Importance of donor number in determining solvating ability of polymers and transport properties in gel-type polymer electrolytes

Abstract

The importance of donor number affecting ion-solvating ability of polymers and transport properties was demonstrated in gel-type polymer electrolytes. Four different polymers, poly(ethylene oxide) (PEO), poly(methylmethacrylate) (PMMA), poly(acrylonitrile) (PAN), and poly(vinylidene fluoride) (PVdF)-hexafluoropropane (HFP) copolymer, were employed as the gel-forming polymer matrix, and the mixture of ethylene carbonate (EC) and propylene carbonate (PC) was used as the liquid constituent. The infrared spectroscopic study indicated that the polymers can partially solvate Li+ ions with different solvating ability, PEO>PMMA>PAN>P(VdF-HFP) in a decreasing order, in both the solvent-free and gel states. This trend was not correlated with the dielectric constant of the polymeric functional groups but strongly with their donor number. The free-ion fraction was higher when the polymers of a higher donor number were used as the gel-forming matrix, but the cation transference number showed the opposite trend, presumably due to the retardation effect exerted by stronger Li+–polymer interactions. The importance of donor number was further demonstrated in the poly(ethylene glycol dimethacrylate)-modified PMMA- and P(VdF-HFP)-based gel electrolytes, where the addition of an ethylene oxide unit of a higher donor number brings about an increase in the free-ion fraction but a decrease in the cation transference number.