From vehicular to structural diffusion: Insights into concentration-dependent Li-ion transport in LiFSI/DMC electrolytes

Achieving high cation transference numbers (t + ) in liquid electrolytes is essential for advancing high-power lithium-ion batteries, yet the molecular origin of the experimentally observed rise in t + at elevated salt concentrations remains incompletely understood. Here we investigate the transport behavior of lithium bis(fluorosulfonyl)imide (LiFSI) dissolved in dimethyl carbonate (DMC) using classical molecular dynamics simulations. We find that the Li + solvation environment becomes increasingly heterogeneous and diverse with salt concentration, exhibiting a broader distribution of local coordination motifs as the solvation shell grows progressively more anion-rich and solvent-deficient. Decomposition of the ionic conductivity into ion-ion correlations indicates that cation (Li + ) dynamics dominate over anion (FSI − ) dynamics at high salt concentrations, yielding an overall increase in t + . The cation–anion cross correlation switches from positive at low concentration to weakly negative at higher concentrations, reflecting a transition from correlated cation–anion co-motion to anti-correlated ion dynamics. Analyses of the characteristic diffusion length and trajectories show that Li + travels progressively shorter distances before its initial solvation shell dissolves, signifying a transition in the dominant transport mechanism from vehicular to structural diffusion, and that Li + migration at high concentrations occurs primarily through exchange of coordinating oxygen atoms, consistent with a structural diffusion mechanism. Our results point to solvation-shell exchange kinetics as a key design lever, suggesting that anions and solvents engineered to promote structural (rather than vehicular) Li + transport could unlock liquid electrolytes with simultaneously high conductivity and high t + .