Spinel lithium manganese oxide was synthesized via co-precipitation. The prepared lithium manganese oxide powder was further heated at 700°C for 15h under pressurized (3bar) oxygen atmosphere. The resultant exhibited a highly crystalline cubic spinel phase with space group Fd3m, as confirmed by X-ray diffraction. The spinel compound exhibited a slightly smaller lattice constant than a conventional spinel compound, even though the cationic ratio of Li/Mn is the same for both compounds. Chemical titration of the Mn component showed that heat treatment under a 3bar oxygen atmosphere resulted in slightly higher average Mn oxidation state, indicating that the amount of Mn4+ increased after the treatment. The Li1.05Mn1.95O3.99 electrode exhibited improved cycling performance, namely, 96.3% of capacity retention during 100 cycles at elevated temperature (60°C). The details of the structure and electrochemistry of the electrode are discussed.