Polymer electrolytes based on poly(ester diacrylate), ethylene carbonate, and LiClO4: a relationship of the conductivity and structure of the polymer according to IR spectroscopy and quantum chemical modeling data

Abstract

New polymer electrolytes based on poly(ester diacrylate) (PEDA), LiClO4, and additives of ethylene carbonate (EC) have a Li+ ion conductivity comparable with that of liquid electrolytes. The conductivity first decreases by an order of magnitude at an EC content of ∼5 wt.% and then increases by three orders of magnitude at 55 wt.% EC. To understand the nature of this extreme dependence, a comprehensive study using IR spectroscopy and quantum chemical modeling was performed. It was found that the changes in the IR spectra with an increase in the EC content were stepwise to form at final stage the same absorption peaks that were observed for the IR spectra of LiClO4 solutions in EC. The density functional theory studies of the energy and structures of mixed Li+ complexes and LiClO4 with EC and PEDA, which was modeled by oligomers H-((CH2)2COO(CH2)2O)n-CH3 (n ≤ 10) showed a stronger binding of the lithium ion with the polymer matrix in the mixed complexes with one EC molecule at a low content of EC resulting, most likely, in a decrease in the conductivity. Less stable mixed complexes with three EC molecules can be formed with an increase in the EC fraction and they become unstable in EC excess because of the transition of the Li+ ions to solvate complexes containing only EC molecules.