LiMn2O4 has been synthesized using LiOH and Mn3O4 for 20h in air by the melt-impregnation method. It showed pure X-ray diffraction (XRD) patterns without any impurities at 750∼900°C. This material exhibited a large 3.2V plateau in its discharge curve when synthesized above 800°C. The 3.2V plateau showed a strong relation with the oxygen deficiency in the spinel structure, which means a degree of oxygen deficiency of the LiMn2O4 spinel. We confirmed that the oxygen deficiency and Mn dissolution are very important factors for inducing a capacity loss in the stoichiometric spinel at elevated temperature.