LiNi0.4Mn1.6O4 was prepared under air and oxygen atmospheres using fine Mn3O4 particle and large MnO2 particle at various temperatures. The sample prepared from Mn3O4 at 750 degrees C under air or oxygen atmosphere exhibited an ideal electrochemical behavior, which was based on three redox couples of Mn3+/Mn4+, Ni2+/Ni3+, and Ni3+/Ni4+. On the other hand, the sample prepared from MnO2 had a larger capacity at 4 V plateau and smaller one at 5 V plateau. However, when using oxygen atmosphere, the sample exhibited more ideal behavior, which is similar to the sample prepared from Mn3O4. This means that some defects exist in LiNi0.4Mn1.6O4, depending on preparation conditions. In order to confirm this point, the chemical composition and the valence state of Mn of the prepared sample was analyzed. From these results, it can be said that electrochemical reactions of LiNi0.4Mn1.6O4 are well explained based oil three redox couples of Mn3+/Mn4+, Ni2+/Ni3+, and Ni3+/Ni4+ by considering a presence of oxygen defects. (c) 2005 Elsevier B.V. All rights reserved.