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A disordered rocksalt Li-excess cathode material, Li 1.25 Nb 0.25 Mn 0.5 O 2 , was synthesized and investigated. It shows a large initial discharge capacity of 287 mAh g − 1 in the first cycle, which is much higher than the theoretical capacity of 146 mAh g − 1 based on the Mn 3+ /Mn 4+ redox reaction. In situ X-ray diffraction (XRD) demonstrates that the compound remains cation-disordered during the first cycle. Electron energy loss spectroscopy (EELS) suggests that Mn and O are likely to both be redox active, resulting in the large reversible capacity. Our results show that Li 1.25 Nb 0.25 Mn 0.5 O 2 is a promising cathode material for high capacity Li-ion batteries and that reversible oxygen redox in the bulk may be a viable way forward to increase the energy density of lithium-ion batteries. [ABSTRACT FROM AUTHOR]

[Display omitted] Cation-disordered Li-excess oxides/oxyfluorides have opened the door for designing alternative, high energy cathodes. However, achieving long cycle life and high rate capability represents a major challenge for disordered rocksalt cathodes (DRX). Herein, we develop Li 2 Mn 3/4 Cr 1/4 O 2 F (LMCOF) DRX materials through a distinct redox mechanochemical method. The LMCOF contains trivalent Cr3+ and Mn3+, which allows for simultaneously accessing stable Mn/Cr dual redox at a narrow voltage window acceptable for conventional carbonate electrolytes. Coupled with the mitigated and reversible oxygen chemical changes, the LMCOF delivers high capacity, rate capability, as well as long cycle life upon extensive 1,000 cycles at various current densities. The multiscale synchrotron and neutron scattering, spectroscopic, and imaging analyses demonstrate that the capacity originates from the Mn/Cr dual redox with minimal contribution from oxygen redox, and that the good cycle life is attributed to the stable global crystal structure and local oxygen chemical environment. [ABSTRACT FROM AUTHOR]