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 oxide) are important for possible use in rechargeable batteries for both electric vehicle and consumer electronics applications. We measure a complete set of transport properties nics for one particular binary salt solution: sodium trifluoromethanesulfonate in poly(ethylene oxide), over 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 transference 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 coefficients of NaCF3SO3 are similar in magnitude to those of LiCF3SO3 in poly(ethylene oxide). The transference number and thermodynamic factor are found by combining concentration cell data with the results of galvanostatic polarization experiments. This novel method of measuring the transference number is straightforward to perform experimentally and 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 numbers derived from the experiments retain fundamental significance in applications involving both steady and transient processes and in systems coupling the polymer electrolyte with electrodes of all types (stoichiometries). The relevant transference numbers can be determined independently 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 concentrated 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 results are interpreted in terms of complexation of the sodium ion with the anionic species.