(103c) A Comparison Between Polymer Salt and Ionomer for Battery Applications

Salts solvated in a Polyethylene oxide (PEO) matrix are considered to be the most promising candidates for solid state polymer electrolytes in battery applications.  Since the anions have high mobility, and do not participate in chemical reactions, they accumulate at the electrode/electrolyte interface, causing a concentration gradient.  This degrades battery performance after several charge/discharge cycles.  An electrolyte in which the cation is the only conducting species is desired.  PEO based ionomers solvate the cations, and reduce anion motion because the anions are covalently bonded to the polymer backbone.   Like their PEO-salt counterparts, these single ion conductors suffer from low conductivity.

In this study, we examine a PEO-ionomer system, and the corresponding PEO-salt system using molecular dynamics simulation.  The PEO-ionomer system has been experimentally synthesized and studied at Penn State.   To provide a clean comparison, we consider a hypothetical PEO-salt system, in which the cation and anion are the same, but the anion is no longer incorporated on the polymer backbone.  Thus, the cation identity, the cation-anion, and the cation-PEO interactions are the same as in the PEO-ionomer system.  Comparison of the two systems isolates the effect of attaching the anion to the polymer backbone by covalent bonding.  A wide range of ion environments is observed in both systems. The number of ion crosslinks in the two two systems are similar, but 1.5 times more cations are solvated by the PEO matrix in the salt compared to the ionomer.  As ion concentration increases in a PEO salt, the PEO chains slow significantly.  In the ionomer, this mechanism is active, but ions also slow the polymer through ionic cross-linking.  Thus, covalently bonding the anion to the PEO backbone slows its mobility in comparison to the PEO-salt system.  This in turn impacts the mobility of the cations and anions. We also find that the cations in the PEO-salt system change their coordination more frequently than in the ionomer.  We conclude that the use of single ion conductors requires a shift in mechanism, such as ion motion facilitated by string-like clusters.  We show examples of such movement in the ionomer and suggest ways to increase this type of ion motion.