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

The electronic Structures of bulk and (001) Surfaces of lambda-MnO2 and LiMn2O4 have been Studied with density functional theory. In lambda-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 0 atoms. The technically undesired Mn3+ ions form Lit LixN4Mn2O4 (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 LiMn2O4 cathodes in Li-ion batteries.