Tight-Binding Modelling of Deep Eutectic Solvent Based Electrolytes

Abstract

Deep eutectic solvents (DES) have in recent years gained momentum in the development of new electrolytes for lithium-ion batteries (LiBs)1. Due to their easy creation, tuning possibilities, wide electrochemical stability windows, and low vapor pressures, they are not only able to create safer LiB electrolytes as compared to traditional organic electrolytes, but more importantly, they may enable high voltage LIB cells2. One aspect not yet fully understood is the connection between the symmetry and entropy of the DES local structure and their macroscopic properties. Exploring this connection could assist in rational design of more performant electrolytes, including controlled electrochemical properties, by symmetry and entropy tuning.Herein, we have studied in detail a few simple DES electrolytes created using the hydrogen bond donor N-methyl-acetamide (MAc) combined with each of the three different lithium salts: LiBF4, LiDFOB, and LiBOB, in 1:4 molar ratios – as previously shown advantageous 3. We chose these salts as they exhibit pseudospherical (BF4-), asymmetrical linear (DFOB), and symmetric linear (BOB) anion geometries and thus are plausible to introduce local symmetry differences. Molecular dynamic simulations were run using DFTB+ 4 and the xTB method 5, and as a first level geometrical analysis we look at the (partial) coordination and solvation numbers of the lithium ions.