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 Na2Fe2F7weberite
polymorphs, and their predicted (de)intercalation behavior. We find
that the as-prepared Na2Fe2F7samples
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 Na2Fe2F7exhibits a respectable initial capacity yet steady
capacity fade, a consequence of the transformation of the Na2Fe2F7weberite phases to the more stable perovskite-type
NaFeF3phase upon cycling, as revealed byex situsynchrotron 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.