In the past decade, lithium-enriched compounds, Li2
(Me = Mn4+
etc.), have been extensively studied for high-capacity positive electrode materials of lithium batteries. Although the origin of high reversible capacities was a debatable subject for a long time, recently it has been evidenced that charge compensation is partly achieved by solid-state redox of non-metal anions, i.e
., oxide ions (anionic redox), coupled with solid-state redox of transition metal ions (cationic redox), which is the basic theory used for classical lithium/sodium insertion materials. Competition between cationic and anionic redox reactions is often evidenced for the lithium-enriched materials because the energy level of oxygen 2p band is lowered by the presence of excess lithium ions with high ionic characters in the crystal lattice. Reversibility of anionic redox reactions is also influenced by ionic and covalent characters for chemical bonds of transition metal ions.[2, 3] In contrast, when the energy of metal 3d band is low enough than that of oxygen 2p, pure cationic redox is realized even for lithium-excess system. In this presentation, we discuss the material design strategy with the concept of cationic and anionic redox reactions to develop new high-capacity lithium/sodium insertion materials for battery applications.
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