Molecular-level heterogeneity in deep eutectic electrolytes

Deep eutectic electrolytes (DEEs) have in recent years gained momentum as prospective compounds for next-generation (lithium) batteries. Yet, the connection between molecular-level properties and macroscopic performance of DEEs is poorly understood. We have here, by molecular dynamics simulations, studied in detail a few simple DEEs created using the hydrogen bond (HB) donor N-methyl-acetamide (NMA) and one of three different common lithium salts: LiBF₄, LiDFOB, and LiBOB, all in a 1 : 4 molar ratio composition. By altered anion size and symmetry, we observe varying molecular-level heterogeneity (MLH), reflecting differences in local structure and coordination as well as dynamic disorder, and especially the heterogeneity of the HB network. The HB network becomes more localized the more asymmetric and larger the anion is, which also affects the ion-pairing and aggregation of the DEEs. From a dynamic point of view, DEEs with larger MLH show slower ion self-diffusion, due to steric hindrances caused by the larger anions and the localized HB network. Overall, both the MLH and the HB network, inherent properties of DEEs, must be properly balanced to allow for adequate ion transport.