High energy density and charge–discharge efficiency of uniaxial stretched poly(vinylidene fluoride-hexafluoropropylene) film with electroactive phase conversion

Abstract

It is significantly urgent for the development of polymer film capacitor with large energy density and high charge–discharge efficiency. The key project for this strategy is the fabrication of polymer insulated film in capacitor with high dielectric constant, low dielectric loss and large polarization. Here we highlight the large energy density with high release efficiency of poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) film via uniaxial stretching technology. The effects of uniaxial deformation conditions, including stretching ratio, environmental temperature and extension rate, on the crystal structures and dielectric performance of P(VDF-HFP) films are investigated systematically. The relative degree of β-phase increases greatly in stretched film, and reaches maximum of 93.1% compared with the original content of only 42.3% in pristine film. The crystallinity is also enhanced because nonpolar phases and amorphous are transferred to electroactive phase during tensile improvement. Correspondingly, the morphologies of P(VDF-HFP) films examined by atomic force microscopy corroborate that the pristine spherulites are deformed along external force direction, and then micro-strips parallel is developed in the matrix. Due to the coordination between fluorine and hydrogen atoms of the adjacent chains as well as high density oriented dipoles, the dielectric constant of stretched P(VDF-HFP) is enhanced by 170%, and reaches 28.5 at 100 Hz with low loss as 0.007. The energy density of 2.7 J/cm3 is obtained in stretching film with high charge–discharge efficiency as 91%. The uniaxial deformation approach provides a facile and productive solution to fabricate polymer dielectrics with large energy density and high discharge efficiency for flexible film capacitor.