锂离子电池因其循环寿命长、能量密度高和自放电率低等优势，被广泛应用于电子产品、交通工具和电网系统等领域。然而，锂离子电池自身材料体系中包含大量易燃成分，滥用条件会引发电池热失控，导致燃烧、爆炸现象的发生，其中过充是导致电池热失控的主要原因之一。近年来，由过充锂离子电池热失控引发的火灾事故频发，严重阻碍了锂离子电池的推广应用。因此，亟需对过充锂离子电池热失控时的火灾安全特性进行研究，为锂离子电池安全使用、火灾预警与防控提供理论支撑。 本文开展了18650型LiNi0.5Co0.2Mn0.3O2（NCM523）过充锂离子电池热失控试验。利用充放电循环仪以不同的截止电压（4.2 V、4.5 V、4.8 V和5.1 V）、充电倍率（0.5 C、1 C、2 C）以及循环次数（1次、10次和20次）对锂离子电池进行过充处理，搭建了锂离子电池热失控试验平台，开展了过充锂离子电池热失控火灾试验。记录并分析过充电池热失控的燃烧现象、质量损失、电池表面温度、羽流温度、热失控节点时间和气体含量等特征参数，讨论截止电压、充电倍率以及循环次数对锂离子电池火灾特性的影响。试验结果表明：过充锂离子电池热失控过程分为被动加热阶段、安全阀破裂阶段、烟气阶段、燃爆阶段和熄灭阶段五个阶段；过充的截止电压越高、过充倍率越大、循环次数越多，锂离子电池热安全性越低，越容易发生热失控，且与热失控剧烈程度成正相关；过充的截止电压越高、过充倍率越大、循环次数越多，电池的表面最高温度越低，羽流最高温度越高；循环次数对电池安全性影响最大，过充循环20次后的电池热失控燃烧现象最剧烈、质量损失率最高，其与正常充电电池相比，热失控时间提前255 s，热失控温度降低69.5 ℃。 本文开展了过充锂离子电池热失控仿真模拟研究。基于COMSOL Multiphysics软件建立了过充锂离子电池热失控模型，模拟分析了不同截止电压（4.2 V、4.5 V、4.8 V和5.1 V）、充电倍率（0.5 C、1 C、2 C）、循环次数（1次、10次和20次）过充下的18650型NCM523锂离子电池高温热失控过程。结果表明：温度变化曲线与试验过程中温度变化曲线基本一致；热失控模拟过程中副反应的发生时间会随着过充截止电压、充电倍率、循环次数的增加而提前；副反应是引起电池热失控的直接因素，正极和电解液副反应的产热是电池热失控的主要热源。

Lithium-ion batteries are widely used in electronic products, transportation and power grid systems due to their long cycle life, high energy density and low self-discharge rate. However, the lithium-ion battery itself contains a large number of flammable components in the material system, the battery is abused conditions will trigger thermal runaway, resulting in combustion, explosion phenomenon. Overcharging is one of the main causes of thermal runaway in batteries. In recent years, fire accidents caused by thermal runaway of overcharged lithium-ion batteries have occurred frequently, seriously hindering the popularization and application of lithium-ion batteries. Therefore, there is an urgent need to study the fire safety characteristics of overcharged lithium-ion battery thermal runaway, to provide theoretical support for the safe use of lithium-ion batteries, fire prevention and control. In this paper, thermal runaway test of 18650 type LiNi0.5Co0.2Mn0.3O2 (NCM523) overcharged lithium-ion battery was carried out. A charge-discharge cycler was utilized to overcharge lithium-ion batteries with different cut-off voltages (4.2 V, 4.5 V, 4.8 V and 5.1 V), charge multiplicities (0.5 C, 1 C, 2 C) and cycle times (1, 10 and 20 times). A thermal runaway test platform for lithium-ion batteries was set up, and a thermal runaway fire test for overcharged lithium-ion batteries was carried out. Characteristic parameters such as combustion phenomena, mass loss, cell surface temperature, plume temperature, thermal runaway node time, and gas content of thermal runaway in overcharged batteries were recorded and analyzed, and the effects of cutoff voltage, charge multiplication, and number of cycles on the fire characteristics of lithium-ion batteries were discussed. The test results show that: the thermal runaway process of overcharged lithium-ion battery is divided into five stages: passive heating stage, safety valve rupture stage, smoke stage, ignition stage and extinguishing stage; the higher the cut-off voltage of overcharging, the larger the overcharging multiplicity, and the higher the cycle times, the lower the thermal safety of lithium-ion battery, the more prone to thermal runaway, and it is positively correlated to the degree of thermal runaway; the higher the cut-off voltage of overcharging, the larger the overcharging multiplicity, and the higher the cycle times, the more likely to be thermal runaway of the battery. The higher the overcharge cut-off voltage, the higher the overcharge multiplier, the more cycles, the lower the maximum surface temperature of the battery, the higher the maximum temperature of the plume; the number of cycles has the greatest influence on the safety of the battery, and the thermal runaway combustion phenomenon is the most intense and the highest quality loss rate of the battery after 20 cycles of overcharge, and the time of thermal runaway is advanced by 255 s, and the temperature of the thermal runaway is lowered by 69.5 ℃ compared to that of the normal rechargeable batteries. In this paper, a simulation study of thermal runaway of overcharged lithium-ion batteries was carried out. Based on COMSOL Multiphysics software, a thermal runaway model of overcharged lithium-ion batteries was established, and the high-temperature thermal runaway of 18650-type NCM523 lithium-ion batteries was simulated and analyzed under different cutoff voltages (4.2 V, 4.5 V, 4.8 V, 5.1 V), charging multiplicities (0.5 C, 1 C, and 2 C), and the number of cycles (1, 10, and 20 times) of overcharge. process. The results show that: the temperature change curve is basically consistent with the temperature change curve during the test; the occurrence time of the side reaction in the thermal runaway simulation process will be advanced with the increase of the overcharge cut-off voltage, the charging multiplication rate, and the number of cycles; the side reaction is a direct factor causing the thermal runaway of the battery, and the heat generated by the side reaction of the negative electrode, positive electrode, and the electrolyte is the main source of heat in the battery thermal runaway.

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