Differences between polymer/salt and single ion conductor solid polymer electrolytes

Abstract

PEO/salts and PEO-based ionomers have been previously investigated for battery applications. Although both contain a PEO backbone, reports so far show different polymer dynamics, cation solvation, and ion conduction mechanisms. The different properties may be attributed to their lattice energy, ion content, molecular weight, and molecular structure. As the ion identities from which this information has been obtained differ, we have used molecular dynamics simulation to study a PEO/salt system that has identical cations and anions as a PEO-based ionomer previously studied in our group. This enables us to isolate the effect of covalent bonding of the anion to the backbone. Our study shows that the ionomer structure reduces PEO flexibility, PEO mobility, and cation solvation. In the PEO/salt, PEO segments crosslinking via ion aggregation does not occur because the anions are not incorporated in the polymer. The absence of ionic crosslinking results in PEO mobility evenly distributed along the polymer backbone in contrast to the gradient mobility exhibited in the PEO-based ionomer. Due to the more mobile PEO chains in the PEO/salt, ion transport is controlled by the dynamics of the polymer matrix, whereas ion hopping is more important within more rigid ionomer systems. To improve cation mobility in the PEO/salt, the focus should be on polymer dynamics. For the PEO-based ionomer, additional attention should be focused on attaining cation states that promote cation hopping.