New High-Capacity Electrode Materials for Rechargeable Lithium Batteries: Li3NbO4-LiMeO2 (Me = Mn3+, Fe3+, and V3+) System with Cation Disordered Rocksalt Structure
Authors: Yabuuchi Naoaki , Takeuchi Mitsue , Komaba Shinichi , Nakayama Masanobu , Shiiba Hiromasa , Sato Kei , Ogawa Masahiro , Yamanaka Keisuke , Ohta Toshiaki 
Published: February 2020

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.