Structure and Properties of the Lithium Bis(fluorosulfonyl)imide (LiFSI) in Sulfolane. Molecular Dynamics Simulation

Abstract

The structure and transport properties of lithium bis(fluorosulfonyl)imide (LiFSI) solutions in sulfolane are studied using molecular dynamics simulation in the concentration range of 0.04 to 5.2 M. It is established that the composition of the first coordination sphere of Li⁺ cation depends significantly on the salt concentration: in diluted solutions (<1 M), the lithium cation is solvated by sulfolane molecules; at high concentrations (>1 M), the composition of the solvate shell contains the solvent molecules and the anion–ion associates and polynuclear complexes form. Li⁺ and FSI^– diffusion coefficients decrease sharply when passing from diluted (0.1 M) to concentrated (2 M) solutions because of the increase in interactions between ions. The maximum ionic conductivity (~3.13 mS/cm) is reached at ~0.94 M. In high-concentration solutions (3–5.2 M), the specific ionic conductivity decreases because of the formation of a rigid ion matrix in spite of the increase in the Li⁺ transport number. The results of our study show that electrolytes based on lithium bis(fluorosulfonyl)imide in sulfolane are characterized by a high ionic conductivity, which confirms that they are perspective if they are used as electrolytes for lithium current sources.