Effect of overcharge on the electrochemical and thermal safety behaviors of LiNi0.5Mn0.3Co0.2O2/graphite lithium-ion batteries

• The thermal safety behavior of the overcharged lithium-ion battery was investigated. • The relationship between battery overcharge and thermal safety behavior was analyzed. • The increase in overcharge cycles delayed the rupture of the battery safety valve. • The increase in overcharge cycles shortened the time for smoke and thermal runaway. • The increase in overcharge cycles increased the thermal runaway flame temperature. Lithium-ion battery (LIB) safety problems caused by overcharge have seriously affected its application. In this study, the electrochemical and thermal safety characteristics of overcharged LIBs are studied. The battery is charged and discharged for different cycles under the overcharge cut-off voltage of 4.8 V. Then, through the stimulation of the external heat source, the thermal safety and internal mechanism under different overcharge cycles are investigated. The results show that when the overcharge cycle is 1, the change in battery capacity and safety is relatively small. However, with the increasing of overcharge cycles, the battery exhibits obvious capacity loss, and apparently cathode damage and side reactions can be detected. When the overcharge cycle reaches a certain limit, the battery capacity begins to decrease sharply. The dV/dQ and EIS results indicate that the increase of overcharge cycle accelerates the damage of electrode materials and the side reactions, which result in the blockage of lithium ion transport channel, the substantial increase in impedance, and the degradation of chemical kinetics. The above inferences are also confirmed by the XRD and SEM results. These changes lead to the consumption of electrolyte, resulting in a decrease in the internal air pressure of battery during the heating process, and a delay in the opening time of the battery safety valve. However, the consumption of electrolyte and the growth of solid electrolyte interphase layer also lead to the production of combustible gas and the decrease in the thermal stability of battery. Therefore, as the overcharge cycle increases, the interval time between the gas release and the thermal runaway (TR) becomes shorter and shorter, which finally leads to the acceleration of the TR reaction and the increase in the TR combustion intensity.