Solvation Structure, Ion Association, and Transport in Sodium Glyme Electrolytes: From Monoglyme to Pentaglyme and Binary Mixtures

Abstract

Glyme-based electrolytes have emerged as promising candidates for sodium-ion batteries owing to their ability to form stable solvation structures around Na+ via multidentate coordination with ether oxygen atoms. While the solvation and transport properties of sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) in short-chain glymes have been systematically investigated, solutions of pentaglyme (G5) remain poorly understood and, to the best of our knowledge, are unexplored in binary glyme mixtures. Herein, molecular dynamics (MD) simulations are used to investigate the solvation structure, ion association, and transport properties of NaTFSI across six systems: monoglyme (G1), triglyme (G3), and pentaglyme (G5), as well as three binary G1:G3 mixtures of varying composition. A combination of experiments and simulations is employed to provide a comprehensive structural and dynamic characterization, including ionic conductivity, viscosity, van Hove correlation functions, and Walden analysis. The results reveal that G3 preferentially displaces G1 from the Na+ coordination shell, even when present in small amounts, thereby generating free G1 molecules that reduce viscosity in G1-containing systems. Analysis of the Na+–Na+ spatial correlations reveals a glyme-bridged Na+–G3–Na+ structural motif unique to G3 and persisting across all G3-containing mixtures. In contrast, G5 exhibits fundamentally different dynamical behavior from G1(0.5):G3(0.5), despite providing the same oxygen-to-Na+ ratio, displaying the highest viscosity and lowest conductivity among all systems studied while maintaining long-lived Na+-Na+ structural correlations. Within the binary mixtures, the G1(0.25):G3(0.75) electrolyte achieves the optimal balance, exhibiting the smallest deviation from the ideal Walden line. These findings demonstrate that molecular architecture, as well as chain length, governs electrolyte behavior, providing fundamental insights for the rational design of glyme-based sodium-ion battery electrolytes.