Theoretical insights into the dissolution of LiFSI in weakly and strongly solvating solvents

Abstract

Cyclopentyl methyl ether, CPME, a weakly solvating solvent for lithium bis(fluorosulfonyl)imide (LiFSI) electrolyte used in Li ion batteries, has been found experimentally to lead to the formation of a more stable solid electrolyte interface (SEI) compared to 1,2-dimethoxyethane (DME). In this study, force field based molecular dynamics simulations and density functional theory based ab initio calculations were used to study the interaction and clustering behaviour in CPME and DME based electrolytes at different concentrations. Treatment of the solvent polarization in force fields via the electronic continuum model reproduces results that largely agree with experimental data. Investigations of the solvent clustering around Li⁺ and the strength of the interaction between the anion and Li⁺ in solution sheds light on the concentration dependence of the formation of FSI⁻/Li⁺ aggregates, which are known to influence the stability of the SEI. In essence, the strength of Li⁺-DME interaction demands that Li⁺ first bind to all available DME molecules before binding to FSI⁻. Cluster survival time analysis further points to a more stable and dominant cluster Li⁺-(DME)₂ compared to the counter-ion pair FSI⁻/Li⁺ at low concentrations of LiFSI.