New High-Capacity Electrode Materials for Rechargeable Lithium Batteries: Li3NbO4-LiMeO2 (Me = Mn3+, Fe3+, and V3+) System with Cation Disordered Rocksalt Structure

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 Li₂Ru_1-xSn_xO₃ system.  Nearly 1.6 moles of lithium ions are reversibly extracted/inserted from/into Li₂Ru_0.75Sn_0.25O₃ 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.