The phase transitions of the bare and AlPO4-coated delithiated LixCoO2 (x = 0.4 and 0.24) according to the coating concentration (thickness) after heat-treatment at 300 degrees C was investigated using nuclear magnetic resonance and X-ray absorption spectra. It was found that the bare and coated LixCoO2 predominantly decomposed into spinel LixCo2O4 or Co3O4 phases depending on the charging voltage. As the charging voltage was increased from 4.3 to 4.6 V, the bare and 1 wt % AlPO4-coated LixCoO2 decomposed into the LixCo2O4 phase while the 2.4 wt % AlPO4-coated sample decomposed into the Co3O4 phase. The improvement in the thermal stability of the 2.4 wt % AlPO4-coated LixCoO2, compared to the bare and 1 wt % AlPO4-coated samples, could be explained by the dominant local formation of the Co3O4 phase over the LixCo2O4 phase. (c) 2005 The Electrochemical Society.