Quantifying the isotope effect. Several studies have investigated the effects of isotopic substitution on the magnitude of ionic transport in Li + conducting borate glasses, but, in general, there are not many studies of isotope effects in solid-state Li + battery materials. In the study of reaction kinetics, it is commonly the ratio of the reaction rate constants that is used to quantify the isotope effect. In that case, constant factors cancel out by taking the ratio. Herein, we investigate a different isotope effect defined as the difference in activation barriers Δ𝐸 A = 𝐸 A ( 7 Li) -𝐸 A ( 6 Li).