Metastable Cubic Structure Exceeds Capacity Limit of Antifluorite Li5FeO4 Cathode Using Small Polarized Oxygen Redox

Cost‐effective Fe‐based cathode materials have attracted significant attention. Recently, antifluorite‐type Li5FeO4 has been investigated as a high‐capacity cathode material owing to its high lithium content and solid‐state oxygen redox reactions. However, its reversible capacity is restricted to transition‐metal redox reactions because of its irreversible structural transformation and oxygen evolution. This study demonstrates the reversible oxygen redox reaction in metastable cubic antifluorite Li5FeO4 obtained by mechanochemical treatment. It exhibits a reversible capacity of more than 300 mAh g‐1, corresponding to an approximately 1.7‐electron reaction, without oxygen gas evolution. Combined analyses of X‐ray absorption spectroscopy (XAS) and theoretical calculations reveal that both Fe3+/Fe4+ and O2‐/O22‐ redox reactions proceed under semi‐coherent structural transformations between antifluorite and rocksalt, which differs from the known orthorhombic Li5FeO4 phase. Although the cyclability of oxygen redox is poor, its solid solution with Co2+ enhances cyclability by suppressing oxygen evolution. Tuning the structural symmetry and transition‐metal content can overcome the reversible capacity limit of antifluorite cathode materials, which is useful for the development of high‐energy cathode materials by the effective utilization of solid‐state oxygen redox reactions. Antifluorite‐type Li5FeO4 has attracted significant attention as a cost‐effective and high‐capacity cathode material. This study demonstrates a reversible oxygen redox reaction, with a specific capacity of more than 300 mAh g–1, in metastable cubic antifluorite Li5FeO4 obtained by mechanochemical treatment. Additionally, cathode performance is enhanced using its solid solution by stabilizing the oxygen redox reaction.