Laser irradiation illuminates uncertain degradation, leading to thermal runaway of 18650 cells charged/discharged at low temperatures

Abstract

The safety of Li-ion secondary cells decreases during charging and discharging at low temperatures due to the deposition of Li metal that reduces the cell thermal stability. Generally, the thermal stability of Li-ion cells is evaluated by an accelerating rate calorimeter (ARC). Therefore, in this study, degraded cells with low capacities have been prepared by charge/discharge cycling at a low temperature. After confirming the decrease in thermal stability by ARC heating, the degraded cells are charged at room temperature, and a thermal runaway is initiated by both ARC heating and laser irradiation. During ARC heating, the exothermic behavior of the degraded cell is identical to that of a new cell; however, the degraded cell undergoes a thermal runaway after laser irradiation at an input energy 100 times lower than that required to induce the runaway of a new cell. Therefore, the degradation of cells due to charge/discharge cycling at low temperatures can be detected by the laser irradiation method despite the decreased thermal stability undetectable by the ARC.