Theoretical insights into ion transport mechanisms in fluoroacetonitrile-based electrolytes for Li/Na/K ion batteries: a molecular dynamics and quantum chemical study

Abstract

Designing electrolytes with a weak solvation strategy is a promising route to high-performance alkali-based ion batteries. However, it often compromises ionic conductivity due to incomplete salt dissociation. Therefore, achieving fast cation transport while preserving weak ion interactions thus remains a key challenge. This work employs fluoroacetonitrile (FAN), a solvent characterized by low solvation energy, a low Li⁺ migration barrier, and a small molecular size, and combines molecular dynamics (MD) simulations with quantum chemical (QC) calculations to systematically investigate three FAN-based electrolytes containing different alkali metal ions (Li⁺, Na⁺, and K⁺). The coupled effects of concentration and temperature on ion transport behavior and microscopic structural evolution are elucidated. FAN exhibits consistently low solvation energies toward Li⁺, Na⁺ and K⁺. Within the ranges of 0.5–3 M and 233–298 K, both ionic conductivity and cation diffusion coefficients display a non-monotonic concentration dependence, reaching a maximum near 1 M. Ion speciation analysis shows that temperature has little impact on the relative populations of solvent-separated ion pairs (SSIPs), contact ion pairs (CIPs) and aggregates (AGGs), indicating that local equilibrium structures are largely temperature-insensitive. The optimal transport at 1 M is explained through speciation analysis and the role of solvent-s ion pairs (SSHIPs). Residence-time analysis reveals that cooling from 298 K to 233 K significantly prolongs FAN residence in the first solvation shell, reducing cation hopping frequency despite nearly unchanged speciation. These insights establish a clear link between microscopic solvation dynamics and macroscopic transport, providing design guidelines for wide-temperature, high-conductivity alkali metal electrolytes based on weakly solvating solvents.