Ionic Transport in Polymer Electrolytes Based on PEO and the PMImI Ionic Liquid: Effects of Salt Concentration and Iodine Addition

We find a strong impact of ion pairing on ionic transport in potential Gratzel-cell electrolytes based on poly(ethylene oxide) (PEO) and 1-propyl-3-methylimidazolium iodide (PMImI). Furthermore, the addition of free iodine enhances both mass and charge transport, which can be explained by the reduced pair-formation tendency of the bulky triiodide ion. These results arise from conductivity and diffusion measurements on amorphous complexes with EO/PMImI molar ratios of 20 and 30 and their evaluation in a comprehensive ion-transport model. In particular, the charge diffusivity D-sigma was compared with the PMIm diffusivity D*(cat) determined by pulsed-field-gradient nuclear magnetic resonance and the iodine diffusivity D*(an) obtained from radiotracer depth profiling. Simultaneous fitting of these diffusion coefficients in complexes with and without iodine additive yields best values for the model parameters. The results characterize not only the mobility of free ions and pairs as a function of temperature and composition but also the degree of ion pairing.