Abstract: The safety of lithium-ion batteries is very important. In this study，the adiabatic rate calorimeter was used to explore the thermal runaway behavior of 21700 lithium-ion batteries with states of charge（SOC） of 0%，25%，50%，75% and 100% respectively. Meanwhile，the thermal stability of the separator was analyzed. It was demonstrated that with the increasing of SOC，the onset temperature of side reaction was lower，the initial temperature point of the thermal runaway was lower，and the maximum temperature was higher. The higher the SOC，the higher the temperature rise rate when thermal runaway occurred，the more violent the reaction，and the more mass loss. The results showed that the internal short circuit caused by the separator melting was not the underlying reason for the thermal runaway of lithium-ion batteries. The heat released by the short circuit would accelerate the occurrence of side reactions. The underlying reason for the thermal runaway is the reaction between the electrode material and electrolyte， and the heat released by subsequent exothermic side reactions.

Key words: lithium-ion battery, state of charge, calorimeter, thermal runaway