A sample of LiCoO2 was prepared as a cathode for a lithium battery, and subjected to a large number of charge/discharge cycles that induced an irreversible loss in capacity. A transmission electron microscopy study of this material showed that the initial O3 crystal structure transformed partially to a cubic spinel phase, especially on the surfaces of the particles. We suggest that this spinel phase formation could be responsible in part for the irreversible capacity fade. (C) 2003 Elsevier Science B.V. All rights reserved.