市販リチウムイオン二次電池の見かけの劣化活性化エネルギーと充放電活性化エネルギーの比較

抄録

An investigation of the thermal deterioration characteristics of a lithium-ion secondary cell is inevitable for its utilization in electric vehicles. An accelerating rate calorimeter study revealed that a thermal runaway of the cell occurs >130°C. We considered that the analysis of the thermal behavior under high-temperature conditions as well as the thermal runaway is indispensable from the viewpoint of safety, but an analysis of the deterioration behavior in the non-heating domain is essential from the viewpoint of the battery’s lifetime. In this study, in order to analyze in detail the deterioration of the non-heating region, thermal-deterioration characteristics of the lithium-ion secondary cell stored at 70 – 100°C were investigated by varying the state of charge (SOC). To evaluate the thermal-deterioration characteristics, we estimated the activation energy from the discharge capacities before and after a heat hazard. On the other hand, impedance spectroscopy measurements of lithium-ion secondary cells at elevated temperature were carried out to determine the activation energy of charge/discharge, which is determined from the electrochemical parameters by using an equivalent circuit. Based on this analysis, the activation energy of the deterioration is known to be about two-times greater in magnitude than the activation energy of the charge/discharge. The charge/discharge and deterioration reactions are independent of each other; however, a comparison of these activation energies is an important element in order to understand the thermal deterioration of lithium-ion secondary cells that takes place in the non-heating region.