The cycling performance of a lithium-manganese oxide spinel (prepared from MnO2 and Li2CO3) has been investigated as a function of preparation temperature in a lithium polymer electrolyte cell. Excellent reversibility and high rate performance have been observed from a spinel phase prepared at 450-degrees-C and of general formula Li(x)Mn(2-z)O4 (0-less-than-or-equal-to-x-less-than-or-equal-to-1.33, 0-less-than-or-equal-to-z-less-than-or-equal-to-0.33). This phase has a characteristic cubic lattice parameter of approximately 8.19 angstrom: less than that found in the stoichiometric spinel material LiMn2O4.