Lithium niobium tungsten oxide samples were prepared by electrochemically lithiating 210±20 mg pellets of Nb18W8O69 over the course of 60-100 h in coin cells following the procedures previously described. After lithiation, the cells were brought into the inert atmosphere of the glovebox where the electrodes were extracted, washed with DMC (3 ´ 2 mL), dried under the vacuum of the prechamber, ground in an agate mortar and pestle, and packed into 4.0 mm zirconia rotors that were then sealed into J. Young NMR tubes. Signal-to-noise (S/N) is particularly important for PFG NMR because it measures signal decay so the initial (g = 0) S/N ratios are given for all compositions and temperatures (Supplementary Table ). All diffusion measurements were performed on Nb18W8O69-I due to the similarity of the electrochemical rate performance of the three samples and to the time-consuming nature of variable temperature and variable composition PFG NMR experiments on partially discharged battery electrode materials.