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, Li 2 MeO 3 (Me = Mn 4+ , Ru 4+ , etc.), have been extensively studied as highcapacity 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 LiMeO 2 (Me = Co 3+ , Ni 3+ , etc.). Herein, as a compound with further excess lithium contents, a cation-ordered rocksalt phase with lithium and pentavalent niobium ions, Li 3 NbO 4 , 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.battery | lithium | anion redox | positive electrode T o realize sustainable energy development in the future, it is widely admitted that the substitution of energy sources for fossil fuels must be considered. An efficient energy storage system using an electrochemical method, such as rechargeable lithium batteries (Li-ion batteries, LIBs), potentially provides the solution to meet these tough challenges. Now, electric vehicles equipped with an electric motor and LIB have been launched in the market, and LIBs are starting to substitute for fossil fuels as power sources in the transportation system using the technology of internal combustion engines. Since their first appearance as power sources for portable electronic devices in 1991, the technology of LIBs has now become sufficiently sophisticated. Nevertheless, the demands for a further increase in energy density are still growing to extend the driving distance for electric vehicles.In 1980, LiCoO 2 with a cation-ordered rocksalt structure (layered type) was first proposed as a positive electrode material for LIBs (1) , etc.), which are also classified as having cation-ordered rocksalt-type structures (2), have been extensively studied as potential high-capacity electrode materials, especially for the Mn 4+ system (Li 2 MnO 3 ) (3-7). Li 2 MnO 3...

Further increase in energy density of lithium batteries is needed for zero emission vehicles. However, energy density is restricted by unavoidable theoretical limits for positive electrodes used in commercial applications. One possibility towards energy densities exceeding these limits is to utilize anion (oxide ion) redox, instead of classical transition metal redox. Nevertheless, origin of activation of the oxide ion and its stabilization mechanism are not fully understood. Here we demonstrate that the suppression of formation of superoxide-like species on lithium extraction results in reversible redox for oxide ions, which is stabilized by the presence of relatively less covalent character of Mn4+ with oxide ions without the sacrifice of electronic conductivity. On the basis of these findings, we report an electrode material, whose metallic constituents consist only of 3d transition metal elements. The material delivers a reversible capacity of 300 mAh g−1 based on solid-state redox reaction of oxide ions.

The binary system, xLi3NbO4-(1 - x)LiVO2, was first examined as an electrode material for rechargeable lithium batteries. The sample (x = 0.43) crystallizes into a cation-disordered rocksalt structure and delivers a reversible capacity of ca. 230 mA h g(-1), which originates from V(3+)/V(5+) redox with electrochemically inactive niobium ions.

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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 enriched materials, Li2MeO3-type layered materials (Me = Mn4+, Ru4+ etc.), which are classified as one of cation-ordered rocksalt-type structures, have been extensively studied as potential high-capacity electrode materials, especially for the tetravalent manganese system (Li2MnO3).  Li2MnO3 had been originally thought to be electrochemically inactive because oxidation of manganese ions beyond the tetravalent state in Li cells is difficult.  However, the fact is that Li2MnO3 is electrochemically active, presumably because of the contribution of oxide ions for redox reaction.  Although the oxidation of oxide ions in Li2MnO3 results in the partial oxygen loss with irreversible structural changes, it has been reported that the solid-state redox reaction of oxide ions is effectively stabilized in Li2Ru1-x Sn x O3 system.  Nearly 1.6 moles of lithium ions are reversibly extracted/inserted from/into Li2Ru0.75Sn0.25O3 with excellent capacity retention, indicating that unfavorable phase transition is effectively suppressed in this system. The use of oxide ion redox is the important strategy to further increase the reversible capacity of positive electrode materials for LIBs because the lithium content is potentially further enriched with a lower amount of transition metals in the framework structure.  Reversible capacity as electrode materials is not limited by the absence of oxidizable transition metals as a redox center.  Negatively charged oxide ions can potentially donate electrons instead of transition metals.  However, oxidation without transition metals inevitably result in the release of oxygen molecules, for instance, electrochemical decomposition of Li2O2. Based on these considerations, we have decided to investigate the rocksalt phase with pentavalent niobium ions, i.e., Li3NbO4.  Increase in oxidation numbers of transition metals from “tetravalent to pentavalent” states (or even higher than pentavalent) allows us to enrich a lithium content in the close-packed framework structure of oxide ions with fewer transition metals.  Similar to Li2MeO3, Li3NbO4 with pentavalent niobium ions is also classified as one of the cation-ordered rocksalt structures.  Although Li3NbO4 crystallizes into the lithium-enriched rocksalt-type phase, it is electrochemically inactive because of its insulating character without electrons in a conduction band (4d0 configuration for Nb5+).  Therefore, to induce electron conductivity in Li3NbO4, transition metals are partly substituted for Nb5+ and Li+.  In this study, x Li3NbO4 – (1-x) LiMeO2 (Me = Mn3+, Fe3+, and V3+) system has been studied as a new series of electrode materials.  Among these samples, the Mn3+-substituted sample can deliver large reversible capacities of 250 – 300 mAh g-1 at elevated temperatures (50 – 60 oC).  Moreover, the large reversible capacity partly originates from the solid-state redox reaction of oxide ions, which has been evidenced by DFT calculation and soft X-ray absorption spectroscopy.  Together with these results, electrode performance and reaction mechanisms are also compared with those of Fe3+- and V3+-substituted samples.   From these results, we will discuss the possibility of the new series of positive electrode materials for rechargeable batteries, beyond the restriction of the solid-state redox reaction based on the transition metals used for past three decades.