The synthesis aspects and electrochemical behaviors of Li–Mn–O compounds prepared at high temperatures of 950–1100°C were investigated. Heating temperature, oxygen content in reaction atmosphere, and cooling rate affect on the product structure and composition. High heating temperatures favor the expansion of unit cell volume and shrinkage of surface area, and slow cooling rate can diminish only oxygen vacancies and the oxygen absorptions are not enough to affect the lattice parameter and surface area severely. The greatly oxygen-deficient LiMn2O4 spinels were prepared by following a slow cooling for 10h to room temperature. In both room and high temperature CV studies, a new oxidation peak at ca. 3.97V was observed for oxygen-deficient spinel, which was considered coming from an oxidation process related with the 3.2V discharge plateau. Meanwhile, splits phenomenon of all the two pairs of redox peaks relative to normal LiMn2O4 spinel was found during high temperature CV measurements at 85°C. dQ/dV–V plot confirmed the splits. Analysis of the increased capacity at 3.2V plateau during room and high temperature discharges coupled with surface area revealed that dissolution reaction of Li–Mn–O spinel in electrolyte, which intensified by elevated temperature, caused the growth of oxygen vacancy in spinel structure.