To improve the cycle performance of LiMn2O4 (Fd (3) over bar m) as the cathode of 4 V class lithium secondary batteries, the cathode properties of the quaternary cubic spinel phases LiMgyMn2-yO4 synthesized at 750 degrees C were examined. Although the cycle performance of the LiMgyMn2-yO4 was improved by the substitution of Mg2+ for Mn3+ in the octahedral sites, the first discharge capacity was reduced considerably compared with that of the parent LiMn2O4. In order to compensate for the theoretical capacity reduction in LiMg1/6Mn11/6O4 nonstoichiometric spinel oxides LiMg11/6Mn11/6O4-delta were prepared under controlled oxygen partial pressures at 750 degrees C. The single-phase region of delta in LiMg1/6Mn11/6O4-delta was 0 less than or equal to delta < 0.04, which was larger than that of parent LiMn2O4 (0 less than or equal to delta < 0.018) in our previous work. From density data, a metal excess model (Li)(8a)[Lidelta/(4-delta)Mgdelta/6(4-delta)Mn11 delta/6(4-delta)](16c)[Mg1/6Mn11/6](16d)O-4 was proposed as the defect structure in the LiMg1/6Mn11/6O4-delta The Chemical diffusion coefficient of lithium ion for nonstoichiometric LiMg(1/6)Mn(11/6)O(4-)delta was smaller than that for the stoichiometric LiMg1/6Mn11/6O4. This also supported the metal excess model, because excess metals in 16c sites prevented an easier diffusion of lithium in the 8a-16c-8a diffusion path.