Effect of concentration and temperature on the structure and ion transport in diglyme-based sodium-ion electrolyte

Abstract

Glyme-based sodium electrolytes show excellent electrochemical properties and good chemical and thermal stability compared with existing carbonate-based battery electrolytes. In this investigation, we perform classical molecular dynamics (MD) simulations to examine the effect of concentration and temperature on ion-ion interactions and ion-solvent interactions via radial distribution functions (RDFs), mean residence time, ion cluster analysis, diffusion coefficients, and ionic conductivity in sodium hexafluorophosphate (NaPF6) salt in diglyme mixtures. The results from MD simulations show the following trends with concentration and temperature: The Na+---O(diglyme) interactions increase with concentration and decrease with temperature, while the Na+---F(PF6-) interactions increase with concentration and temperature. The mean residence time suggests that Na+---O(diglyme) are significantly longer lived compared with that of Na+---F(PF6-) and H (diglyme)---F(PF6-), which shows the affinity of diglyme to the Na+ ions. The ion cluster analysis suggests that the Na+ ions largely exist as solvated ions (coordinated to diglyme molecules), whereas some fractions exist as contact-ion pairs, and negligible fractions as aggregated ion pairs, with the latter two increasing slightly with temperature and more with ion concentration. The magnitude of the diffusion coefficients of Na+ and PF6- ions decreases with concentration and increases with temperature, where the Na+ ion has slightly lower mobility compared with the PF6- anion. The simulated total ionic conductivities show qualitative trends comparable to experimental data and highlight the need for the inclusion of ion-ion correlations in the Nernst-Einstein equation, especially at higher concentrations and lower temperatures.