Experimental Study on Effect of State of Charge on Thermal Runaway Characteristics of Commercial Large-Format NCM811 Lithium-Ion Battery

The application of Li(Ni_0.8Co_0.1Mn_0.1)O₂ (NCM811) cathode-based lithium-ion batteries (LIBs) has alleviated electric vehicle range anxiety. However, the subsequent thermal safety issues limit their market acceptance. A detailed analysis of the failure evolution process for large-format LIBs is necessary to address the thermal safety issue. In this study, prismatic cells with nominal capacities of 144Ah and 125Ah are used to investigate the thermal runaway (TR) characteristics triggered by lateral overheating. Additionally, TR characteristics under two states of charge (SoCs) (100% and 5%) are discussed. Two cells with 100% SoC exhibit similar characteristics, including high failure temperature, high inhomogeneity of temperature distribution, multi-points jet fire, and significant mass loss. Two cells with 5% SoC demonstrate only a slight rupture of the safety valve and the emission of white smoke. However, there is a difference: the 144Ah cell still has a significant temperature rise inflection point. Moreover, apart from the early voltage drop before the sudden temperature rise, the environmental pressure shows an even earlier rise. The above phenomenon means the pressure signal could also be used as a valid indicator to predict potential thermal failure events. Interestingly, the ablation traces on the aluminum casing of the TR cells show a spreading arch shape on both the side and bottom surface. In summary, the results indicate that the batteries with a high SoC exhibit a more severe mode of failure and a faster failure evolution process. The research provides a comprehensive insight into the TR evolution process and has specific guiding implications for future TR warnings and thermal safety accident traceability work.