Daisuke Endo

High-capacity electrode materials for rechargeable lithium batteries: Li3NbO4-based system with cation-disordered rocksalt structure.

Significance This study describes new and promising electrode materials, Li3NbO4-based electrode materials, which are used for high-energy rechargeable lithium batteries. Although its crystal structure is classified as a cation-disordered rocksalt-type structure, lithium ions quickly migrate in percolative network in bulk without a sacrifice in kinetics. Moreover, the large reversible capacity originates from the participation of oxide ions for a charge compensation process, which has been confirmed by first-principles calculations combined with X-ray absorption spectroscopy. This finding can be further expanded to the design of innovative positive electrode materials beyond the restriction of the solid-state redox reaction based on the transition metals used for the past three decades. Rechargeable lithium batteries have rapidly risen to prominence as fundamental devices for green and sustainable energy development. Lithium batteries are now used as power sources for electric vehicles. However, materials innovations are still needed to satisfy the growing demand for increasing energy density of lithium batteries. In the past decade, lithium-excess compounds, Li2MeO3 (Me = Mn4+, Ru4+, etc.), have been extensively studied as high-capacity positive electrode materials. Although the origin as the high reversible capacity has been a debatable subject for a long time, recently it has been confirmed that charge compensation is partly achieved by solid-state redox of nonmetal anions (i.e., oxide ions), coupled with solid-state redox of transition metals, which is the basic theory used for classic lithium insertion materials, such as LiMeO2 (Me = Co3+, Ni3+, etc.). Herein, as a compound with further excess lithium contents, a cation-ordered rocksalt phase with lithium and pentavalent niobium ions, Li3NbO4, is first examined as the host structure of a new series of high-capacity positive electrode materials for rechargeable lithium batteries. Approximately 300 mAh⋅g−1 of high-reversible capacity at 50 °C is experimentally observed, which partly originates from charge compensation by solid-state redox of oxide ions. It is proposed that such a charge compensation process by oxide ions is effectively stabilized by the presence of electrochemically inactive niobium ions. These results will contribute to the development of a new class of high-capacity electrode materials, potentially with further lithium enrichment (and fewer transition metals) in the close-packed framework structure with oxide ions.