From Clustered Motion to Percolation: Unveiling Correlated Lithium Ion Dynamics in Ionic Liquid Mixtures via an Ab Initio Force Field

Understanding the local solvation structure and correlated ion motion in lithium salt/ionic liquid mixtures is critical for designing high-performance electrolytes. Accurately modeling the complex electrostatic environment of these concentrated systems requires explicit treatment of electronic polarization─an effect often inadequately represented by conventional nonpolarizable force fields. Here, we present a first-principles polarizable force field for lithium ions in ionic liquid environments, derived from ab initio Symmetry-Adapted Perturbation Theory (SAPT) calculations. Particular emphasis is placed on the rigorous determination of atomic polarizabilities, especially in systems with small, high-charge-density species such as Li⁺, where standard force fields tend to overestimate short-range induction. The resulting force field accurately predicts lithium diffusion coefficients and ionic conductivities at low Li⁺ concentrations, and significantly improves quantum-mechanical cluster interaction energies compared to nonpolarizable force fields. We demonstrate that the local solvation structure in these systems is highly force field dependent, challenging prior assumptions of structural insensitivity. At elevated Li⁺ mole fractions, we further uncover correlated ion-motion and clustering behavior that gives rise to a nonmonotonic trend in the lithium transference number: an initial decrease due to Li⁺-anion cluster dynamics, followed by an increase beyond χ_Li⁺ ≈ 0.5 associated with the formation of a percolated ionic network. These results provide new physical insight into the interplay between polarization, solvation, and ion transport in concentrated lithium electrolytes, offering valuable guidance for designing high-performance electrolytes with enhanced lithium-ion mobility.