Time-resolved X-ray diffraction (XRD) studies were used to monitor structural changes in charged Li1-xNi0.8Co0.15Al0.05O2 cathodes in the presence of electrolyte as a function of temperature. The electrolyte accelerated thermal decomposition of the charged cathode material. The presence of the electrolyte changed the paths of the structural changes and lowered the temperatures for onset of the reactions. As the degree of Li-ion deintercalation increased, the thermal decomposition took place at less elevated temperature. At low states of charge (x < 0.5) the XRD pattern of Li1-xNi0.8Co0.15Al0.05O2 cathode heated at 450&DEG;C is similar to NiO structure. The formation of the NiO-like rock salt structure is accompanied by the formation of Li2CO3. In the presence of electrolyte the solvent acts as a reducing agent and also lithium extracted from the structure reacts with the solvent resulting in the formation of Li2CO3. Time-resolved XRD results for Li1-xNi0.8Co0.15Al0.05O2 at higher states of charge (x > 0.5), when heated from 25 to 450degreesC, show the conversion to a disordered spinel structure from a layered structure at elevated temperature. When subsequently heated at higher temperature, the disordered spinel structure converts to NiO-like rock salt structure. (C) 2004 The Electrochemical Society.