Operando synchrotron X-ray diffraction. Operando synchrotron X-ray diffraction measurements were carried out at Argonne National Laboratory on beamline 17BM-B. The samples were measured at ambient temperature in transmission geometry at 51 keV (0.24099 Å) with an area detector. All operando measurements were performed in the AMPIX cell, which has been described elsewhere. Briefly, it contains a hard, conductive glassy carbon window to prevent inhomogeneous electrochemical reactions, which are a concern with flexible and/or non-conductive X-ray windows. Self-standing electrodes were fabricated with a 5:4:1 ratio of NaNb7O18:carbon black (Vulcan XC72R):polytetrafluoroethylene (PTFE, Sigma, 1 µm powder) uniaxially pressed into a pellet at 150 MPa. The active material loading of the 10-cm-diameter electrode was 11.5 mg cm -2 . AMPIX cells were constructed in an argon glovebox with lithium metal counter electrodes (Sigma, 99.9%), glass fiber separators (Whatman GF/B), and 1.0 M LiPF6 dissolved in 1:1 v/v ethylene carbonate/dimethyl carbonate (EC/DMC, Sigma, battery grade). The cell was cycled at a rate of C/3.67 corresponding to a current density of 0.61 mA cm -2 . Two-dimensional diffraction images were converted to conventional one-dimensional diffraction patterns through integration in GSAS-II after calibration with LaB6. Background subtraction, primarily from the glassy carbon window, and normalization to X-ray intensity were performed on the one-dimensional integrated diffraction data.