Effects of Strongly and Weakly Solvating Electrolytes on Ionic Conductivity in Sodium-Ion Battery Electrolytes

The ion transport in sodium-ion battery electrolytes is significantly influenced by the synergistic effects of strongly solvating electrolytes (SSEs) and weakly solvating electrolytes (WSEs), yet the underlying mechanism remains to be illustrated. In this study, molecular dynamics simulations were conducted to investigate the solvation structures and ionic conductivities (σ's) of 1 M NaPF6 in mixed SSE/WSE electrolytes composed of diglyme (DG) and tetrahydrofuran (THF), with various DG/THF molar ratios of 0/1, 1/8, 1/6, 1/4, 1/3, 1/2, 1/1, 2/1, and 1/0. As DG increases in the electrolyte, it progressively replaces THF and PF6- in the solvation shell of Na+, leading to increasing solvent-separated ion pairs (SSIPs) and decreasing contact ion pairs and aggregates. The proportion of SSIPs in the electrolyte is found to align with the degree of uncorrelated ion motion estimated by the ratio of ionic conductivity from the Green-Kubo relation against that from the Nernst-Einstein relation. On the other hand, when more DG is in the electrolyte, the relaxation of the solvation shell of Na+ becomes slower, limiting the self-diffusivity of Na+. Consequently, σ initially increases and subsequently decreases with increasing DG/THF molar ratios, reaching a maximum σ of 14.05 ± 0.59 mS·cm-1 at DG/THF = 1/3, as a balance between the degree of salt dissociation and relaxation of the solvation shell of Na+. We hope this study provides molecular-level insights into the rational design of electrolytes.