Similarly, such rearrangement of charge was known when metal 3 d orbital was heavily hybridized with oxygen 2 p orbital, for instance Fe 4+ in SrFeO 3 ( 12). The possibility of charge compensation by oxide ions on lithium extraction was also discussed for late-transition metal oxides before 2000 ( 10, 11). The voltage as positive electrode materials is also much more attractive for the oxide ions. Because sulfide ions are relatively soft and polarizable anion compared with oxide ions, oxidation of sulfide ions to persulfide ions is easier than that of oxide ions. Historically, such charge compensation by nonmetal anions has already been evidenced in sulfides before 1990 ( 9). Nearly 1.6 mol of Li + are reversibly extracted from Li 2Ru 0.75Sn 0.25O 3 with excellent capacity retention, indicating that unfavorable phase transition is effectively suppressed in this system. Although the oxidation of oxide ions in Li 2MnO 3 results in partial oxygen loss with irreversible structural changes ( 5, 6), solid-state redox reaction of oxide ions is effectively stabilized in the Li 2Ru 1- xSn xO 3 system ( 8). However, the fact is that Li 2MnO 3 is electrochemically active, presumably because of the contribution of oxide ions for redox reaction. Li 2MnO 3 had been originally thought to be electrochemically inactive because oxidation of Mn ions beyond the tetravalent state is difficult. In the past decade, Li-enriched materials, Li 2MeO 3-type layered materials (Me = Mn 4+, Ru 4+, 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 2MnO 3) ( 3– 7). After this finding, lithium insertion materials with cation-ordered rocksalt-type structures, LiMeO 2 (Me = Co 3+, Ni 3+, etc.) have been extensively studied as electrode materials. In 1980, LiCoO 2 with a cation-ordered rocksalt structure (layered type) was first proposed as a positive electrode material for LIBs ( 1) and is still widely used for high-energy mobile applications. 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. It is proposed that such a charge compensation process by oxide ions is effectively stabilized by the presence of electrochemically inactive niobium ions.
Approximately 300 mAh⋅g −1 of high-reversible capacity at 50 ☌ is experimentally observed, which partly originates from charge compensation by solid-state redox of oxide ions.
Rechargeable lithium batteries series#
Herein, as a compound with further excess lithium contents, a cation-ordered rocksalt phase with lithium and pentavalent niobium ions, Li 3NbO 4, is first examined as the host structure of a new series of high-capacity positive electrode materials for rechargeable lithium batteries. 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.). In the past decade, lithium-excess compounds, Li 2MeO 3 (Me = Mn 4+, Ru 4+, etc.), have been extensively studied as high-capacity positive electrode materials. However, materials innovations are still needed to satisfy the growing demand for increasing energy density of lithium batteries.
Lithium batteries are now used as power sources for electric vehicles. Rechargeable lithium batteries have rapidly risen to prominence as fundamental devices for green and sustainable energy development.
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