The Evolution of Lithium-Ion Cell Thermal Safety with Aging Examined in a Battery Testing Calorimeter

The effect of calendar aging on the thermal safety of 4.6 Ah pouch cells with a LiMn2O4 (LMO) cathode was investigated by a battery test calorimeter (BTC) that can be used to determine the heat evolved during an uncontrolled exothermic runaway reaction. Cells were stored at 55 degrees C and 100% state of charge (SOC) for accelerated aging, and they were taken out after 10, 20, 40, 68, and 90 days of storage to obtain different aging states. Those cells were then put into the BTC for thermal safety tests. The results show the cell thermal safety improves after aging: (1) the self-heating temperature increases; (2) the thermal runaway temperature increases; and (3) the exothermal rate during the process of thermal runaway decreases. The cell voltage drops to zero about 40 degrees C earlier than the thermal runaway, indicating the voltage can be used as a signal for cell safety monitoring.