Impact of Frictional Interactions on Conductivity, Diffusion, and Transference Number in Ether- and Perfluoroether-Based Electrolytes*

Little is known about the underpinnings of ion transport in fluorinated polymer electrolytes. We use NMR and full electrochemical characterization in tetraglyme mixed with LiTFSI (H4) and in a fluorinated tetraglyme analog mixed with LiFSI (F4). Conductivity is significantly lower in F4 than in H4, and F4 exhibits negative transference numbers, while those of H4 are positive at most salt concentrations. By analyzing Stefan-Maxwell diffusion coefficients, we conclude that at low salt concentrations, the difference in transference numbers is due to differences in anion-polymer interactions. We also develop several Nernst-Einstein-like equations relating ionic conductivity to Stefan-Maxwell, instead of self, diffusion coefficients. In F4, we find that conductivity is well-described by a new regime in which the only Stefan-Maxwell diffusion coefficient influencing conductivity is the cation-anion diffusion coefficient. This implies that in F4, ionic conductivity is controlled completely by cation-anion frictional interactions