Influence of Zn2+ and Water on the Transport Properties of a Pyrrolidinium Dicyanamide Ionic Liquid

In order to expand our understanding of a potential zinc-based battery electrolyte, we have characterized the physical and transport properties of the ionic liquid (IL) 1-butyl-1-methylpyrrolidinium dicyanamide ([C(4)mpyr][dca]) containing various levels of both Zn2+ and H2O. Detailed measurements of density, viscosity, conductivity, and individual anion and cation diffusion coefficients using pulsed-field-gradient (PFG) NMR combined with NMR chemical shifts and spin-lattice relaxation (T-1) NMR experiments provide insights into the motion and chemical environment of all molecular species. We find that the various techniques for probing ion transport and dynamics form a coherent picture as a function of electrolyte composition. Zn2+ addition causes a moderate reduction in the self-diffusion of the IL anion and cation, whereas the addition of H2O increases ion mobility by increasing the liquid's overall fluidity. Temperature-dependent C-13 T-1 experiments of the dca carbon analyzed using Bloembergen-Purcell-Pound fits show monotonic slowing of anion dynamics with Zn2+ addition, suggesting increased Zn2+/dca(-) association. T, experiments show minimal change in the spin-lattice relaxation of cation or anion upon H2O addition, suggesting that H2O is playing no significant role in Zn2+ speciation. Finally, we employ a novel electrophoretic NMR technique to directly determine the electrophoretic mobility of the C(4)mpyr cation, which we discuss in the context of impedance-based conductivity measurements.