Samples of the layered cathode material, Li[NixCO1 Mn--2x(x) 02 (X = 0. 1, 0.2, 0.35, 0.45, and 0.5), were synthesized at 900 degrees C, fabricated into electrodes, and charged to both 4.2 and 4.4 V vs. Li/Li+. The charged electrode materials were rinsed to remove the electrolyte salt and then added, along with ethylene carbonate (EC)/diethyl carbonate (DEC) solvent or I M LiPF6 EC/DEC, to stainless steel accelerating rate calorimetry (ARC) sample holders that were welded closed. ARC shows that there is no significant exothermic reaction between Li-0.5[Ni-0.1 Co0.8Mn0.1]O-2 (x = 0. 1, 4.2 V) and EC/DEC solvent or 1M LiPF6 EU/DEC until approximately 170 degrees C, compared to a 140 degrees C onset temperature for LiCoO2 charged to 4.2 V. As x increases from 0.1 to 0.2, the onset temperature for materials charged to 4.2 V does not change significantly, but the main exothermic peak shifts significantly to higher temperatures. However, the thermal stability of Li[NixCo1 Mn--2x(x)] 02 with x = 0. 1 is significantly worse for samples charged to 4.4 V. By contrast, samples with x = 0.2, 0.35, 0.45, or 0.5 show almost identical excellent thermal stability at both 4.2 and 4.4 V. (c) 2005 The Electrochemical Society. [DOI: 10.1149/1.1859611] All rights reserved.