Tight-Binding Modelling of Deep Eutectic Solvent-Based Lithium Battery Electrolytes

Abstract: In recent years, there has been a growing interest in utilizing liquid electrolytes such as deep eutectic solvents (DES) as electrolytes for lithium batteries, owing to their numerous advantageous properties, including ease of preparation, tunability, wide electrochemical stability windows, and low vapor pressures. However, an aspect that remains poorly understood is the intricate relationship between the local structural symmetry, entropy, and the macroscopic properties of DES electrolytes. Using simulations is vital in facilitating comprehension of the properties and dynamics of various materials and is here applied to DES electrolytes.  Herein different semi-empirical computational tools have been used to study the local structure connection to macroscopic properties of different electrolytes, both more standard sodium-based highly concentrated electrolytes and lithium-based deep eutectic electrolytes (DEEs). Molecular dynamic simulations were carried out using different software such as CP2K utilizing the xTB method and MOPAC. Through studying DEEs made by combining N-methyl-acetamide (MAc) as the hydrogen bond donor with each of three distinct lithium salts: LiBF4, LiDFOB, and LiBOB, at a 1:4 molar ratio, the role of anion geometry and symmetry for the (partial) coordination and solvation numbers of the cations and calculating the diffusivity of Li+ and anions were revealed.  We find that in the DES electrolytes, the spherical BF4- anion results in more solvated cations, while the bulkier and non-spherical anions (BOB, DFOB) tend to promote the formation of ion pairs and aggregates. This markedly differs from the observations made with conventional electrolytes employing the same salts.