Metastable Cubic Structure Exceeds Capacity Limit of Antifluorite Li_5FeO_4 Cathode Using Small Polarized Oxygen Redox

摘要

Cost-effective Fe-based cathode materials have attracted significantattention. Recently, antifluorite-type Li_5FeO_4 has been investigated as ahigh-capacity cathode material owing to its high lithium content and solidstateoxygen redox reactions. However, its reversible capacity is restrictedto transition-metal redox reactions because of its irreversible structuraltransformation and oxygen evolution. This study demonstrates the reversibleoxygen redox reaction in metastable cubic antifluorite Li_5FeO_4 obtainedby mechanochemical treatment. It exhibits a reversible capacity of morethan 300 mAh g~(-1), corresponding to an approximately 1.7-electron reaction,without oxygen gas evolution. Combined analyses of X-ray absorptionspectroscopy (XAS) and theoretical calculations reveal that both Fe~(3+)/Fe~(4+)and O~(2-)/O_2~(2-) redox reactions proceed under semi-coherent structuraltransformations between antifluorite and rocksalt, which differs from theknown orthorhombic Li_5FeO_4 phase. Although the cyclability of oxygen redoxis poor, its solid solution with Co2+ enhances cyclability by suppressingoxygen evolution. Tuning the structural symmetry and transition-metalcontent can overcome the reversible capacity limit of antifluorite cathodematerials, which is useful for the development of high-energy cathodematerials by the effective utilization of solid-state oxygen redox reactions.