While electric fields primarily result in migration of charged species in electrolytic solutions, the solutions are dynamically heterogeneous. Solvent molecules within the solvation shells of the cation will be dragged by the field while free solvent molecules will not. We combine electrophoretic NMR measurements of ion and solvent velocities under applied electric fields with molecular dynamics simulations to interrogate different solvation motifs in a model liquid electrolyte. Measured values of the cation transference number (𝑡 ! " ) agree quantitatively with simulation-based predictions over a range of electrolyte concentrations. Solvent-cation interactions strongly influence the concentration-dependent behavior of 𝑡 ! " . We identify a critical concentration at which most of the solvent molecules lie within solvation shells of the cations. The dynamic heterogeneity of solvent molecules is minimized at this concentration where 𝑡 ! " is approximately equal to 0.

Improving transport properties of the electrolyte is important for developing lithium-ion batteries for future energy storage applications. In Newman’s concentrated solution theory, electrolytes are characterized by three transport parameters, conductivity,...

Accurate measurement of the cation transference number is critical for designing batteries with a given electrolyte. A promising approach for measuring this parameter is electrophoretic nuclear magnetic resonance (eNMR). In the standard approach, the average cation, anion, and solvent velocities under an applied electric field are used to estimate the cation transference number with respect to the solvent velocity, t + 0. In this study, we show that t + 0 can be determined from measurements of the electric-field-induced velocities of individual species. The t + 0 values obtained from eNMR experiments on a model electrolyte (LiTFSI/tetraglyme) based on single species velocities are consistent with the standard approach. An important parameter that enters into the analysis is the velocity of the electrode–electrolyte interface which must be finite in an eNMR experiment. Agreement is only obtained after accounting for this velocity. The single-species approach is particularly valuable when one or more components of the electrolytic mixture are not easily accessible by NMR, for example zinc and magnesium cations.