The phase diagram of LiMn2O4 over a wide oxygen pressure-temperature region was determined, and the thermodynamics of the oxygen extraction (the heat of reaction, Delta H, etc.) were estimated using thermal analysis, x-ray diffraction, sample quenching and a high pressure oxygen treatment. The oxygen stoichiometry, delta, in LiMn2O4+/-delta varied with the selection of treatment conditions. The extent of the cooperative Jahn-Teller effect in the LiMn2O4-delta and various kinds of spinel Mn oxides are discussed in terms of the Madelung potential and Mn valency using the DV-X alpha calculation : The performance of coin-type cells containing cathodes of the synthesized spinel was investigated and compared to that of cells containing standard LiMn2O4 spinel cathodes.