Polymorphism in Weberite Na₂Fe₂F₇ and its Effects on Electrochemical Properties as a Na-Ion Cathode

Weberite-type sodium transition metal fluorides (Na₂M²⁺M'³⁺F₇) 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 Na₂Fe₂F₇, 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 Na₂Fe₂F₇ weberite polymorphs, and their predicted (de)intercalation behavior. We find that the as-prepared Na₂Fe₂F₇ 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 Na₂Fe₂F₇ exhibits a respectable initial capacity yet steady capacity fade, a consequence of the transformation of the Na₂Fe₂F₇ weberite phases to the more stable perovskite-type NaFeF₃ 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.