7 Li pulsed field gradient nuclear magnetic resonance (PFG NMR) spectroscopy is commonly applied to liquid and superionic solid electrolytes where lithium diffusion is sufficiently fast and nuclear relaxation is sufficiently slow that it is possible to measure lithium motion over micrometer distances. In principle, PFG NMR should be extremely useful for measuring ionic transport properties in mixed ionic-electronic conductors such as battery electrode materials but prior to Nb16W5O55, this had proven impossible due to relatively slow Li + diffusion and electronic effects. In light of the fast electrochemical intercalation of lithium through the bulk particles of Nb18W8O69, As expected, the value of p is correlated with the extracted diffusion coefficients. Fitting the PFG diffusion data with a biexponential function increases the uncertainty of these correlated parameters so the impact of the degrees of freedom (one, two, or three parameters) was evaluated systematically (Supplementary Figure ) and 95% confidence intervals were calculated (Figure , Supplementary Figure ). The best fits to all the data sets were achieved with values of p fixed between 0.3-0.5 while letting the diffusion coefficients refine (i.e. a constrained biexponential), with very little difference between goodness-of-fit parameters being obtained in this range of p (Figure , Supplementary Figure ). Monoexponential, unconstrained biexponential, and constrained biexponential fits and their implications for the extracted diffusion coefficients are detailed in the Supporting Information (Supplementary Figures and). Li diffusion coefficients from 10 -13 m 2 •s -1 to 4´10 -11 m 2 •s -1 were observed at room temperature (298