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.