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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 by ex 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.

The development of multielectron redox-active cathode materials is a top priority for achieving high energy density with long cycle life in the next-generation secondary battery applications. Triggering anion redox activity is regarded as a promising strategy to enhance the energy density of polyanionic cathodes for Li/Na-ion batteries. Herein, K2Fe(C2O4)2is shown to be a promising new cathode material that combines metal redox activity with oxalate anion (C2O42–) redox. This compound reveals specific discharge capacities of 116 and 60 mAh g–1for sodium-ion batterie (NIB) and lithium-ion batterie (LIB) cathode applications, respectively, at a rate of 10 mA g–1, with excellent cycling stability. The experimental results are complemented by density functional theory (DFT) calculations of the average atomic charges.