Thermal stability of a lithium-rich layered oxide cathode material with composition 0.5Li(4/3)Mn(2/3)O(2)-0.5LiNi(1/3)Co(1/3)Mn(1/3)O(2) (LMO-NCM) is investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Investigated material shows higher thermal stability (higher onset temperature) than LiCoO2. The state of charge and previous cycling history of LMO-NCM, particularly activation of lithium manganese oxide (LMO) "component" during first charge, affect its thermal stability. Activation of LMO has an ambivalent effect on cathode thermal stability: on the one hand, the onset temperature of cathode decomposition is increased, probably due to the change in oxidation states of Ni, Co and Mn after the first cycle. On the other hand, the enthalpy of decomposition increases, presumably due to formation of unstable oxygen in the lattice. X-ray diffraction (XRD) analyses of cathodes before and after thermal decomposition at 600 degrees C indicate differences in decomposition reactions of charged cathodes, depending on state of charge and cycling history. (C) 2014 Elsevier B.V. All rights reserved.