To improve the storage performance of Li-Mn spinel at high temperatures, we investigated a mixture of Li-Mn spinel (Li1.1Mn1.9O4) and Li-Ni-Co-Mn oxide (LiNi0.4Co0.3Mn0.3O2) as a positive electrode material. We found that a battery using such a mixture as a positive electrode material showed better storage characteristics at 45degreesC for 30 days than batteries using both a single material Li-Mn spinel and a single material Li-Ni-Co-Mn oxide. This improved storage performance resulted from the suppression of capacity loss of LiMn-spinel in the mixture. We compared the differences in Li-Mn spinel to clarify the reasons for its improved storage performance in the mixture and found two changes during charging in the potential and lattice constant of Li-Mn spinel in the mixture that were different from those of single material Li-Mn spinel. In single material, changes in potential and lattice constants of Li-Mn spinel showed two-step change when charged, while Li-Mn spinel in a mixture showed continuous change. (C) 2004 The Electrochemical Society.