(Alfa, 99.8%, g-phase, light green) ground together with an agate mortar and pestle, uniaxially pressed into a free-standing pellet, and annealed in a covered Pt crucible via three different heating procedures described in the main text. X-ray diffraction was measured with a Panalytical diffractometer equipped with a Cu Ka source in Bragg-Brentano geometry. Data were collected at ambient temperature at 0.83°×min -1 from 3-150° 2q whilst the sample stage was continuously rotated to improve powder averaging. Rietveld refinement was performed with GSAS-II, structural representations were drawn in VESTA, and bond valence sum maps were calculated with 3DBVSMAPPER Electrode active materials mass loadings were 2.0 ± 0.2 mg•cm -2 . Electrochemistry was performed in a temperature-controlled room at 293 K with a Biologic galvanostat/potentiostat running EC-Lab software. Accessible capacity and, therefore, C-rate are not well-defined in the Wadsley-Roth phases due to the sloping voltage profiles that lead to a strong dependence of capacity on the voltage window; multielectron redox of both Nb 5+ to Nb 3+ and W 6+ to W 4+ are known. In these phases, we believe the most convenient method is to defined C-rate relative to one electron transfer per transition metal: for Nb18W8O69 (to Li26Nb18W8O69): 1C = 164.1 mA×g -1 , 20C = 3282 mA×g -1 .