Unlike the LiMn2O4 spinel, the lithium-substituted LiMn2-yLiyO4+/-delta (0.05less than or equal toyless than or equal to0.15) spinel oxides exhibit capacity in the 5 V region. The 5 V capacity increases with increasing amount of excess lithium y. In contrast to a recent literature report, the data demonstrate that excess oxygen is not needed to impart the 5 V capacity. With the samples having a significant amount of excess lithium y>0.1, the 4 V capacity increases with cycling while the 5 V capacity decreases. The 5 V capacities of the LiMn2-yLiyO4+/-delta spinels are explained on the basis of an electrochemical oxidation of Mn4+ or O2-, which subsequently results in a self-discharge reaction involving the evolution of oxygen from the lattice and a reduction of Mn4+ to Mn3+. (C) 2003 The Electrochemical Society.