Effect of Alkali Metal Ions and Salt Composition on Structural and Dynamical Properties of Ternary Electrolytes.

Abstract

Molecular dynamics simulations were employed to investigate the influence of alkali-metal ions (Li+, Na+, K+, and Cs+) and salt loading on the solvation structure, coordination environment, and transport properties of ternary electrolytes composed of a short-chain polyethylene-block-poly(ethylene oxide) (PE-PEO) surfactant, the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]), and alkali-metal salts M(TFSI). Radial distribution functions, coordination-number analysis, and denticity distributions reveal a continuous evolution in solvation structure across the alkali-metal series. At low salt loading, Li+ and Na+ are predominantly coordinated by PEO oxygen atoms, whereas increasing ionic size promotes greater participation of TFSI anions in the first coordination shell. Cs+ exhibits the largest contribution from TFSI coordination, with more than 65% of its first-shell oxygen coordination originating from anions. Increasing salt loading enhances competition between PEO and TFSI oxygen atoms for coordination sites, resulting in progressively more mixed coordination environments and increased multidentate anion binding. Self-diffusion coefficients and ion-pair lifetime analyses demonstrate that ion mobility is strongly influenced by coordination persistence and ion association. Li+ and Na+ form the most persistent coordination environments and exhibit the largest reduction in mobility with increasing salt loading. In contrast, K+ and Cs+ display shorter-lived coordination interactions and retain comparatively higher diffusivities. Across the alkali-metal series, increasing ionic size (decreased charge density) promotes greater TFSI participation as well as weaker interaction to the surrounding coordination environment, with K+ exhibiting intermediate behavior and Cs+ representing the limiting case. At elevated salt loading, stronger ion association and longer ion-pair lifetimes are accompanied by increasingly similar diffusivities among the ionic species, while K+ and Cs+ maintain higher individual diffusivities than at low salt loading. Increasing temperature enhances ion mobility across all systems by accelerating coordination-shell exchange and weakening interactions between different species. These results establish molecular-level understanding between alkali-metal ion series, solvation structure, coordination dynamics, and transport behavior in PE-PEO/[EMIM][TFSI]/M(TFSI) electrolytes, providing insight which may aid the design of polymer-ionic liquid electrolytes for energy-storage applications.