Crystal phase transition dependence of the energy storage performance of poly(vinylidene fluoride) and poly(vinylidene fluoride-hexafluoropropene) copolymers

Large-scale mechanical stretching has been performed to modify the crystal phase structures of the pristine poly(vinylidene fluoride) (PVDF) and its copolymer poly(vinylidene fluoride-hexafluoropropylene) [P(VDF-HFP)] with various molar contents HFP in an attempt to improve their energy storage performances. It is found that the physical stretched PVDF and P(VDF-HFP) 95.5/4.5 mol % films have a phase transition from the nopolar α-phase to highly polar β-phase, which is different from the P(VDF-HFP) films with relative high HFP molar contents (α to γ phases). The following results show that the phase transition in these PVDF-based polymers has a significant effect on their dielectric and energy storage performances. On account of the reformation of the crystalline property and elimination of the impurity defects, an ultra-high breakdown electric field of ∼900 MV/m has been obtained in all the stretched samples. Consequently, the higher discharged energy densities of 27.1 and 27.7 J/cm³ are calculated from the D–E loops of the β-PVDF and β-P(VDF-HFP) 95.5/4.5 mol % films, respectively. Regarding their excellent discharging energy density of ∼10 J/cm³ under 600 MV/m for thousands of times, the stretched PVDF and its copolymer P(VDF-HFP)s are promising candidates for high power capacitors applications.