Probe ion diffusivity measurements in salt-in-polymer electrolytes: Stokes radii and the transport number problem

In order to explore the factors which distinguish ionic motions in salt-in-polymer electrolytes from those in molecular and aqueous solvents, self-diffusivity measurements of cations and anions have been made utilizing the electrochemical technique of chronoamperometry. Stokes' law radii have been calculated using both the macroscopic and microscopic solvent viscosities and found to differ greatly from the crystal radii. The friction acting on the small cation Ag+ is found to be more than a factor of 20 greater than calculated from the microscopic viscosity. Nernst-Einstein conductivities have been calculated for the probe ion species and are found to be much greater than the measured host conductivity on the supposition of comparable host ion diffusivities. The excess conductivity correlates with the ion pairing propensity of the host anions, triflate and perchlorate. Correcting the measured probe diffusivities for ion pair contributions, our data imply host cation transport numbers as low as 0.1.