Methanol Environment Induced Smaller Crystallite Size to Enhance Energy Storage Properties of Poly(vinylidene fluoride)

Polyvinylidene fluoride (PVDF) dielectric capacitors are critical to their applications due to their low dielectric loss and excellent energy storage density. The energy storage properties of PVDF are closely related to its crystallographic properties, such as crystal pleomorphism, size of crystals, and crystal confinement. In this case, the crystal structure is mainly based on α-crystalline PVDF films with various crystal sizes made by methanol treatment at various temperatures. It reduced the crystallite sizes, which significantly improved the dielectric and energy storage properties of PVDF films. At approximately the same crystallinity, the breakdown performance of polymer films with larger crystallite sizes is better. In contrast, the polarization rate of films with smaller crystallite sizes is more significant, and the discharge energy density is more remarkable. When the methanol environment temperature was 40 °C, the PVDF films showed the largest crystallite size of 24.3 nm compared to the other methanol-treated PVDF films. The higher breakdown strength can also be maintained while a specific polarization rate is maintained. The smaller crystal size of the treatment for PVDF by methanol greatly reduces its dielectric loss. In addition, the ferroelectric domain coupling is attenuated due to the restraining effect in the smaller nanoscale grains. As a result, the performance of the energy density and that of the efficiency with the environment temperature at 40 °C in methanol increased by 2.7 times and more than 70%, respectively, compared with ordinary PVDF films with larger crystallite size. This study proposes a convenient and valuable solution for the application of PVDF in energy storage to obtain optimal dielectric films.