Note that relevant exponent in the BPP model (see Supplemental Note 2) is the quantity (1 + 𝛽), but 𝛽 is referred to here as the asymmetry parameter. The activation energies obtained by a leastsquares fitting to the BPP equation are 0.113 ± 0.005 eV for 6 Li10SnP2S12 and 0.134 ± 0.007 eV for Li10SnP2S12. The symmetric BPP model (𝛽 = 1) corresponds to three-dimensional Li + migration, which is expected in this temperature range. A further confirmation for this constraint on 𝛽 is that the magnitudes of the activation barriers agree closely with those obtained from the high-temperature SAE measurements. In the case of 7 Li10SnP2S12, our results also agree with those of Kaus et al., who reported a Li activation energy of 0.13 eV. It should be noted that the 𝑇 1 relaxation method could not be used at low enough temperatures to verify the activation barrier associated with Li + transport in the channels. Already, the 𝑇 1 relaxation times at the lowest two temperatures were not included in the regression analysis (of both datasets). Nevertheless, these results provide confidence that there is an isotope effect on ion transport in these Li10SnP2S12 materials and, specifically, a change in the activation barrier.