Revealing a novel intercalation nature of high-capacity conversion cathode materials for Fluoride-Ion Batteries by Operando Studies

To rationalize and improve the performance of high energy density electrode materials for all-solid-state fluoride ion batteries and abstract the increasing demands of energy storage, it is important to gain a fundamental understanding of structure/phase evolution during cell operation, which is closely correlated to capacity fading upon cycling. Here, we design an operando cell for laboratory X-ray diffraction, by which we monitor the real-time structure evolution of BiF3 cathode materials and the degradation of superior ionic conductor BaSnF4 over the negative cell potential range against Sn/SnF2 anode at an elevated temperature (100 °C). The results, together with ex-situ XRD measurements, reveal that BiF3 undergoes multiple-step phase transformations upon defluorination (from original o-BiF3 to c-BiF3 and then o’-BiF3) before the formation of Bi metal, showing an intercalation-type mechanism. In addition, the formation of Bismuth oxidefluoride (BiOF) at later discharged state (beyond a capacity of 200 mAh g-1) is observed, which is attributed to oxygen impurities from the solid electrolyte introduced by the applied solid state synthesis method. Operando XAS measurements confirm the continuous reduction of the Bi oxidation state from +3 to 0 and the formation of multi-intermediate phases during the defluorination process. Through Rietveld analysis on operando XRD patterns we obtain the quantification of phase fractions and crystal structure information of intermediate phases during the defluorination process, from which we are able to derive a model for the first defluorination process of BiF3 at elevated temperatures. Furthermore, the comparison between the results from operando XAS and operando XRD indicates that BaSnF4 plays a crucial role on the transport process of not only F- but also oxygen impurities within the BiF3 cathode composite, contributing to the formation of the oxidefluoride phase. In addition, BaSnF4 shows a rather wide potential window, with degradation taking place at a potential below -200 mV (against Sn/SnF2 anode).