Anionic Redox Activity as a Key Factor in the Performance Degradation of NaFeO₂ Cathodes for Sodium Ion Batteries

The origin of the irreversible capacity of O3-type NaFeO₂ charged to high voltage is investigated by analyzing the oxidation state of Fe and phase transition of layered NaFeO₂ cathodes for sodium-ion batteries during the charging process. In-situ X-ray absorption spectroscopy results revealed that charge compensation does not occur through the Fe³⁺/Fe⁴⁺ redox reaction during sodium extraction as no significant shift to high energy was observed in the Fe K-edge. These results were reinforced with ex-situ near-edge X-ray absorption spectroscopy, which suggests that oxygen redox activity is responsible for charge compensation. Formation of Fe₃O₄ product occurs because of oxygen release at high voltage when more than 0.5 Na is extracted from the structure; this is observed by transmission electron microscopy. NaFeO₂ irreversibility is due to the formation of Fe₃O₄ with oxygen release, which inhibits Na insertion into the structure.