The electrochemical profile of oxygn-deficient LiMn2O4-delta in both aqueous and nonaqueous electrolyte was extensively investigated. LiMn2O4-delta exhibited better cyclic capability in 0.5 M Li2SO4 aqueous electrolyte than 1 M LiFP6 nonaqueous electrolyte. The cycling stability in the aqueous electrolyte is critically dependent on the pH value and charge cutoff potential. The difference in cycling stability between various electrolytes was demonstrated to be due to the oxidation of Mn on the surface of electrode by active oxygen atoms. X-ray photoelectron spectroscopy demonstrated that the Mn oxidation state of LiMn2O4-delta after cycling in aqueous electrolyte increased from 3.44 to 3.64. The amounts of oxygen atoms generated on the surface of LiMn2O4-delta after cycling in were estimated from the graphical representation of the Tafel plots. (C) 2009 The Electrochemical Society.