Nano-sized composite powder which consisted of two manganese-based oxides, alpha manganese dioxide (alpha-MnO2) and spinel Li-Mn-O, was successfully formed by intergrowth of the spinel phase inside alpha-MnO2. This composite oxide was synthesized by precipitation and heat treatment in air; a-manganese dioxide powder was firstly prepared by oxidative precipitation of Mn(II) with K2S2O8 in an aqueous solution, and then a mixture of the obtained manganese oxide powder and LiOH methanol solution was heat-treated in air. Electron microscopy and diffraction observations confirmed that the manganese oxide composite consisted of nano-sized grains of the spinel LiMn2O4 and alpha-MnO2 phases. It was found that this alpha-MnO2/spinel LiMn2O4 composite electrode exhibited highly reversible lithium insertion compared to the pristine alpha-MnO2 and conventional LiMn2O4, that is, the composite demonstrated high discharge capacity of 148 mAh g(-1) as a cathode material of lithium cells in the potential range of 2.5-4.3 V with no significant capacity fading. It was thought that the intimately mixing of two oxides on a nanometer scale helped to maintain structural integrity on charge-discharge cycling, which leads to excellent capacity retention for both of the spinel and alpha-type manganese oxide. (c) 2004 Elsevier Ltd. All rights reserved.