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

Abstract

The conventional design paradigm for lithium-ion battery electrolytes prioritizes high-dielectricconstant solvents to promote salt dissociation and enhance ionic conductivity. Here, we challenge this dielectric-centric dogma through systematic molecular dynamics simulations of ternary mixtures comprising 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]), ethylene carbonate (EC), dimethyl carbonate (DMC), and LiTFSI. Contrary to the prevailing expectation that EC—possessing a static dielectric constant approximately thirtyfold greater than DMC—should dominate the lithium cation solvation shell, our simulations reveal a persistent preference for DMC coordination across all mixture compositions. We demonstrate that this anomalous behavior originates from competing steric and entropic factors that override purely dielectric considerations. Radial distribution function analysis shows Li+–O(DMC) first-shell distances of 2.17 Å with coordination numbers ranging from 1.37 to 3.34, while Li+–O(EC) distances extend to 2.32 Å with lower coordination numbers. Spatial distribution functions reveal that DMC molecules occupy geometrically favorable tetrahedral positions around Li+, whereas EC's bulkier structure imposes steric penalties. Increasing carbonate content paradoxically enhances cation–anion interactions: the [EMI]+–[TFSI]- coordination number increases from 12.37 to 7.89 as carbonate concentration rises, indicating that organic co-solvents do not simply dilute ionic associations but restructure them. Mean square displacement analysis shows ion dynamics decrease monotonically with carbonate content, with Li+ diffusion coefficients ranging from 0.079 × 10-12 m2/s (C1, pure carbonate) to 26.27 × 10-12 m2/s (G1, pure IL), revealing a nearly two-orders-of-magnitude dynamical range accessible through composition tuning. These findings establish a new conceptual framework wherein steric accessibility and entropic driving forces supersede dielectric constants in determining ion solvation and transport, providing design principles for next-generation hybrid electrolytes.