Lithium salt/amide-based deep eutectic electrolytes for lithium-ion batteries: electrochemical, thermal and computational study

Deep eutectic solvents (DESs) have recently attracted significant attention as inexpensive materials with similar characteristics to ionic liquids. For practical applications of DESs in electrochemical devices such as lithium-ion batteries (LIBs), the manipulation of the melting point and electrochemical stability is important as they are important parameters that determine device performance. In this study, we investigated a family of Li-salt/amide-based electrolytes (DEEs) comprised of five amide derivatives (urea, acetamide, N,N′-dimethylpropyleneurea, 2-imidazolidinone and tetramethylurea) and two representative Li-salts (LiCl and LiTFSI), in terms of thermal and electrochemical properties. To verify the effect of the coordination state on the melting point, the coordination state between lithium salt and amide was calculated by a molecular dynamics simulation using four representative DEEs. Regarding electrochemical stability, the HOMO and LUMO were calculated by density functional theory and the correlation with the experimental result of cyclic voltammetry was verified. Hydrogen bonding donor (HBD)-free DEEs comprised of amides without any N–H bonds (e.g. 1,1,3,3-tetramethylurea and 1,3-dimethyl-2-imidazoline) were found to be superior to those containing HBDs derived from amides having N–H bonds (e.g. urea, acetamide and 2-imidazolidinone), in terms of reduction stability. Among various DEEs evaluated in this study, the DEE derived from LiTFSI : 1,1,3,3-tetramethylurea = 1 : 5 mol% was the best electrolyte in terms of melting point, electrochemical stability and ionic conductivity. The results of this study provide important guidelines for designing DESs as LIB electrolytes.