IONIC CONDUCTIVITY AND LITHIUM TRANSFERENCE NUMBER OF POLY(ETHYLENE OXIDE):LiTFSI SYSTEM

Polymer electrolytes comprising LiN(CF3SO2)2 salt (LiTFSI) and high molecular weight poly(ethylene oxide) PEO are prepared by casting from solution. A wide range of salt concentration is studied, from molar ratio EO:Li of 50:1 to 1.5:1. Samples of electrolytes are measured in symmetrical Li|electrolyte|Li cell. The values of lithium transference number t+ of electrolytes are determined using a novel procedure, which involves impedance spectroscopy as well as two potentiostatic polarization segments with opposite sign of d.c. bias. The principles of this procedure are based on Bruce-Vincent method, which has been extended in order to compensate the possible differences in properties of the two Li|electrolyte interfaces, and ensure proper conditioning of the cell between different stages of the measurement process. The results are analyzed using Bruce-Vincent and Watanabe approaches, which give similar values of lithium transference number. The reported values of t+ initially decrease with rising concentration of salt, but after passing a minimum at molar ratio EO:Li of around 6:1, a considerable increase is recorded, eventually leading to lithium transference number higher than 0.5 for “polymer in salt” systems. Such behavior of the studied system is discussed either in terms of formation of PEO:salt complexes, or formation of ionic aggregates.