Electrical Spectroscopy Studies of Lithium and Magnesium Polymer Electrolytes Based on PEG400

Abstract

Four classes of solvent-free polymer electrolytes were prepared in order to study the mechanism of ionic motion and the interactions existing in polymer electrolytes. The following electrolytic complexes were studied PEG400/(MgCl₂)_x (0.00329 ≤ x ≤ 0.7000) (PEG400 = poly(ethylene glycol) 400); poly[PEG400-alt-DEOS]/(MgCl₂)_x (6.28 × 10^-2 ≤ x ≤ 13.16) (DEOS = diethoxydimethylsilane); [EDTA][PEG400]₂/(LiCl)_2.26; [EDTA][PEG400]₂/(MgCl₂)_1.98; [EDTA][PEG400]₂/(LiCl)_2.26(MgCl₂)_1.9; [EDTA]₃[PEG400]₇/(LiCl)_6.39; [EDTA]₃[PEG400]₇/(MgCl₂)_8.23; and [EDTA]₃[PEG400]₇/(LiCl)_6.39(MgCl₂)_6.16 (EDTA = ethylenediaminetetraacetic acid). The studies were carried out by impedance spectroscopy in the 20 Hz to 1 MHz range at different temperatures. Real and imaginary components of conductivity spectra in terms of equivalent circuit analysis and correlated ionic motion analysis based on a generalized universal power law were investigated. Results revealed that in the PEG400/(MgCl₂)_x and poly[PEG400-alt-DEOS]/(MgCl₂)_x systems the ionic species formed in the bulk materials are crucial for the overall conductivity. Indeed, in PEG400/(MgCl₂)_x, conductivity takes place through hopping of the cationic species Mg²⁺ and [MgCl]⁺ between the coordination sites present along the polyether chains. In poly[PEG400-alt-DEOS]/(MgCl₂)_x systems, two types of mechanisms were detected:? (a) the migration of cationic species Mg²⁺ and [MgCl]⁺ at low salt concentrations and (b) the hopping of Cl^- anions between magnesium species coordinated by the oxygen donor atoms of polyether chains at high salt concentrations. This latter phenomenon results in cation migration without any substantial geometric site relaxation. Studies of the lithium and magnesium polymer electrolytes based on [EDTA][PEG400]₂ and [EDTA]₃[PEG400]₇ polymers revealed that the type and geometry of coordination sites present in the polymer host are of crucial importance in modulating the conductivity of the polymer electrolytes. In particular, the chelating EDTA sites, which are able to strongly coordinate cations such as Mg²⁺, are excluded from the overall conductivity mechanism.