Structure and transport properties of polymer gel electrolytes based on PVdF-HFP and LiN(C2F5SO2)2

Gel polymer electrolytes composed of PVdF/HFP and a non-aqueous lithium electrolyte solution EC/DEC/LiN(C2F5SO2)2 (BETI) were prepared to investigate the conduction properties of the gel materials. Structural and micro-structural characterization was carried out by means of modulated differential scanning calorimetry (MDSC) and scanning electron microscopy (SEM). The samples with polymer/solution weight ratios higher than 0.5 contain at least an amorphous swollen gel phase and a crystalline one, while a pure liquid phase appears, which occupies cavities in the micrometer range in the gels with lower polymer content. The transport properties of the gels were investigated by means of impedance spectroscopy for conductivity and pulsed-field gradient nuclear magnetic resonance (PFG-NMR) for diffusion coefficients. The results were compared with those of corresponding films prepared with LiN(CF3SO2)2 (TFSI). The BETI-based samples showed diffusion values lower than the TFSI-based ones. Estimation of the carrier concentration change with the gel composition revealed that the interaction between the host polymer and the electrolyte in the gel especially in the polymer rich region (>50 wt%).