Sodium ion batteries continue to be of immense interest as potential substitution for lithium-ion battery due to its abundance on earth, contrary to limited source of lithium. The beneficial effect on the cost efficiency will increase if other element in the cathode is also earth abundant materials. Many type of iron based layered oxides such as secondary, tertiary, quaternary layered cathode compound for Na ion batteries have been studied. Most of these studies showed severe loss of cycle performance at high voltage. To improve performance of iron based layered oxides, especially at high voltage region, it is necessary to better understand the redox reaction of Fe³⁺/Fe⁴⁺ during sodium extraction. Thus, O3 type NaFeO₂ was synthesized and variety of analysis was conducted such as in-situ synchrotron hard X-ray absorption spectroscopy, ex-situ soft X-ray absorption spectroscopy, and transmission electron spectroscopy.

In situ hard X-ray absorption spectra showed that Fe K-edge doesn’t shift to higher energy during Na extraction. The shift in Fe K-edge is sensitive with oxidation state of Fe that mean Fe³⁺/Fe⁴⁺ redox reaction does not contribute much during electrochemical test. Instead of Fe, the emerging of new peak on the pre-edge of the oxygen K-edge, indicating charge compensation of oxygen during the whole Na extraction/insertion process. Moreover, we directly reveal the limited reversibility of the oxygen redox reaction and correlation of performance degradation with surface degradation of α-NaFeO₂ cathode in sodium-ion cells especially at high voltage. These finding provide more deep understanding for future development of iron-based cathode materials for sodium-ion batteries.