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 Li x Mn2O4.014, during Li+ insertion was found at 85°C. 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°C in both reduction and oxidation branches at room and elevated temperatures using a lower scan rate of 0.02mV/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.