Infrared Spectra and Structure of Solid Polymer Electrolytes Based on Poly(vinyl alcohol) and Lithium Halides

Abstract

Films obtained from the aqueous solutions of poly(vinyl alcohol) and MX salts, where M = Li, Na, K, and Cs and X = Cl, Br, and I (salt concentration is 10 mol % in relation to the polymer) are studied by Fourier transform infrared spectroscopy. It is shown that the degree of crystallinity of PVA films containing non-lithium salts is higher by a factor of 1.5 than that of the film without salt (51–57 and 37%, respectively), while films with lithium salts contain an almost completely amorphous polymer (0–7%). It is shown that lithium salts completely dissolve in the polymer, while the non-lithium ones exhibit only a partial dissolution. Salt addition leads to the shift of maximum of the band corresponding to the stretching vibrations of PVA hydroxyl groups. Direction and value of the shift depend on the size of anion and cation of the added salt. This fact is explained by the breakage of ОН···ОН hydrogen bonds and formation of OH···Х– bonds and donor-acceptor M+···OH bonds between ions of salts and ОН groups of the polymer. These bond strengths decrease in the sequence Cl– > Br– > I– and Li+ > Na+ > K+ > Cs+. A high solubility of lithium salts in PVA and the suppression of its crystallization by these salts are provided by a high affinity of the oxygen atom of ОН group of the polymer for the lithium ion.