A highly oxidized lithium manganospinel, Li2Mn4O8+z , was prepared from LiMnO4 and MnCO3 precursors. The effect of synthesis temperature, time, and oxygen partial pressure on the extent of oxidation of the material was investigated by thermogravimetry (TG). X-ray diffraction (XRD), potentiometric titration, thermogravimetry, and elemental analyses were used to determine the stoichiometry of the product.