Characterizing the Lithium Ion Coordination in Ionic Liquid Mixtures and Polymer Electrolytes: A Molecular Dynamics Study

Polymer electrolytes (PEs) are actively studied for their potential applications in lithium batteries. In salt-doped dry PEs, negative lithium transference numbers are observed in high salt concentration. This leaves a question about how to effectively control the cation-anion aggregation by polymer-ion solvation in PEs compared to the mixtures of ionic liquids (ILs) with lithium salts. In this work, we develop and import a predictive model for IL electrolytes consisting of Li+, [EMIM+], and [TFSI-], and their mixture with a set of poly(ethylene oxide) (PEO)-based polymers in molecular dynamics simulation. We focus on elucidating the solvation structure of lithium ions with the anion and the polymeric solvent, which provide a molecular interpretation of its effect on the negative transference number. The extent of lithium-anion clustering and its coordination structure is highly sensitive to the electrostatic parameters of the anion/solvent model, which is an unseen trend in neat ILs. Furthermore, in PEs based on PEO the system undergoes rebalancing of lithium solvation by the anion and EO moieties, which had strong influence on the ion transport coefficients of the PE.

This study opens a perspective for fine-tuning molecular interactions to develop high-transference PEs.