A new type of oxygen stoichiometric and Mg-doped LiMn2O4 spinel with improved crystallinity and decreased surface area was synthesized by a special “two-step” method: first, calcinate the mixture of metal oxides at “ultra-high” temperatures (950–1100°C) to obtain an intermediate product with improved crystallinity, larger particle size and oxygen defects; then, anneal the intermediate at relatively low temperatures (600–800°C) with the addition of extra LiOH to achieve oxygen stoichiometry. These spinels with general formula Li1+x Mg y Mn2−x−y O4+δ or (Li, Mg, Mn)3O4+δ are oxygen-rich based on chemical analysis (O/(Li+Mg+Mn) ratio larger than 4:3), and they can be called oxygen stoichiometric spinels with metal cation vacancies and rewritten as [Li]8a [Li n Mg m Mn2−n−m−p □ p ]16d [O4]32e . This new kind of materials with controlled oxygen stoichiometry exhibited greatly improved cycling performance and reduced Mn dissolution at elevated temperatures over that of other Mg-doped materials prepared by conventional “one-step” method.