Oxygen-deficient spinel (LiMn)3O4−z was successfully synthesized by melt-impregnation method. The chemical composition of each spinel sample was carefully analyzed and the value of z was correlated with its discharge capacity at about 3.2V (C 3.2V). The linear relationship between C 3.2V and z was theoretically deduced from the structural consideration of (LiMn)3O4−z .