Careful control over the defect chemistry of crystalline compounds is typically critical to transport phenomena (ion, electron, phonon). In one-dimensional (1D) ion conductors, even small concentrations of defects in the diffusion pathway can be pernicious. Wadsley-Roth block phases are 1D ion conductors with high redox capacity and among the highest diffusivity of any battery electrode materials. The origin of their high-rate transport has been attributed to parallel tunnels that could impart defect tolerance, but direct evidence is limited. Herein, a new lithium-ion battery negative electrode material, NaNb