Advanced Nuclear Magnetic Resonance Techniques for Characterizing Ionic Liquids for Lithium Ion Battery Applications; High Presssure NMR and Fast Field Cycling Relaxometry

Abstract

Ionic liquids (ILs) have received widespread acclaim for their potential use as a safe and effective alternative to unstable and flammable organic solvents used in electrolytes for lithium-ion secondary batteries. The thermal and chemical stability of ILs, as well as the tunable properties of ILs by altering cation and anion species, have made them promising materials for use in multiple applications: such as textile dyeing, nuclear waste recycling, and solar cells. A limiting characteristic for IL use in secondary lithium-ion batteries is a notable lower ionic conductivity in most known species. Characterizing the effect of cation and anion structure on ionic diffusion in ILs, as well as studying molecular dynamics in IL species is of great importance for increasing their potential utility in battery chemistries. To this end, the techniques of Fast Field Cycling Nuclear Magnetic Resonance (FFCNMR) and pressure dependent NMR diffusometry are used to study ionic liquid species of interest. Fast field cycling measurements of spin-lattice relaxation times (T1) allow for access to time dependent molecular dynamics in ionic systems, as functions of both magnetic field strength and temperature. FFC measurements are complementary to high-pressure NMR experiments, which reveal pressure dependence of reorientation and tumbling processes, as well as the effects of alkyl chain length on ionic mobility in specific species. Presented here is the application of these two techniques to 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (CnMIM TFSA) ILs. Specifically, a selectively deuterated species of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (BMIM TFSA) is observed and characterized. FFC results were used to extract cation self-diffusion coefficients, showing good agreement with Pulse Field Gradient (PFG) NMR measurements. High pressure T1 data on selectively deuterated cation isotopologues revealed site dependent interactions on the cation alkyl chain. The work at BNL (JFW and JH) was supported by the US-DOE Office of Science, Division of Chemical Sciences, Geosciences and Biosciences under contract DE-SC0012704.