Exploring the Limits of Fe-Rich Chemistries in Na-Based CaFe 2 O 4 -Type Postspinel Oxides.

Structural trends, physical properties, and electrochemical performances of the NaFe _x Ru _{2- x} O ₄ 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 ⁵⁷ Fe Mössbauer spectroscopy data, and elemental analysis by electron microprobe, the existence of a confined compositional solid solution (1 ≤ x ≤ 1.3) adopting the CaFe ₂ O ₄ -type postspinel structure is demonstrated. This is contrasted with the NaFe _x Ti _{2 - x} O ₄ system, for which no evidence of a solid solution is observed. However, for all explored synthetic routes of NaFe _x Ru _{2- x} O ₄ 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 CaFe ₂ O ₄ -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 NaM ₂ O ₄ -based positive electrode materials with similar structure types.