Investigating the Transport Properties and Solvation Chemistry of Tertiary Ionic Liquid-solvate Ionic Liquid Mixtures

Lithium-metal batteries are of interest as a post-lithium-ion battery chemistry due to their high theoretical energy density. Both lithium-ion and lithium-metal batteries suffer from safety concerns due to use of flammable organic electrolytes. Ionic liquids are non-volatile, non-flammable molten salts that are liquid below 100°C. Solvate ionic liquids are mixtures of a salt and polyether solvent that have similar properties to ionic liquids. Both ionic liquids and solvate ionic liquids have good electrochemical stability windows leading to their interest as a material for battery electrolytes. However, poor Li+ mobility in the electrolyte due to high viscosity limits their applicability in lithium-based batteries. In this work, ionic liquids with polyether side-chains and various imide anions are synthesized and added to solvate ionic liquids in an attempt to modify the solvation environment around the Li+ ion. The tertiary electrolytes show improved ionicity with the addition of ionic liquids compared to the binary solvate ionic liquid. Using Walden’s rule to plot the measured ionicity against the ideal KCl solution suggested increased ionicity in tertiary mixtures. Nuclear magnetic resonance (NMR) spectroscopy was used to measure 1H, 19F, 7Li and 13C and the spectra obtained informs about the local chemical environments present in the electrolyte compositions. Raman spectroscopy allows the investigation of the Li+ - imide anion environment to show anion interaction with the glyme and ionic liquid solvent. Pulsed field gradient NMR probing 1H, 19F, and 7Li nuclei to experimentally determine the diffusion coefficients of each species and couple with other techniques allows for better understanding of species mobility in the various electrolyte compositions.