High Li⁺ Self-Diffusivity and Transport Number in Novel Electrolyte Solutions

With an interest in exploring the limits of relative cation/anion mobilities in nonaqueous electrolyte solutions, we have measured the diffusivities of Li- and F-containing species in 0.5 M solutions of the new lithium salt, lithium bis(perfluoropinacolato) borate. LiBPFPB, which contains a giant anion with 24 fluorine atoms. Using the pulsed field gradient spin echo method on the NMR resonances of ⁷Li and ¹⁹F in the temperature range 30-95°C we find, for the first time in nonaqueous salt-in-molecular solvent solutions, lithium diffusivities that are higher than those of the anion-containing species. Furthermore, solutions in propylene carbonate (PC) appear to be fully dissociated, since the conductivities calculated from the Nernst-Einstein equation exceed the measured conductivities by only 23% at ambient temperature and 41% at 95°C. These values are comparable with those observed for molten salts such as LiNO₃, NaNO₃, and aqueous LiCl solutions. Since such deviations are known to be due to interionic friction alone, transport numbers for Li' may be calculated from the diffusivities without correction for neutral species. We obtain a value of 0.55 for PC solutions at 50°C. In the lower dielectric constant 1,2-dimethoxyethane solutions the ratio of calculated to measured conductivity is much higher. Here it would appear that ion association is still a problem and must be corrected for in calculating the transport number. For this case we obtain the value 0.53. We discuss means of increasing this value toward unity and show that this must involve abandoning simple salt solutions as electrolytes.