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The electronic structures of bulk and (001) surfaces of λ-MnO2and LiMn2O4have been studied with density functional theory. In λ-MnO2, the Mn oxidation state is +4 both in the bulk and at the (001) surface. In LiMn2O4, however, Mn atoms exhibit mixed Mn3+/Mn4+oxidation states in the bulk, while only Mn3+at the (001) surface, due to lower coordination with O atoms. The technically undesired Mn3+ions form at LixMn2O4(001) even for very small Li concentrations. Upon covering the LiMn2O4(001) surface with Al2O3, the oxidation state of surface Mn atoms changes from +3 to +4, which explains the improved performance of coated LiMn2O4cathodes in Li-ion batteries.