Nanosize Cation-Disordered Rocksalt Oxides: Na2TiO3-NaMnO2 Binary System

To realize the development of rechargeable sodium batteries, new positive electrode materials without less abundant elements are explored. Enrichment of sodium contents in host structures is required to increase the theoretical capacity as electrode materials, and therefore Na-excess compounds are systematically examined in a binary system of Na2TiO3-NaMnO2. After several trials, synthesis of Na-excess compounds with a cation disordered rocksalt structure is successful by adapting a mechanical milling method. Among the tested electrode materials, Na1.14Mn0.57Ti0.29O2 in this binary system delivers a large reversible capacity of approximate to 200 mA h g(-1), originating from reversible redox reactions of cationic Mn3+/Mn4+ and anionic O2-/O-n(-) redox confirmed by X-ray absorption spectroscopy. Holes in oxygen 2p orbitals, which are formed by electrochemical oxidation, are energetically stabilized by electron donation from Mn ions. Moreover, reversibility of anionic redox is significantly improved compared with a former study on a binary system of Na3NbO3-NaMnO2 tested as model electrode materials.