New Insights into the Performance Degradation of Fe-Based Layered Oxides in Sodium-Ion Batteries: Instability of Fe³⁺/Fe⁴⁺ Redox in α-NaFeO₂

The emergence of sodium-ion batteries (SIBs) employing cathodes based on earth abundant sodium and iron is expected to be ideal for large-scale electrical energy storage systems, for which the cost factor is of primary importance. However, these iron-based layered oxides still show unsatisfactory cycle performance, and the redox of the fleeting Fe³⁺/Fe⁴⁺ couple needs to be better understood. In this study, we examine the quasi-reversibility of the layered α-NaFeO₂ cathode in sodium-ion cells. A NaFeO₂ powder sample that has the O3-type layered structure was synthesized via a solid-state synthesis method. The changes in Fe oxidation states and crystallographic structures were examined during the electrochemical sodium cycling of the NaFeO₂ electrodes. Ex situ Mössbauer spectroscopy analysis revealed the chemical instability of Fe⁴⁺ in a battery cell environment: more than 20% of Fe⁴⁺ species that was generated in the desodiated Na_1-xFeO₂ electrode was spontaneously reduced back to Fe³⁺ states during open circuit storage of the charged cell. The in situ synchrotron X-ray diffraction further revealed the nonequilibrium phase transition behavior of the NaFeO₂ cathode. A new layered phase (denoted as O″3) was observed in the course of sodium deintercalation, and an asymmetric structural behavior during cycling was identified. These findings explain the quasi-reversibility of α-NaFeO₂ in the sodium cell and provide guidance for the future development of iron-based cathode materials for sodium-ion batteries.