Controlling the Ion Transport Number in Solvent-in-Salt Solutions

Solvent-in-salt (SIS) systems present promising materials for the next generation of energy storage applications. The ion dynamics is significantly different in these systems from that of ionic liquids and diluted salt solutions. In this study, we analyze the ion dynamics of two salts, Li-TFSI and Li-FSI, in highly concentrated aqueous and acetonitrile solutions. We performed high-frequency dielectric measurements covering the range of up to 50 GHz and molecular dynamics simulations. The analysis of the conductivity spectra provides the characteristic crossover time between individual charge rearrangements and the normal charge diffusion regime resulting in DC conductivity. Analysis revealed that the onset of normal charge diffusion occurs at the scale of ∼1.5-3.5 Å, comparable to the average distance between the ions. Based on the idea of momentum conservation, distinct ion correlations were estimated experimentally and computationally. The analysis revealed that cation-anion correlations can be suppressed by changing the solvent concentration in SIS systems, leading to an increase of the light ion (Li⁺ in our case) transport number. This discovery suggests a way for improving the light cation transport number in SIS systems by tuning the solvent concentration.