An exploration of salt solvation and ionic conduction in oligomeric polyether-based electrolytes

Abstract

Polyether-based electrolytes possess a unique mode of ionic conduction, quite unlike that of conventional low molecular weight electrolytes or of crystalline solid electrolytes. The goal of this thesis was to clarify the ionic conduction mechanism in polyether-based electrolytes, specifically by identifying which ionic species occur in polyether solvents, and by investigating the factors that influence the mobilities of these charged species. The solvation of CoBr$\sb2$ in the oligomeric polyethers, polyethylene glycol (PEG), polyethylene glycol dimethyl ether (PEGDE), and polytetramethylene glycol (PTMG), was investigated through DSC, $\sp{13}$C NMR, FAR-IR spectroscopy, and UV-visible spectroscopy. It was established that the following species coexist in the polyether solutions: CoL$\sb6\sp{2+},$ CoBr$\sb2$L$\sb2,$ CoBr$\sb3$L$\sp-,$ CoBr$\sb4\sp{2-},$ and Br$\sp-.$ The populations of solvated species as a function of overall salt content were determined quantitatively for the CoBr$\sb2$-polyether electrolytes. Similar populations were found for the PEG and PEGDE solutions, indicating that the nature of the polyether end group is not critical in determining solvation phenomena. However, the polyether repeat unit is a critical parameter; neutral species predominate at all concentrations in the PTMG solutions, while free anions and cationic octahedral species are prevalent in the PEG and PEGDE solutions. Such differences were explained on the basis of the chelate effect, and it was concluded that the polyether repeat unit -C-C-O- produces the most effective salt dissociation. For MBr$\sb2$-PEG solutions (M = Ca, Co, Zn, and Cd), it was found that more highly viscous systems are also more highly conductive. This peculiar trend was explained using a qualitative model involving immobile cationic crosslinkers and anionic charge carriers. It was proposed that, for these systems, higher viscosities signify more complete salt dissociation. It was also shown that the simple relationship between ionic mobility and electrolyte viscosity given by Walden's rule is inappropiate for macromolecular electrolytes. Instead, the ionic mobility in such systems appears to be determined primarily by free volume effects, with the monomeric friction coefficient $\zeta$ a relevant parameter. The relationship $\Lambda(\Delta\eta)\ \alpha\ \Sigma$nq was proposed, and shown to be consistent with conductimetric and spectroscopic results of CoBr$\sb2$-PEG and CoBr$\sb2$-PTMG systems.

Subject Area

Materials science|Polymers|Chemistry

Recommended Citation

Mendolia, Michael S, "An exploration of salt solvation and ionic conduction in oligomeric polyether-based electrolytes" (1993). Dissertations available from ProQuest. AAI9321443.
https://repository.upenn.edu/dissertations/AAI9321443

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