Electrochemical cells of LiCoO2 were charged to 4.7 V versus lithium, and held at this high voltage for various times. After this high voltage aging, the cells suffered irreversible losses of capacity and suppressed kinetics. Transmission electron microscopy showed the formation of some cubic spinel phase in the LiCoO2 material subjected to high voltage aging, and an increased density of dislocations and internal strains. (C) 2004 Elsevier Ltd. All rights reserved.