Cell pellets were prepared in a three-layers cell configuration by uniaxial pressing anode composite, electrolyte, and cathode composite using a Specac Atlas 25T manual hydraulic press in an argon-filled glovebox (2 t for 90 s). The mass loadings of electrolyte, anode composite and cathode composite were 200 mg, 10 mg and 5 mg, respectively, i. e., the anode composite was used in excess as compared to the cathode composite. The galvanostatic cycling was performed at 100 °C with a current density of 20 μA cm -2 (unless specified otherwise) on potentiostats from Biologic Science Instruments (VSP or VMP-300 for cycling only and SP-150, VSP-300 for operando XAS and XRD measurements, respectively). The specific capacities were calculated based on the weight of active material in the cathode composite (BiF3). Due to the different electrochemical operations and measurement limitations (space of set up, time etc.) for ex-situ and operando characterization techniques, three types of cell designs were custom-made and used. Figure shows a typical high-temperature Swagelok type cell , which is used for charging/discharging cell pellets for ex-situ analysis.