The thermal stability of a high-voltage spinel cathode (LiMn1.5Ni0.5O4), synthesized via a sol-gel method, was investigated using Accelerating Rate Calorimetry (ARC) and compared to that of LiMn2O4. Both cathode materials crystallize in the Fd (3) over barm space group with nearly identical surface areas (similar to 0.65 m(2) g(-1)), but they show different microstructures and morphologies that affect their reactivity. In the presence of 1 M LiPF6 in ethylene carbonate (EC): diethyl carbonate (DEC) (1: 2 v/v) electrolyte, both materials show an exothermic surface reaction that is dependent on the cathode morphology, at relatively low temperatures (below 200 degrees C). The onset temperature of the self-heating reaction for Li1-xMn1.5Ni0.5O4 sample is found to be as low as 60 degrees C (compared to 140 degrees C for Li1-xMn2O4), significantly affecting the thermal stability of a whole battery containing LiMn1.5Ni0.5O4 as the cathode. The decomposition of the spinel material takes place at 195 degrees C for Li1-xMn1.5Ni0.5O4 and at 215 degrees C for Li1-xMn2O4, with significantly higher self-heating rates for Li1-xMn1.5Ni0.5O4 than for LiMn2O4. Our results show that, above 200 degrees C, Ni4+ is reduced to the more stable Ni2+ oxidation state and the oxygen released from the cathode during this reaction fuels the combustion of carbonate solvents. (C) 2016 The Electrochemical Society. All rights reserved.