In this paper, we thus demonstrate an operando study of the first discharge process of the cell BiF3|BaSnF4|Sn over a wide potential range at 100 °C with X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS), respectively. For the first time, we reveal that the phase transition of the initial orthorhombic phase of BiF3 (o-BiF3) into metallic Bi is not a direct reduction process. In contrast to a simple reconstructive conversion reaction, three intermediate phases are identified during this process. operando XRD measurements provide real-time monitoring of the structure evolution of the BiF3 composite, correlating the transition process to the corresponding cell potential (capacity). This approach differs from ex-situ analysis and even reveals the formation of an intermediate orthorhombic Bi-based phase, which has not been reported so far. This allows for the deduction of a reaction mechanism, providing new insights into the degradation of the electrolyte material and overall cell performance, which will contribute to further improving the cycling stability of high-capacity BiF3 cathode material while reducing the operating temperature closer to ambient conditions. were sealed in a ZrO2 milling vial (50 mL) with ZrO2 milling balls (5 mm diameter, ball-to-powder ratio of 17:1) under argon atmosphere in the glovebox and milled at 600 rpm for 4 h. After ball milling, the powder mixture was annealed at 300 °C for 2 h under dynamic vacuum (10 -2 mbar) using a Büchi Glass Oven B-585. The mechanical milling and the annealing process were repeated three times to enhance the doping process. Bismuth trifluoride (BiF3) (99 %), Sn nanopowder (>99 %, <150 nm particle size (SEM)) and carbon nanofibers (CNF) (>98 %) from Sigma Aldrich were used to prepare the BiF3 cathode and Sn anode composites, as described by Reddy et al. . The cathode composite consisted of 40 wt% BiF3, 50 wt% BaSnF4, and 10 wt% CNF, while the anode composite contained 50 wt% Sn, 40 wt% BaSnF4, and 10 wt% CNF. Again, BiF3 and CNF were dried at 190 °C under dynamic vacuum for 24 h using the vacuum furnace before synthesis process.