In this study, we analyzed the thermal characteristics of commercial cylindrical lithium-ion cells using an accelerated rate calorimeter (ARC) and by analyzing the electrochemical impedance spectroscopy (EIS) measurements. During the thermal runaway analysis, the cells were tested at different state of charges (SOCs) and the thermal deterioration was determined by monitoring the impedance at 1kHz and the open circuit voltage as a function of the temperature. The mapping of the thermal runaway was obtained, ant the non-self heating, self-heating and thermal runaway regions were identified at different SOCs. The self-heating and thermal runaway behaviors showed a temperature dependence. In order to clarify the deterioration reaction in the non-self heating region, high temperature storage tests were carried out, storing the cell in the temperature range of 70–100°C. We estimated the activation energy from the discharge capacities before and after the high temperature storage. In order to determinate the charge/discharge activation energy of the cells, EIS measurements were recorded. Based on our study, the activation energy of the deterioration was about two times greater in magnitude than the activation energy of the charge/discharge.