The exothermic reaction mechanism of the delithiated cathode LiNi0.8Co0.15Al0.05O2 in the presence of an electrolyte has been investigated. The contributions of the delithiated cathode and electrolyte to the overall thermal runaway mechanism were evaluated using an accelerating rate calorimeter and differential scanning calorimeter (DSC). The cathode LiNi0.8Co0.15Al0.05O2 was also chemically delithiated to evaluate the influence of each component of the composite cathode as well as the structural changes during the exothermic reaction. Thermogravimetric analysis results confirmed that delithiated LiNi0.8Co0.15Al0.05O2 lost its weight at elevated temperatures, varying with the state of charge. The X-ray diffraction and DSC studies show that the exothermic reaction of the delithiated cathode sample started very close to the onset temperature of the structural change in the delithiated cathode as detected by the X-ray powder diffraction. Based on the results of this study, a reaction mechanism leading to the thermal runaway of the delithiated cathode is also proposed.