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 Na1–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.