Metastable and Nanosized Li_1.2Nb_0.2V_0.6O₂ for High-Energy Li-ion Batteries (Supporting Information)

A Li-excess cation-disordered rocksalt oxide, Li_1.2Nb_0.2V_0.6O₂, with higher theoretical capacity than traditional stoichiometric and layered oxides, is synthesized and tested as positive electrode materials for battery applications. Although Li_1.2Nb_0.2V_0.6O₂ cannot be synthesized by conventional calcination method, a single phase and metastable oxide is successfully synthesized by high-energy mechanical milling. Electrode performance of metastable and nanosized Li_1.2Nb_0.2V_0.6O₂ is significantly improved by heat treatment at 600 °C. Heat treated Li_1.2Nb_0.2V_0.6O₂ with a partial cation ordered layered structure delivers a high reversible specific capacity of 320 mAh g⁻¹ on the basis of highly reversible two-electron redox of V ions. Moreover, inferior cyclability originating from the dissolution of V ions is successfully improved by using concentrate electrolyte solution, and over 90 % capacity retention is achieved after 50 cycles. This finding opens a new way to design high-capacity metastable Li-excess oxides for advanced Li-ion batteries with higher energy density.