A layered Li-Co-Mn oxide was synthesized from a host layered Na-Co-Mn oxide by ion-exchange technique. Its electrode performance showed anomalous high redox potential of ca. 4.5 V vs. Li/Li+ for the intercalation and deintercalation of lithium, although the end members of solid solution LiCoO2 and LiMnO2 did only 4.0 V. The oxidation state of the cobalt was measured by X-ray photoelectron spectroscopy and was trivalent. (C) 2001 The Electrochemical Society.