Despite the common concern that large volume changes of CAMs observed for conversionbased phase transformation (recrystallization) lead to loss of contact and poor capacity retention upon cycling, the detailed phase transformations occurring upon charge/discharge processes are only little understood. In the case of BiF3, previous investigations on reaction mechanism were mostly carried out on LIBs (BiF3 + 3 Li + + 3 e - 3 LiF + Bi) and/or FIBs (BiF3 + 3 e - Bi + 3 F -) based on liquid electrolytes . Konishi's group proposed that the poor cycling performance of BiF3 electrode in Lithium ion batteries (LIBs) could be attributed to insufficient discharge reactions as well as the production of Bi metal and organic electrolyte impeding surface charge reactions . Yamanaka's group observed different reactivity and defluorination mechanisms of different BiF3 modifications (orthorhombic structure (o-BiF3) and cubic structure (c-BiF3)) in FIBs with liquid electrolytes by in-situ Raman microscopy . They proposed that the defluorination of o-BiF3 occurred via a direct defluorination mechanism while the defluorination of c-BiF3 was most likely controlled by the competition between a direct mechanism and a dissolution-deposition mechanism. Li and his group presented an aqueous dual-ion battery in which the pre-addition of Bi7F11O5 to the BiF3 electrode composite can enhance the structural stability and reversibility of Bi-F-O phases by formation of intermediate phase Bi7F11O5 during defluorination, which can reduce the volume changes and improve electronic conductivity . However, there is a poor understanding of the evolution of the BiF3/Bi electrode in all-solid-state fluoride ion batteries, both theoretically and experimentally. Rongeat et al. reported the formation of an oxidefluoride phase, referred to as BiF3-2xOx (Fm-3m, 0.41 ≤ x ≤ 0.52), during the fluorination of Bi2O3 (19 wt% detected by X-ray diffraction (XRD)) present in the commercially purchased Bi powder. Based on their X-ray diffraction analysis, they suggested that BiF3-2xOx (0.41 ≤ x ≤ 0.52) cannot be defluorinated within a subsequent discharging. They noted that a more detailed data analysis was hindered due to intense and broad reflections from the used electrolyte phase La1-xBaxF3-x (0.05 ≤ x ≤ 0.15, denoted as LBF), which overlapped with signals from the Bi-based phases. Later in 2017, Grenier et al. studied the involvement of various bismuth oxidefluoride phases in electrochemical reactions using symmetric cells (BiF3/Bi -LBF -Carbon) via synchrotron-based XRD and pair distribution function (PDF) analysis. The CAMs consisted of 24 wt% BiO0.55F1.9 (R-3m) and Bi2O3 (relative 29-44 wt%, P21/c). To date, it has been observed that electrochemical reactions within BiF3/Bi active materials can involve multiple species and phases depending sensitively on different electrolytes and cells testing conditions, however, these reactions have only been clarified in part. Moreover, rechargeable all-solid-state FIBs (ASSFIBs) with novel roomtemperature solid electrolytes, such as BaSnF4, PdSnF4 and recently CsPb0.9K0.1F2.9 have been reported. The electrochemical compatibility of BiF3/Bi as CAMs with those electrolytes has been successfully demonstrated at lower temperatures. However, a deeper understanding of the involved phases and their structure evolution is still required. Thus, there are many open questions remain to be addressed to improve the cyclability and stability of ASSFIBs using conversion-based electrode materials. These include the stability with the electrolyte, phase transformations, and the maintenance of active interfaces within CAM particles. Such experimental studies are typically time-consuming, and time-efficient in-situ/operando analyses of fluoride ion batteries have not yet been reported. Further, since these cells are operated at elevated temperature, ex-situ studies at room temperature may not accurately represent the behavior of the materials at operation conditions.