Lithium-Ion Battery Electrolyte Structural Transport: Anion Dependent Exchange Mechanisms

Li + moves through an electrolyte both by diffusing with its intact solvation shell (vehicular transport) and by exchanging its solvent partners (structural transport). Structural diffusion depends on the frequency of solvent dissociation from Li + and how far the ion moves when it does, yet neither is well characterized experimentally. Li + transport in propylene carbonate solutions of LiTFSI, LiBF 4 , and LiPF 6 , from battery-relevant to superconcentrated levels, was studied via a vibrational probe molecule, phenyl selenocyanate, with ultrafast two-dimensional infrared chemical exchange spectroscopy. Li + diffusion coefficients significantly exceed the Stokes-Einstein prediction. Combining this excess diffusion with solvent dissociation times yields the the step length Li + moves per exchange event. Dissociation times correlate with the reorientation time of unbound probe molecules, indicating that productive exchange requires reorganization of the surrounding solvent. For all salts, the step length decreases with concentration. In relatively dilute LiTFSI it is ~1.3 Å, matching the distance predicted from an associative mechanism through a five-coordinate intermediate, whereas in LiPF 6 it is 0.7–0.8 Å at every concentration, consistent with a predominantly dissociative mechanism through a three-coordinate intermediate. Because PF 6- forms the fewest contact ion pairs, it leaves the least free solvent available to act as an entering group, suppressing the associative pathway. Combined with the longest dissociation times, these short steps make LiPF 6 electrolytes generally the most reliant on vehicular transport despite their high viscosity. Thus, the anion influences not only how often Li + exchanges solvent but also the mechanism, and therefore how far Li + moves with each step.