The characterization of both stoichiometric LiCoxMn2-xO4 synthesized by solid-state reaction and nonstoichiometric LiCoxMn2-xO4-delta prepared under controlled partial pressure of oxygen was studied on structure, morphology, thermal stability, and electrochemistry by X-ray diffractometry (XRD), scanning electronic microscopy (SEM), distribution of particle size, differential thermal analysis (DTA), thermogravimetry (TG), and charge-discharge measurement. Stoichiometric LiCoxMn2-xO4 showed the single cubic spinel phase even when the doping amount of Co for Mn reached x = 0.5. The lattice parameter decreased linearly with increasing amount of Co. SEM observation and particle size analysis showed that all samples had almost the same particle size and morphology. TG-DTA showed a linear relationship between the Co-doped amount and the decomposition temperature to form nonstoichiometry in an air atmosphere. More Co was beneficial to extend the region of the single phase of LiCoxMn2-xO4-delta. The cubic lattice parameter of LiCoxMn2-xO4-delta increased linearly with the increase of delta. The initial capacity and the high rate capability can be improved by the introduction of defective structure, and more Co dopant also improves the rate capability. However, the cyclability of nonstoichiometric spinels is slightly poorer than that of stoichiometric spinels. (C) 2003 The Electrochemical Society.