Stretching-induced γ-β phase transition of poly (vinylidene fluoride) for high energy density capacitors

Abstract

Poly (vinylidene fluoride) (PVDF) of large dielectric constant has been widely used in electric energy storage applications. However, its low energy density limits broader utilization. Since stretching has been proven to be an effective method for enhancing the energy density of polymers, investigating its effect on γ-PVDF films is particularly valuable, given that γ-crystals exhibit the highest electric breakdown strength among all PVDF polymorphs. In this paper, γ–PVDF films were stretched at 80 ^oC and 110 ^oC, respectively, with their microstructural evolution thoroughly characterized. The results demonstrate that the onset of γ-β phase transition occurs at a smaller strain during stretching at 80 ^oC, whereas complete γ-β phase transition is achieved at 110 ^oC. Films stretched at 80 ^oC possess smaller crystal sizes while maintaining comparable crystallinity to those stretched at 110 ^oC. Furthermore, the stretched films exhibit enhanced dielectric constant, polarization and breakdown strength attributed to the increased chain orientation, higher interphase content and suppressed leakage current. Notably, PVDF films stretched at 80 ^oC with a stretching ratio of 4 achieve a discharged energy density of 26.08 J/cm³ and an energy efficiency of 66.94%. This work provides an alternative strategy for fabricating high-performance PVDF films for energy storage applications.