Lithium Salt Effects on the Liquid Structure of Choline Chloride–Urea Deep Eutectic Solvent

Abstract

Deep eutectic solvents (DESs) added with lithium salts are emerging as alternative electrolytes for lithium-ion batteries (LIBs). Yet, to design, optimize, and develop efficient DES-based electrolytes for LIBs, an in-depth understanding of the role played by the lithium cations in the intermolecular interactions between all species in the mixture is crucial. A joint approach of experimental NMR techniques and polarizable molecular dynamics (MD) simulations is used here to gather a comprehensive picture of the structure and dynamics of the prototypical system composed of the DES choline chloride:urea (ChCl:U, xChCl = 0.33) and the lithium salt containing the same anion, LiCl. Strong coordination of lithium cations by chloride anions, resulting in the formation of LiCl32– units, is revealed. Other species (especially, urea) are present in the second coordination shell of lithium, creating an extensive hydrogen-bond network. The effect of small quantities of water, typically absorbed by DES from air moisture, on the studied properties is discussed.