Two kinds of nongraphitizable carbon were applied as active materials of negative electrodes in Li-ion batteries. The thermal properties of cycled electrodes mixed with or without commercial 1M LiPF6/EC–DMC electrolyte were investigated by TG–DSC from room temperature to 400°C at a heating rate of 5°C/min. Both kinds of lithiated electrodes showed exothermic peaks at about 310°C due to decomposition of SEI by lithiated carbon powder. For the mixture of delithiated electrodes and electrolytes, the heat generation was attributed mainly to thermal decomposition of the electrolyte at about 280°C. But for the mixture of lithiated electrodes and electrolytes, thermal risk mainly came from the reaction between original/secondary SEI and intercalated Li ions, which caused drastic heat generation at about 285°C. Moreover, the thermal behavior of the mixture of cycled electrodes and electrolytes was directly related to the ratio between electrodes and coexisting electrolytes. When the ratio between electrodes and electrolytes was suitable, an endothermic reaction between SEI and electrolytes became dominant and depressed other exothermic heat, which reduced the thermal risk of the mixture of electrodes and electrolytes at elevated temperature.