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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 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.

Sodium (Na)-ion batteries are the most explored 'beyond-Li' battery systems, yet their energy densities are still largely limited by the positive electrode material. Na3FeF6 is a promising Earth-abundant containing electrode and operates through a conversion-type charge-discharge reaction associated with a high theoretical capacity (336 mA h g-1). In practice, however, only a third of this capacity is achieved during electrochemical cycling. In this study, we demonstrate a new rapid and environmentally-friendly assisted-microwave method for the preparation of Na3FeF6. A comprehensive understanding of charge-discharge processes and of the reactivity of the cycled electrode samples is achieved using a combination of electrochemical tests, synchrotron X-ray diffraction, 57Fe Mössbauer spectroscopy, X-ray photoelectron spectroscopy, magnetometry, and 23Na/19F solid-state nuclear magnetic resonance (NMR) complemented with first principles calculations of NMR properties. We find that the primary performance limitation of the Na3FeF6 electrode is the sluggish kinetics of the conversion reaction, while the methods employed for materials synthesis and electrode preparation do not have a significant impact on the conversion efficiency and reversibility. Our work confirms that Na3FeF6 undergoes conversion into NaF and Fe(s) nanoparticles. The latter are found to be prone to oxidation prior to ex situ measurements, thus necessitating a robust analysis of the stable phases (here, NaF) formed upon conversion.

Sodium (Na)-ion batteries are the most explored 'beyond-Li' battery systems, yet their energy densities are still largely limited by the positive electrode material. Na3FeF6 is a promising Earth-abundant containing electrode and operates through a conversion-type charge-discharge reaction associated with a high theoretical capacity (336 mA h g-1). In practice, however, only a third of this capacity is achieved during electrochemical cycling. In this study, we demonstrate a new rapid and environmentally-friendly assisted-microwave method for the preparation of Na3FeF6. A comprehensive understanding of charge-discharge processes and of the reactivity of the cycled electrode samples is achieved using a combination of electrochemical tests, synchrotron X-ray diffraction, 57Fe Mössbauer spectroscopy, X-ray photoelectron spectroscopy, magnetometry, and 23Na/19F solid-state nuclear magnetic resonance (NMR) complemented with first principles calculations of NMR properties. We find that the primary performance limitation of the Na3FeF6 electrode is the sluggish kinetics of the conversion reaction, while the methods employed for materials synthesis and electrode preparation do not have a significant impact on the conversion efficiency and reversibility. Our work confirms that Na3FeF6 undergoes conversion into NaF and Fe(s) nanoparticles. The latter are found to be prone to oxidation prior to ex situ measurements, thus necessitating a robust analysis of the stable phases (here, NaF) formed upon conversion.

The iron‐based polyanionic fluoro‐oxalate material, KFe(C2O4)F (KFCF), has been synthesized by hydrothermal methods. This compound shows promising reversible lithium and sodium insertion properties as a cathode material. The material delivered a first‐cycle discharge capacity of 120 mAh g−1 at ∼3.3 V (Li+/Li) and 97.4 mAh g−1 at ∼3.0 V (Na+/Na) in LIB and NIB, respectively. Stable cycling performance was observed in both cases. The involvement of reversible Fe2+/Fe3+ redox was confirmed by ex‐situ Mössbauer spectroscopy supported by first‐principles calculations. This study reveals promising performance from a mixed oxalate‐fluoride based polyanionic material thereby opening up further possibilities for materials discovery in the design of new electrode materials. [ABSTRACT FROM AUTHOR]

Vanadium multiredox‐based NASICON‐NazV2−yMy(PO4)3 (3 ≤ z ≤ 4; M = Al3+, Cr3+, and Mn2+) cathodes are particularly attractive for Na‐ion battery applications due to their high Na insertion voltage (>3.5 V vs Na+/Na0), reversible storage capacity (≈150 mA h g−1), and rate performance. However, their practical application is hindered by rapid capacity fade due to bulk structural rearrangements at high potentials involving complex redox and local structural changes. To decouple these factors, a series of Mg2+‐substituted Na3+yV2−yMgy(PO4)3 (0 ≤ y ≤ 1) cathodes is studied for which the only redox‐active species is vanadium. While X‐ray diffraction (XRD) confirms the formation of solid solutions between the y = 0 and 1 end members, X‐ray absorption spectroscopy and solid‐state nuclear magnetic resonance reveal a complex evolution of the local structure upon progressive Mg2+ substitution for V3+. Concurrently, the intercalation voltage rises from 3.35 to 3.45 V, due to increasingly more ionic VO bonds, and the sodium (de)intercalation mechanism transitions from a two‐phase for y ≤ 0.5 to a solid solution process for y ≥ 0.5, as confirmed by in operando XRD, while Na‐ion diffusion kinetics follow a nonlinear trend across the compositional series. [ABSTRACT FROM AUTHOR]

Sodium (Na)-ion batteries are the most explored 'beyond-Li' battery systems, yet their energy densities are still largely limited by the positive electrode material. Na3FeF6 is a promising Earth-abundant containing electrode and operates through a conversion-type charge–discharge reaction associated with a high theoretical capacity (336 mA h g−1). In practice, however, only a third of this capacity is achieved during electrochemical cycling. In this study, we demonstrate a new rapid and environmentally-friendly assisted-microwave method for the preparation of Na3FeF6. A comprehensive understanding of charge–discharge processes and of the reactivity of the cycled electrode samples is achieved using a combination of electrochemical tests, synchrotron X-ray diffraction, 57Fe Mössbauer spectroscopy, X-ray photoelectron spectroscopy, magnetometry, and 23Na/19F solid-state nuclear magnetic resonance (NMR) complemented with first principles calculations of NMR properties. We find that the primary performance limitation of the Na3FeF6 electrode is the sluggish kinetics of the conversion reaction, while the methods employed for materials synthesis and electrode preparation do not have a significant impact on the conversion efficiency and reversibility. Our work confirms that Na3FeF6 undergoes conversion into NaF and Fe(s) nanoparticles. The latter are found to be prone to oxidation prior to ex situ measurements, thus necessitating a robust analysis of the stable phases (here, NaF) formed upon conversion. [ABSTRACT FROM AUTHOR]

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., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

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, K 2 Fe(C 2 O 4 ) 2 is shown to be a promising new cathode material that combines metal redox activity with oxalate anion (C 2 O 4 2- ) redox. This compound reveals specific discharge capacities of 116 and 60 mAh g -1 for 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., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)