The theoretical capacity and cation vacancy of metal ion (M)-doped LiMn2-xMxO4 spinel compounds serving as positive electrodes in a 4-V lithium ion batteries are calculated. The capacity depends strongly on the mole fraction of doped metal ion and vacancies. The theoretical capacity increases with increasing oxidation number of the doped metal ion in the 16d site of LiMn2O4 at the same doping fraction. The validity of the proposed equation for calculation of the capacity has been initially confirmed using a metal ion with well-known valence, such as the Al ion. The oxidation state of Co, Ni and Cr ions in the spinel structure is found to be trivalent, divalent and trivalent, respectively. Analysis shows that metal ion-doped spinel compounds with low vacancy content promote high capacity.