A systematic investigation of thermal runaway characteristics, inflame evolution and post-mortem analysis caused by overcharging for lithium-ion batteries with different cathode materials

In this work, three pouch batteries based on different cathode materials including LiCoO₂ (LCO), Li(Ni_0.6Mn_0.2Co_0.2)O₂ (NMC622), and LiFePO₄ (LFP) are studied. The thermal runaway (TR) characteristics, flame evolution, internal morphology and functional group changes during the TR process caused by overcharge are comprehensively compared for the first time. The LFP battery takes the shortest time to reach TR under overcharging conditions, followed by the LCO battery, and the NMC622 battery. However, the TR behaviour of LiFePO₄ batteries is the safest, as they do not ignite but instead generate a large amount of smoke. The highest temperature reached is only about 310 °C. In contrast, LCO and NMC622 batteries undergo severe combustion processes with peak temperatures exceeding 1000 °C. Sparks and jet fires are the predominant flame forms, with jet fire in particular being a major factor in the propagation of TR. The dynamic changes of flame characteristic parameters are also quantitatively analysed, including maximum temperature, inflame height, inflame area and inflame probability map. With the increase of charging rate, the flame spread for the LCO and NMC622 batteries is bigger. Under the overcharge condition of 2 C, LCO batteries exhibit the highest flame height of 341.7 mm, while NMC622 batteries exhibit the largest flame area of 0.3 m². After TR, drastic morphological and chemical changes are observed for the LCO and NMC622 cathodes, while the LFP cathode maintained a better dense structure with minor chemical changes.