The high-temperature electrochemical cell behavior of Li-stoichiometric Li-Mn-O spinel material, LiMn2O4.014, has been studied by cyclic voltammetry and coulometric titration curve measurement methods. A split of the reduction peak at x > 0.5, in LixMn2O4.(014), during Lie insertion was found at 85 degreesC. Correspondingly, three plateaus appeared in its coulometric titration curve, which manifest itself as three peaks in dQ/dV-V plot. The peak split phenomenon was also found for oxygen deficient spinel, LiMn2O3.986, prepared at 900 degreesC in both reduction and oxidation branches at room and elevated temperatures using a lower scan rate of 0.02 mV/s. The disproportionation dissolution of stoichiometric spinel LiMn2O4.014 intensified by the elevated temperature in slightly acidic electrolyte is considered responsible for the occurrence of the phenomenon. The finding gives a piece of evidence for oxygen deficient spinel LiMn2O4-z as the insoluble product of disproportionation dissolution.