Thermal runaway in commercial lithium-ion cells under overheating condition and the safety assessment method: Effects of SoCs, cathode materials and packaging forms

Overheating, a common field failure of lithium-ion (Li-ion) batteries, can lead to thermal runaway and catastrophic results. Here, overheating behaviors and thermal runaway features of Li-ion cells with different states of charge (SoCs), cathode materials (LiFePO₄(LFP), Li[Ni_0.5Co_0.2Mn_0.3]O₂ (NCM523)), and packaging forms (pouch and prismatic) are investigated. The overheat-to-thermal runaway process of Li-ion cells can generally be divided into four stages, and the overheating behaviors of the Li-ion cell at each stage are monitored and analyzed. The results show that, under the same overheating condition, with the increase of SoC, the tolerance of LFP Li-ion pouch cell to high-temperature becomes poorer, while the mass loss ratio (i.e. severity of thermal runaway reactions) and the maximum temperature (i.e. thermal runaway hazards) increase. When thermal runaway occurs, the NCM Li-ion cell catches fire and much smoke and sparks are ejected from the rupture, while the LFP Li-ion cells just release a lot of white smoke; the NCM Li-ion cell is more hazardous and exhibits more severe thermal runaway reactions. With respect to the prismatic Li-ion cell, it takes a long time from the opening of the relief valve to the occurrence of thermal runaway although an internal short circuit has occurred; the designed pressure relief valve significantly delays the occurrence of thermal runaway and reduces the thermal runaway risk. In addition, postmortem and SEM analyses are performed on the electrodes harvested from the dead cells after the overheating tests. According to the overheating behaviors and the evolution of characteristic parameters, a modified safety assessment method is proposed. This study can provide important guidance for the safety management of Li-ion cells with different SoCs, cathode materials and packaging forms under the condition of overheating in automotive applications.