Breaking Dielectric Dogma: Steric-Entropic Dominance and Anomalous Ion Pairing in Ternary Ionic Liquid–Carbonate Electrolytes

Abstract

Conventional electrolyte design assumes that bulk dielectric constants (ε) govern ion solvation and that high-ε solvents necessarily dominate the first coordination shell of Li+ while promoting ion dissociation. Here we systematically challenge this electrostatic paradigm using charge-scaled classical molecular dynamics simulations of ternary 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI])–carbonate blends (EC/DMC) containing 1 M LiTFSI. Across five compositions from pure carbonate to neat IL, we uncover a striking, counterintuitive preference: Li+ coordinates preferentially with low-polarity dimethyl carbonate (DMC, ε≈3.1) over highly polar ethylene carbonate (EC, ε≈90). This anomalous behavior originates from local steric-entropic packing—the flexible, acyclic DMC allows an unconstrained tetrahedral geometry around Li+, whereas rigid cyclic EC imposes severe inter-ligand repulsion. Furthermore, carbonate addition paradoxically reinforces short-range Li+–[TFSI]- bidentate clustering and [EMI]+–[TFSI]- hydrogen bonding. This counterintuitive ion-pairing enhancement arises from a dielectric buffering effect: carbonate molecules screen long-range isotropic Coulombic repulsions, lowering kinetic barriers to close approach and deepening local interaction wells. Spatial distribution functions reveal that carbonate addition disrupts the native anisotropic order of neat [EMI][TFSI] (cations stacked above/below the ring plane, anions in-plane), driving isotropization while simultaneously strengthening specific H-bonds. Quantitative coordination numbers identify a structural ―sweet spot‖ at 40–60 % IL content, where balanced Li+–anion exchange and high vehicular transport coexist. These findings overthrow the simple ε-centric design rule, establishing steric-entropic constraints and dielectric mediation as primary variables for rationally designing non-flammable, high-conductivity post-lithium electrolytes.