Ion transport in liquid electrolytes

Abstract

For many years ion transport has been viewed as a hydrodynamic process where ion size and solvent viscosity are the primary factors controlling the movement of ions in solution. However, it will be shown for the electrolytes studied here that the isothermal parameters of interest are the following: (1) the concentration of "free" ions, (2) the solvent dielectric constant, and (3) the solvent functional group. The temperature dependence of ionic conductivity for liquid electrolytes and polymeric electrolytes above the glass transition temperature has also been studied for many years. These conductivities do not follow Arrhenius behavior like those which are observed for solid glassy electrolytes. Therefore, the temperature-dependent conductivities of liquid and amorphous polymer electrolytes are usually represented by empirical equations. However, an empirical representation of the data provides no insight into the fundamental aspects of ion transport. Here, for a family of liquid electrolytes, the temperature-dependent conductivity is written as an Arrhenius expression and it is shown that the experimentally observed non-Arrhenius behavior is due to the temperature dependence of the dielectric constant contained in the exponential prefactor. Scaling the temperature-dependent conductivities to conductivities at a chosen reference temperature so that the dielectric constant remains invariant leads to a "compensated" Arrhenius equation that provides an excellent description of the data, implying that ion transport is governed by a single activated process. An energy of activation Ea can be extracted from the compensated Arrhenius plot for each family of solvents. Dividing the temperature-dependent conductivities by the factor exp(-Ea/RT), where Ea is determined from the compensated Arrhenius plot, gives the prefactors. Plotting the prefactors versus the temperature-dependent solvent dielectric constant results in all of the data points falling on a single "master curve"