In conclusion, on the basis of neutron diffraction and DFT-supported Na NMR spectroscopy, NaNb7O18 can be viewed as a Wadsley-Roth-derived structure with the crystal chemical formula [Na sq.pl. ]0.44[Na cuboct. ]0.56Nb7O18. NaNb7O18 has an extremely high point defect concentration where 16% of the perovskite-A-site-like environments in the tunnels are occupied by cuboctahedral Na + . In a conventional electrode material with 1D ion transport such as LiFePO4, just 1% of tunnel-blocking defects causes 90% capacity loss and 100x increase in