Material Characterisation. The thermogravimetric analyses were collected on a TA Q600 SDT system, the samples were heated to 900 o C under air with a 2 °C/min ramp rate. X-ray diffraction (XRD) patterns were collected on a Brucker D8 Discover diffractometer using Co Kα (λ = 1.79 Å) operating at 35 kV and 40 mA with a PSD Braun detector. CHN elemental analysis was performed on a Perkin Elmer 2400. Before measurements, the samples were degassed overnight at 90 °C. ICP-AES experiments were performed on reaction filtrates using an Agilient 4210 MP-AES fitted with a SPS4 autosampler. SEM images were taken using the lower electron detector in a JEOL 7100F REGSEM. Infrared measurements were taken using a Schimadzu IT Affinity-1 spectrometer, using an attenuated total reflection crystal sampling technique. The investigated spectral range was from 4000 to 400 cm -1 with a resolution of 4 cm -1 . The signal was obtained by averaging 16 scans. Fe Mössbauer spectra were recorded at room temperature in the transmission mode using a constant acceleration spectrometer and a 57 Co(Rh) source. The velocity scale was calibrated using a 6 μm thick α-Fe foil. All the spectra were computer-fitted and the isomer shifts referred to the centroid of the spectrum of α-Fe at room temperature. ELECTROCHEMCIAL EVALUATION. Electrochemical performances were performed in an operando cell (Figure ). To compensate for the low energy of potassium XANES, 60 µm glassy carbon windows were used as both the current collector and X-ray window of the cell. The electrodes consisted of 80 % active material, 10 % C65 carbon black, and 10 % polyvinylidene fluoride (PVDF) binder, which were ground together into a paste and spread on the substrate with a K-Bar at ca. 100 μm thickness. The electrochemical performance of active material was evaluated using cyclic voltammetry (CV) at ambient temperature. The measurements were performed using an Ivium OctoStat30 potentiostat. Measurements were performed in a three-electrode cell. A Pt wire acted as the counter electrode, and a Ag/AgCl (3M NaCl) filled capillary tube as the reference electrode. The electrolyte comprised of flowing 0.1 M KNO 3 (aq). XANES MEASUREMNTS. XANES measurements was performed at B18 Beamline at Diamond Light Source, Didcot, UK. XANES were set-up with a fast-scanning silicon (111) double-crystal monochromator. Operando measurements were taken in fluorescence mode. For ex-situ measurements, sample preparation consisted of spreading a thin layer of material on carbon tape and taken in total electron yield (TEY). XANES data was processed using Athena software of the Demeter package. Linear Combination Fitting of operando XANES on normalised datasets was caried out in the energy window of 3593-3643 eV. Standards for PB and electrolyte were recorded within the operando cell environment to mitigate for self-absorption effect. The PW standard was taken at a fixed -0.1 V potential after the current response stabilised.