Complexation of poly(vinylidene fluoride):LiPF₆ solid polymer electrolyte with enhanced ion conduction in 'wet' form

Poly(vinylidene fluoride) (PVDF):LiPF₆ complex membranes as 'solid' and 'wet' polymer electrolytes are evaluated. X-ray diffraction (XRD) and differential scanning calorimetric (DSC) studies show a decrease in crystalline size and crystallinity of the polymer PVDF with increasing LiPF₆ concentration. Although PVDF is not an ionomer, acid-base complexation between the PVDF polymer at fluorine sites and the Li ions of the LiPF₆ salt is evident from X-ray photoelectron spectroscopy (XPS) spectra. Such interaction is responsible for the reduction in both PVDF crystallinity and sol particle size during solid film formation. As evident from scanning electron micrographs, smaller spherical domains in the dry film are found with increasing lithium salt. Impedance (ac) measurements suggested the ionic conductivity of PVDF:LiPH₆ film increases with increasing lithium salt. By comparison, PVDF:LiPH₆ solid polymer electrolyte (SPE) wetted with Ethylene carbonate/propylene carbonate (EC/PC) shows less dependence of conductivity on salt concentration with a shallow maximum at about 15wt.% salt. The average value is comparable with that of the solvent-free film at 20wt.% LiPF₆. This result suggests the existence of dual conduction mechanisms in 'wet' SPE via the amorphous PVDF matrix and the EC/PC solution trapped in the swollen region of the PVDF porous network.