Density-functional theory calculations of the relative energies and solid-state NMR observables of NaNb7O18 were carried out in the planewave pseudopotential code CASTEP v21. The starting structure was taken from Marinder and Sundberg (ICSD 48105) and then charge balanced with two additional A-site Na + cations per unit cell. Calculations used the Perdew-Burke-Ernzerhof exchange-correlation functional and ultrasoft on-the-fly pseudopotentials generated in CASTEP with a planewave energy cut-off of 800 eV and a (3  2  9) Monkhorst-Pack k-point grid . Geometry optimization was performed with the Limitedmemory Broyden-Fletcher-Goldfarb-Shanno (LBFGS) algorithm until the forces on atoms and stresses on the cell converged to better than 0.05 eV Å -1 and 100 MPa, respectively. Subsequently, the NMR chemical shielding and quadrupolar interaction tensors were calculated using the gauge-including projector augmented-wave (GIPAW) approach. Electrochemistry. Electrodes were made by slurry casting with a 160 µm doctor blade onto 50-µm-thick copper foil (AOT). NaNb7O18 and Super P carbon (Gelon) were ground together in an agate mortar and pestle and then added to a solution of polyvinylidene difluoride (Kynar HSV900) dissolved in N-methyl-2-pyrrolidone (99.9%, anhydrous, NMP, Sigma Aldrich). The final mass ratio of active material:carbon:binder was 80:15:5. Circular electrodes were punched with areas of 1.27 cm 2 and mass loadings of 2.4 ± 0.4 mg cm -2 .