induced by nail penetration

NCM811 (Li(Ni_0.8Co_0.1Mn_0.1)O₂) lithium-ion battery (LIB) at 100 °C is in a critical state of internal chemical reaction and external thermal runaway (TR), and the coupled stimulations of nail penetration under such thermal load will accelerate TR, and coupled stimulations have hindered the development of LIBS. In this paper, an experimental platform for coupled stimulations of heat-penetration on LIBs was built, and revealed the thermal runaway acceleration mechanism, explored the influence of the SOC on the TR behavior of the cells when penetration under the critical thermal load condition. The results show that critical thermal load condition reduces the critical SOC for TR to occur, and 25% SOC NCM811 cell still produces a jet flame, elevating the fire risk of thermal runaway when compared to room temperature conditions. And the maximum temperature of 25% SOC cell was elevated by 76.1 °C, which was 18% higher than that of penetration at room temperature. Meanwhile, at critical 100 °C, as the SOC increases from 0% to 100%, the average temperature rise rate of the cell sharply increases from 1.992 °C/s to 93.033 °C/s, and the maximum temperature of cell increases from 125.9 °C to 652.9 °C, and the mass loss increases from 3.332 g to 31.180 g. The 0%SOC cell undergoes slighter TR, generating a lot of smoke but no flame. However, with the SOC increase of 25%–100%, the flame temperature increases from 437.8 °C to 918.5 °C, the flame area rise ratio reaching 281.77%. Combined with the microscopic performance characterization experiments, the dynamics behavior of particle eruption is mainly dominated by the anode graphite. The results of this study provide scientific guidance for the safety prevention of LIBs.