Impedance growth of LiCoO2 electrodes cycled with an upper cutoff potential of 4.5 V is caused by side reactions involving species on the surface of LiCoO2 and LiPF6-based electrolyte. These detrimental chemical species can apparently be removed by a heat-treatment to 550degreesC because electrodes made with heat-treated materials show excellent capacity retention even when cycled to 4.5 V. The detrimental surface species can be subsequently reformed on heat-treated materials by exposure of LiCoO2 to water, suggesting prolonged air exposure is responsible for their formation on commercial samples. (C) 2003 The Electrochemical Society.