The tunnels within the blocks of Wadsley-Roth crystallographic shear structures enable lithium transport and the 5 ´ 5 blocks of octaheda in Nb18W8O69 are the largest that have been synthesized as an isolated phase. These 2 nm ´ 2 nm blocks give rise to sixteen parallel lithium tunnels with twelve that are adjacent to shear planes (i.e. 'edge' tunnels) and four that are central and ReO3-like (i.e. 'central' tunnels). Bond valence sum maps (Figure ) and calculations on related systems indicate that lithium moves through both types of tunnel. It is not known how the dynamics differ at central vs. edge tunnels; this is an interesting question requiring computational insights, though it is non-trivial as diffusion barriers are highly sensitive to the specific transition metal cation ordering in Wadsley-Roth TiNb2O7. Regardless, the large blocks offer a maximal number of tunnels and rapid Li + conductivity. That the rate performance of Nb18W8O69 is not higher than