Anionic effects on ultrafast dynamics of concentrated lithium electrolyte solutions

Concentrated electrolyte solutions are widely used in systems including material synthesis, energy storage, geochemical mineral transformation, carbon storage, critical materials separations, and biological processes. The chemical properties of ions in solution are directly affected by the dynamics of the solvent, counter ions, and local environment interacting with the ions on the femtosecond-picosecond timescale. While molecular dynamics computations can be performed at such ultrafast timescales, direct experimental measurements are limited by the available molecular spectroscopy techniques. In this work, we use two-dimensional infrared spectroscopy (2D-IR) to investigate the effects of anions on the water-ion and ion-pair dynamics in concentrated solutions of common lithium salts. The dynamics of the lithium ion are measured using methyl thiocyanate as a probe molecule, which displays distinct vibrational spectral features when bound to either a water molecule or lithium ion, allowing the direct measurement of the effect of chemical composition local solvation environment on the ion interaction dynamics at timescales comparable to those of the molecular dynamics calculations.