Lithium transition-metal oxides Li(Mn2-xCox)O-4 (0less than or equal todeltaless than or equal to0.5) have been synthesized by a solid state reaction. The phase transition in the spinel LiMn2O4 was investigated using powder synchrotron radiation diffraction. A transition from cubic (Fd (3) over barm) to orthorhombic (Fddd) was observed around T=220 K. Increasing the average valence of Mn, by doping with Co ion in the [Mn2O4] framework, reduces the concentration of the Mn3+-ions, suppress as the Jahn-Teller distortion, and retains the cubic phase at low temperature. The capacity loss observed in the undoped Li/LiMn2O4 Cell is about 25% after 20 cycles, whereas that for the x=0.1 and x=0.2 doped spinels it is about 0.5% and 1%, respectively. The cycle performance retention of the Li(Mn2-xCox)O-4 (0.1less than or equal toxless than or equal to0.5) electrode is attributed to the chemical substitution of Co at the Mn sites in the spinel structure.