Diffusion, conductivity and DSC studies of a polymer gel electrolyte composed of cross-linked PEO, γ-butyrolactone and LiBF₄

The gel electrolyte system composed of γ-butyrolactone (GBL), LiBF₄, and cross-linked acrylated polyethylene oxide (PEO) with a molecular weight of 4000 (PEO4) was studied using the pulsed field gradient (PFG) NMR method to measure the diffusion coefficients. The NMR spin-lattice relaxation times, ionic conductivities and thermal behaviour were also measured. Seven reference samples were also prepared: pure GBL (sample A), 0.5, 1 and 1.5 M LiBF₄ in GBL (i.e., solution electrolyte; samples B-D), 20 wt % PEO4 in GBL (sample E), 1 M LiBF₄ plus 20 wt % PEO4 in GBL (sample F) and a gel without the salt (sample G), in addition to three gel electrolyte samples containing 0.5, 1, and 1.5 M concentrations of LiBF₄ in GBL with 20 wt % cross-linked PEO4 (samples H-J). Importantly, using ¹H, ⁷Li, and ¹⁹F PFG NMR the diffusion coefficients of all the species present were able to be measured. The diffusion coefficients were found to be sensitive to the salt concentration and the cross-linking of the polymer. The Li and BF₄ ions are solvated with GBL even in the gel state. The deviation of the measured conductivities from the values calculated using the Nernst-Einstein equation reflects the effects of ion association. It was observed that at least, at low salt concentrations, the polymer aids in the dissociation of the salt. By considering all of the experimental data obtained, we show that in the gel system the BF₄ ions exist predominantly in the solvent while the motion of the Li ions, although solvated in GBL, is strongly associated with the polymer. From the combination of the conductivity and diffusion measurements we were able to obtain values for the dissociation constants for the salt dissolved in the GBL and in the gel samples.