THE MEASUREMENT OF A COMPLETE SET OF TRANSPORT-PROPERTIES FOR A CONCENTRATED SOLID POLYMER ELECTROLYTE SOLUTION

Polymer electrolytes based on alkali metal salts in poly(ethylene oxid e) are important for possible use in rechargeable batteries for both e lectric vehicle and consumer electronics applications. We measure a co mplete set of transport properties nics for one particular binary salt solution: sodium trifluoromethanesulfonate in poly(ethylene oxide), o ver a wide range of salt concentrations (0.1 to 2.6M) at a particular temperature (85 degrees C). The transport properties measured include the conductivity, the salt diffusion coefficient, and the sodium ion t ransference number. The mean molar activity coefficient of the salt is also the determined. The conductivity is measured using the standard ac impedance method. The salt diffusion coefficient is found by using the method of restricted diffusion. The conductivity and diffusion coe fficients of NaCF3SO3 are similar in magnitude to those of LiCF3SO3 in poly(ethylene oxide). The transference number and thermodynamic facto r are found by combining concentration cell data with the results of g alvanostatic polarization experiments. This novel method of measuring the transference number is straightforward to perform experimentally a nd yet does not require that the solution be either dilute or ideal. A theoretical analysis of the experimental method based on concentrated ,solution theory is given. Our study verifies that the transference nu mbers derived from the experiments retain fundamental significance in applications involving both steady and transient processes and in syst ems coupling the polymer electrolyte with electrodes of all types (sto ichiometries). The relevant transference numbers can be determined ind ependently of any knowledge of speciation of the polymer electrolyte. The transference numbers found here for the sodium ion are much lower than those reported for the lithium ion, especially in the concentrate d solutions. The transference number of the sodium ion is negative in the more concentrated solutions and levels off at its maximum value of 0.31 in the dilute concentration range. The transference number resul ts are interpreted in terms of complexation of the sodium ion with the anionic species.