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 calcula-tions 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 Mo''ssbauer 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.

Automated summary

Weberite-type sodium transition metal fluorides show potential as high-performance sodium intercalation cathodes, but the inconsistencies in the structure and properties of Na2Fe2F7 have hindered the understanding of structure-property relationships. In this study, a combined experimental-computational approach is used to reconcile the structural characteristics and electrochemical behavior of Na2Fe2F7, revealing the presence of multiple polymorphs and their transformation to more stable phases upon cycling. These findings emphasize the importance of controlling polymorphism and phase stability for improved performance.