NMR Studies of Ionic and Molecular Motion in Diglyme:LiCF3SO3 Complexes

Abstract

Low molecular mass diglyme forms complexes with lithium salts similar to the high molecular mass and much-studied poly(ethylene oxide) (PEO), and thus serves as a useful model for investigating ion solvation,association, and transport in polymer electrolytes. NMR measurements of diglyme:LiCF3SO3 complexes were performed on all three mobile components (solvent, cation, and anion) at 1H, 7Li and 19F Larmor frequencies of 301.0, 116.9 and 283.2 MHz respectively. The diglyme/Li ratio varied from 5:1 to 40:1, and variable temperature measurements were carried out for a 20:1 sample. Spectra, spin-lattice relaxation times (T1) and self-diffusion coefficients (D) were obtained; D measured by standard pulsed field gradient methods. Spin-lattice relaxation times and self-diffusion coefficients for the anion (19F), cation (7Li) and solvent ( 1H) as a function of diglyme concentration at 20oC are displayed in Figures 1 and 2, respectively. As shown in Figure 1 the T1?s are observed to increase with decreasing salt concentration, consistent with the known increase in viscosity of electrolytes at high salt concentrations. The self-diffusion coefficient is a more direct indicator of mass transport, and thus also decreases with increasing salt concentration. At high salt concentrations (n = 5 and 6) all three species gave similar D?s, indicating strongly correlated motion. For diglyme/salt ratios greater than n=6, the proton (solvent) D's were higher than those of both the cation and anion. A comparison of the anions and cations shows the cations having greater D's than the anions above n = 20. These results will be discussed in the context of cation-anion association and cation-diglyme complexation information gained from vibrational spectroscopic data on the same system. Finally, variable pressure self-diffusion measurements were performed for the first time on any polymer electrolyte using a static magnetic field gradient. For the 10:1 diglyme/LiTf complex, diffusion activation volumes for the anion and host were the essentially the same, again indication strongly correlated motion.