Nb18W8O69, with a crystallographic shear structure, hosts superionic lithium ion conductivity that leads to battery rate performance on par with recently reported members of the Nb2O5-WO3 phase diagram including shear phase Nb16W5O55 and bronze-like phase Nb18W16O93. PFG NMR was able to directly capture room temperature lithium diffusion of up to 10 -11 m 2 •s -1 for the majority Li signal contribution in this mixed ionic-electronic conductor. The 5 ´ 5 blocks of (Nb/W)O6 octahedra in Nb18W8O69 provide 16 parallel tunnels for long-range lithium diffusion, which may explain its high Li + diffusion. Nb18W8O69 is a metastable member of the Nb2O5-WO3 phase diagram and variation in synthetic procedures lead to subtly different diffraction patterns and capacity retention. Understanding the nature of the defect chemistry and optimizing the synthesis accordingly could lead to improved electrochemical cycling performance in this and related Wadsley-Roth phases.