or other cs phases regarding the extent or impact of Wadsley defects nor the nature or impact of crystal facets or surface chemistry on the battery performance. Furthermore, systematic studies are needed to correlate electrochemical properties with particle size and morphology in the Wadsley-Roth phases. A related consequence of the large block size in Nb18W8O69 is that it has the lowest density of cs planes in the Wadsley-Roth structures. The shear planes are thought to provide stability by decreasing the structural degrees of freedom and preventing the tilts and distortions common to unstabilized ReO3 and WO3 upon lithiation. An appropriate balance between conducting tunnels and stabilizing shear planes is thus desired for high-rate battery performance but the optimal shear structure is not established. From the small but growing body of literature on Wadsley-Roth phases, there is no clear trend between long-term stability and shear plane density or shear plane arrangement, though many variables stand between the idealized atomic structure and battery performance. In addition to device fabrication and microstructural considerations, an unavoidable complication is that the variation in the blocks is accompanied by changes in composition. Though Wadsley defects are generally expected, a further challenge in understanding this family is that the exact type, concentration, arrangement, and role of defects is not well-known nor easy to determine. Future work could consider other subtle fundamental aspects of the interplay between synthesis-structure-defects-electrochemistry, the relationship between lithium content and reversibility, and practical strategies to increase the cycle stability of superionic Nb18W8O69 and enable its use in ultrafast charging and high power energy storage.