A Li-excess cation-disordered rocksalt oxide, Li1.2Nb0.2V0.6O2, with higher theoretical capacity than traditional stoichiometric and layered oxides, is synthesized and tested as positive electrode materials for battery applications. Although Li1.2Nb0.2V0.6O2 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 Li1.2Nb0.2V0.6O2 is significantly improved by heat treatment at 600 °C. Heat treated Li1.2Nb0.2V0.6O2 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.