(Invited) Investigating Polymorphism in Na2Fe2F7 and Its Effect on Electrochemical Properties
Authors: Foley Emily E , Wu Vincent C , Jin Wen , Cui Wei , Yoshida Eric , Manche Alexis , Clement Raphaele J 
Published: September 2023

With the present materials supply crunch caused by the burgeoning Li-ion battery-based electric vehicle market, more sustainable battery chemistries are needed. When considering possible chemistries, sodium (Na) and iron (Fe) stand out as promising candidates for the intercalant ions and redox centers, respectively. Recently, weberite-type sodium metal fluorides (Na2
M
2+
Mʹ
3+F7) have emerged as potential high performance Na cathode materials1-4 with predicted energy densities in the 600 to 800 Wh/kg range (higher than any known Na cathode material) and high ionic conductivity and structural stability. One of the few weberites that has been electrochemically tested so far is Na2Fe2F7 and while one study has shown it to have great, high-rate performance,2 there have been inconsistent reports regarding the structure and electrochemical properties of Na2Fe2F7.
In this study, we endeavor to reconcile these characteristics using a combined experimental-computational approach. Density functional theory calculations reveal the inherent metastability of the weberite phase and the close energetics of several Na2Fe2F7 polymorphic forms, as well as the predicted (de)intercalation behavior for each Na2Fe2F7 polymorph. Experimentally, we find that Na2Fe2F7 samples inevitably contain a mixture of weberite polymorphs as characterized using a broad suite of long-range and local probes sensitive to both amorphous and crystalline phases. Notably, we present the first ever 23Na nuclear magnetic resonance (NMR) investigation into weberites and showcase NMR’s ability to parse out different local environments despite the presence of polymorphism. Electrochemically, our Na2Fe2F7 material shows respectable early capacities yet exhibits steady capacity fade, which ex situ synchrotron X-ray diffraction and 23Na NMR reveal to be associated with transformation of the weberite phase upon cycling. However, fine tuning of electrode composition and design appears to be effective at reducing this effect.
Overall, this study presents an in-depth characterization of Na2Fe2F7 and provides key insights that pave the way to unlocking weberites as high energy density Na-ion cathode materials.
This work made use of the shared facilities of the UC Santa Barbara MRSEC (DMR 1720256) and Center for Scientific Computing (CNS 1725797, DMR 1720256). E. E. Foley and V. C. Wu were supported by the NSF Graduate Research Fellowship Program under Grant No. DGE 1650114. This work is supported by an NSF CAREER award under Grant No. DMR 2141754.

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Fig. 1
, Charge- discharge curve for Na2Fe2F7 and the weberite structure.

Figure 1