Data for 10.1021/jacs.5b13273 Raw data from the Advanced Photon Source (uploaded as .tiff files), collected in June 2014 (used for Figure 1, 3 - 8). NMR data for electrochemically prepared samples and model compounds (collected in Cambridge)uploaded as .txt files (Figure 1, 3). Electrochemical data for in situ PDF experiment uploaded as a .txt file (Figure 3, middle). Cif files for DFT-predicted structures. X-ray scattering data: data is numbered sequentially (starting 1), collected every 45 minutes. The voltage at the point of each data collection can be found by reference to the electrochemical data (column 2 shows the 45 minute slot each .tiff file is connected to). Corrected and normalised and Fourier transformed data (details in text) is shown for every fourth file in Figure 3. Background measurement and CeO2 measurement are also included. NMR data is normalised with respect to the number of scans and sample weight and named sequentially from 1 in the order shown in Figure 3, starting from the bottom: 1) SuperP carbon discharged to 1 mV 2) Pristine electrode 3) 2hour sodiation 4) 4hour sodiation 5) 8hour sodiation 6) 12hour sodiation 7) 16hour sodiation 8) End of sodiation S1-b 9) End of process D1-a 10) End of process D1-b 11) End of process D1-c 12) End of process S2-a 13) End of process S2-b 14) End of process S2-c 15) End of process S2-d 16) Model compound Na3Sb 17) Model compound NaSb

This research data supports "Tracking sodium-antimonide phase transformations in sodium-ion anodes; insights from operando pair distribution function analysis and solid-state NMR spectroscopy" which has been published in "Journal of the American Chemical Society".