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