Ionic conduction and ion diffusion in binary room-temperature ionic liquids composed of [emim][BF4] and LiBF4

Binary room-temperature ionic liquid (RTIL) samples including a lithium salt were prepared by mixing 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) with LiBF4. The ionic conductivity, viscosity, thermal properties, and ion self-diffusion coefficients in [emim][BF4] and the binary [Li][emim][BF4] at six concentrations of LiBF4 ranging from 0.25 to 1.50 M were measured at various temperatures. The self-diffusion coefficients of the individual components, [emim], BF4, and Li, were measured by using H-1, F-19, and Li-7 pulsed gradient spin-echo NMR, respectively. Since the Walden product holds similar to typical solution electrolytes, the ion conduction mechanism is interpreted using a flux basis electrolyte theory. The ions form associated structures and diffuse under the influence of the counterions in the binary IL systems. An attempt to correlate the ion diffusion with the ionic conduction was made in the framework of the Nernst-Einstein relationship. The Li net transference number and the apparent ion activity are also discussed.