Effect of Concentration and Temperature on the Structure and Ion Transport in Diglyme-Based Sodium-Ion Electrolyte

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.