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 degreesC) to obtain an intermediate product with improved crystallinity, larger particle size and oxygen defects; then, anneal the intermediate at relatively low temperatures (600-800degreesC) with the addition of extra LiOH to achieve oxygen stoichiometry. These spinels with general formula Li1+xMgyMn2-x-yO4+delta or (Li, Mg, Mn)(3)O4+delta 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)Mn(2-n-m-p)rectangle(p)](16d)[O-4](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. (C) 2004 Elsevier Ltd. All rights reserved.