Lithium transition-metal oxides Li(Mn2-xCox)O-4 (0 less than or equal to x less than or equal to 0.5) are synthesized by solid-state reaction. X-ray and electrochemical data show that the replacement of Mn3+ (d(4)) ions by Co3+ (d(6)) in the octahedral framework of the spinel eliminates the local disorder present in the lattice around the [Mn3+O6] octahedra. The capacity loss observed in the undoped Li/LiMn2O4 cell is about 25% after 20 cycles, whereas that for the x = 0.1 and 0.2 doped spinel materials is about 0.48 and 1%, respectively. The good capacity retention of Li(Mn2-xCox)O-4 (0.1 less than or equal to x less than or equal to 0.5) electrode is attributed to the stabilization of the spinel structure by Cc doping for Mn ion sites. The chemical substitution of Co3+ for Mn3+ in LiMn2O4 improves the efficiency in maintaining electrochemical capacity over a large number of cycles without sacrificing initial reversible capacity at room temperature.