Exploring the Limits of Fe-Rich Chemistries in Na-Based CaFe₂O₄-Type Postspinel Oxides

Structural trends, physical properties, and electrochemical performances of the NaFeRu_2-O₄ system have been investigated. Synthesis attempts using both conventional solid-state routes and high-pressure methods were explored for the compositional range 1.0 ≤ x ≤ 1.67. Based on Rietveld refinements against powder X-ray diffraction data, analyses of ⁵⁷Fe Mössbauer spectroscopy data, and elemental analysis by electron microprobe, the existence of a confined compositional solid solution (1 ≤ x ≤ 1.3) adopting the CaFe₂O₄-type postspinel structure is demonstrated. This is contrasted with the NaFeTi_2-O₄ system, for which no evidence of a solid solution is observed. However, for all explored synthetic routes of NaFeRu_2-O₄ compositions, a trivalent iron oxidation state is maintained. Structural analysis and qualitative bond valence energy landscape models reveal that the progressive integration of iron into the postspinel framework results in narrowed sodium ion diffusion channels, restricting electrochemical deintercalation of sodium. Consequently, the CaFe₂O₄-type iron-rich compounds explored in this study demonstrate limited potential as positive electrode materials for sodium batteries. It is expected that this fundamental insight will help guide the exploration of alternative NaM₂O₄-based positive electrode materials with similar structure types.