HT-LiMnO2 was prepared by a solid-state reaction at 900 degrees C. Electrochemical charge-discharge cycling of HT-LiMnO2 showed an initial charge capacity 113 mA h g(-1), followed by a small first discharge of 25 mA h g(-1). The discharge capacity subsequently increased and reached up to similar to 120 mA h g(-1) after 30 cycles. In situ synchrotron diffraction showed that HT-LiMnO2 transformed to a new phase (Phase II) during the first charge process, which was metastable and could easily convert to spinel LiMn2O4 in the following cycles. The progressive formation of this spinel phase was responsible for the capacity increase of the battery in the first cycles. The different cycling performance of HT-LiMnO2 and LT-LiMnO2 was attributed to the large stacking faults in LT-LiMnO2, which may facilitate the material to completely convert to spinel LiMn2O4 in the first charge without the medium transition to Phase II. (c) 2007 Elsevier B.V. All rights reserved.