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