Operando XAS experiments at the Bi LIII-edge (13419 eV) and the Sn K-edge (29200 eV) on a cell pellet of BiF3|BaSnF4|Sn at 100 °C during galvanostatic discharging were carried out at the DESY (Deutsches Elektronen-Synchrotron) beamline P65 in Hamburg, Germany. The current density used for galvanostatic discharging is 20 μA cm -2 . An image of the measurement setup is shown in Figure S 6Figure S 6b. The measurements were performed using a doublecrystal monochromator -Si(111) for Bi, Si(311) for Sn measurements -with a maximum synchrotron beam current of 100 mA in continuous mode (300 s per spectrum). All spectra were recorded in fluorescence mode due to the operando setup. Additionally, for measurements at the Bi edge a Co filter (3 μm thickness) was applied to reduce interference radiation on the detector. For energy calibration, a Bi foil and a Sn foil were measured in advance and after sample spectra to account for possible energy shift due to monochromator movements. The edge energies were determined by the first maximum in the spectrum derivatives. For comparison and analysis of the overall energy shift range during galvanostatic discharging, ex-situ XAS spectra of another fresh-prepared cell pellet (denoted as starting BiF3 in Figure and) and a cell sample discharged to 444 mAh g -1 (also investigated by ex-situ XRD) were recorded in advance.