Polymorphism in Weberite Na(2)Fe(2)F(7) and its Effects on Electrochemical Properties as a Na-Ion Cathode

[Image: see text] Weberite-type sodium transition metal fluorides (Na(2)M(2+)M′(3+)F(7)) have emerged as potential high-performance sodium intercalation cathodes, with predicted energy densities in the 600–800 W h/kg range and fast Na-ion transport. One of the few weberites that have been electrochemically tested is Na(2)Fe(2)F(7), yet inconsistencies in its reported structure and electrochemical properties have hampered the establishment of clear structure–property relationships. In this study, we reconcile structural characteristics and electrochemical behavior using a combined experimental–computational approach. First-principles calculations reveal the inherent metastability of weberite-type phases, the close energetics of several Na(2)Fe(2)F(7) weberite polymorphs, and their predicted (de)intercalation behavior. We find that the as-prepared Na(2)Fe(2)F(7) samples inevitably contain a mixture of polymorphs, with local probes such as solid-state nuclear magnetic resonance (NMR) and Mössbauer spectroscopy providing unique insights into the distribution of Na and Fe local environments. Polymorphic Na(2)Fe(2)F(7) exhibits a respectable initial capacity yet steady capacity fade, a consequence of the transformation of the Na(2)Fe(2)F(7) weberite phases to the more stable perovskite-type NaFeF(3) phase upon cycling, as revealed by ex situ synchrotron X-ray diffraction and solid-state NMR. Overall, these findings highlight the need for greater control over weberite polymorphism and phase stability through compositional tuning and synthesis optimization.