Lithium-Excess Cation-Disordered Rocksalt-Type Oxide with Nanoscale Phase Segregation: Li1.25Nb0.25V0.5O2

A Li-excess cation-disordered rocksalt oxide, Li1.25Nb0.25V0.5O2, is proposed as a new high-capacity positive electrode material for rechargeable lithium batteries. Li1.25Nb0.25V0.5O2 delivers a reversible capacity of over 240 mAh g–1 with excellent capacity retention on the basis of the two-electron redox of V3+/V5+ as evidenced by X-ray absorption spectroscopy. Moreover, a small volume change is noted on electrochemical cycles, presumably associated with reversible vanadium migration from octahedral to tetrahedral sites. Additionally, a heat-treated sample shows unexpectedly good rate capability as electrode materials. Observation by scanning transmission electron microscopy reveals that the sample consists of vanadium-rich and niobium-rich nanoparticles formed by phase segregation, which hinders the kinetics as electrode materials. The heat-treatment process effectively changes the cation distribution in the particles and relieves strain induced by ball milling, leading to good rate capability. These findings open a path to design high-capacity electrode materials with the multielectron redox reaction.