Operando Synchrotron X-ray Diffraction: Operando synchrotron X-ray powder diffraction (SXRPD) measurements were performed on the MSPD beamline of the ALBA synchrotron in Spain with Debye-Scherrer geometry (λ=0.8266 Å) using an in situ coin cell with glass windows. Prior to the operando measurements, the SXRPD pattern of the pristine Na3V2(PO4)3 was collected with a 0.3 mm diameter capillary. Two in situ cells containing Na3V2(PO4)3 electrodes were measured at the same time thanks to the eight-cells holder with an automatic positioning system available at the MSPD beamline. Each SXRPD pattern was collected every 30 minutes with an acquisition time of ~3.5 minutes in the 2θ angular range of 2-40°, with a 2θ step size of 0.006° using a MYTHEN detector for rapid pattern collection. The working electrodes were composed of the Na3V2(PO4)3 powder and carbon black (80/20 in wt%), and Na metal was used as counter/reference electrode. The mass loadings of the active material in the electrodes were 5.33 and 6.12 mg, respectively. One sheet of Whatman glass fiber (GF/D) was used as a separator and the electrolyte was composed of 1 M NaPF6 in ethylene carbonate (EC) / dimethyl carbonate (DMC) (1:1, w/w) with 2 wt.% of fluoroethylene carbonate (FEC). The first cell was measured for one cycle with a slow electrochemical reaction rate of 0.11 C (1 C = 58.2 mA/g, or about 9 h for the exchange of 1Na + / 1e -) with a voltage window of 1 -3.75 V versus Na + /Na. The second cell was measured for six cycles with higher C-rates (i.e., 0.37-0.77-0.77-0.77-0.37-0.77 C). A large overpotential was observed during the measurements of the second cell, hence the voltage windows were adjusted with the respected C-rates (detailed cycling conditions can be found in Table in the supporting information section).