The thermogravimetric (TG) and differential thermal analysis (DTA) were used to investigate the thermal stability of fully charged and discharged LiCoO2 cathode and graphite anode in nitrogen and air atmospheres. The results showed that the weight of charged and discharged LiCoO2 cathode samples exhibited an obvious decrease between 100 and 120°C in two atmospheres. The exothermic decomposition reaction of fully charged LiCoO2 cathode occurred at 250°C in two atmospheres. A small decomposition reaction of the discharged LiCoO2 cathode occurred at 300°C. When the temperature of samples was elevated to 600°C, the weight of fully charged and discharged LiCoO2 cathode in air atmosphere did not change; while the weight of samples in nitrogen atmosphere decreased. This was because the Co3O4 as the decomposition product of the cathodes could be reduced to CoO by the carbon black above 600°C in N2 atmosphere. The solid electrolyte interphase (SEI) film of fully charged and discharged graphite anode was decomposed at 100–120°C in two atmospheres, and the weight loss of fully charged graphite anode at 100–120°C was obviously less than that of the fully discharged graphite anode. When the samples were heated to 300°C, there was no fierce exothermic reaction for the lithiated graphite anode in N2 atmosphere, whereas an exothermic reaction in air atmosphere occurred rapidly.