Cation‐Vacancy Model for MnO2

A cation‐vacancy model for electrochemically active manganese dioxides ( and ) is presented. According to this model, the crystal structure, composed of closely packed O2− ions which are octahedrically coordinated to Mn4+ ions, the octahedra sharing edges and corners, contains Mn vacancies. Each vacancy is coordinated to, and electrostatically compensated by, four protons. The protons are present in the form of OH− ions, these latter replacing O2− in the lattice, without noticeable change in lattice parameters. Many hitherto unexplained phenomenological and experimental facts, relating to chemical composition (water content), density, electrochemical capacity, proton‐transfer rate, electronic conductivity, and electrode potential of , can be reconciled, and quantitatively predicted, on the basis of the present theory. In particular, electrochemical reactivity of in battery electrodes is shown to be due to the presence of cation vacancies in the lattice.