Investigation of the ionic conduction mechanism of composite poly(ethyleneoxide) PEO-based polymer gel electrolytes including nano-size SiO2

New types of polymer gel electrolytes have been prepared by swelling, in a mixed solvent of ethylenecarbonate (EC) and diethylcarbonate (DEC), a membrane composed of a 4 x 10(6) molecular weight linear poly(ethyleneoxide) (PEO) with the addition of lithium tetrafluoroborate (LiBF4) and fumed silica. The swelling procedure led to a membrane weight increase of 350%. These swelled membranes are optically and macroscopically homogeneous and show high ionic conductivity combined with low activation energy for lithium ion transport. The role of the solvents, the structures of the ions and of the matrix polymer in these swelled gel electrolytes have been investigated by pulsed gradient spin echo (PGSE) NMR measurements. The values of the self-diffusion coefficients suggest that the BF4- anion diffuses faster than the Li+ ion due to the interaction of the latter with the polymer gel matrix. The effects of fumed silica on the ion diffusions are also discussed.