Oxidation States of Mn Atoms at Clean and Al₂O₃-Covered LiMn₂O₄(001) Surfaces

The electronic structures of bulk and (001) surfaces of λ-MnO 2 and LiMn 2 O 4 have been studied with density functional theory. In λ-MnO 2, the Mn oxidation state is +4 both in the bulk and at the (001) surface. In LiMn 2 O 4, however, Mn atoms exhibit mixed Mn 3+ /Mn 4+ oxidation states in the bulk, while only Mn 3+ at the (001) surface, due to lower coordination with O atoms. The technically undesired Mn 3+ ions form at Li x Mn 2 O 4 (001) even for very small Li concentrations. Upon covering the LiMn 2 O 4 (001) surface with Al 2 O 3, the oxidation state of surface Mn atoms changes from +3 to +4, which explains the improved performance of coated LiMn 2 O 4 cathodes in Li-ion batteries.