Revealing an Intercalation Nature of High-Capacity Conversion Cathode Materials for Fluoride-Ion Batteries by Operando Studies.

To improve the performance of high-energy-density electrode materials for all-solid-state fluoride-ion batteries (ASSFIBs), it is important to understand the structure and phase evolution during operation, which is closely correlated to capacity fading. In this study, an operando cell is designed compatible with laboratory X-ray diffraction (XRD) to monitor real-time structural changes of bismuth trifluoride (BiF₃) cathodes and degradation of the ionic conductor BaSnF₄ under negative potentials at 100 °C. Supported by ex-situ XRD, our results reveal a multi-step defluorination of BiF₃: from orthorhombic (o-BiF₃) to cubic (c-BiF₃), then to distorted orthorhombic (o'-BiF₃), and finally to metallic bismuth (Bi), indicating partial intercalation-type character. Formation of bismuth oxidefluoride (BiOF) beyond 200 mAh g^-1 is attributed to oxygen impurities introduced via solid-state synthesis. operando X-ray absorption spectroscopy (XAS) confirms a continuous reduction of Bi₃₊ to Bi₀ with intermediate phases. Rietveld refinement quantifies the phase fractions and structural transitions, enabling a model for BiF₃ defluorination. Comparison of operando XRD and XAS reveals that BaSnF₄ contributes transport of both fluoride and oxygen impurities, leading to BiOF formation. BaSnF₄ also exhibits a broad stability window, with degradation occurring below -200 mV, rather than the expected -50 mV vs. Sn/SnF₂.