Influence of crystalline properties on the dielectric and energy storage properties of poly(vinylidene fluoride)

Poly(vinylidene fluoride) (PVDF) films with various crystal phases (α, β, and γ phases) and varied crystallinities were fabricated via different processes. The influence of the crystalline properties, such as the crystallinity and crystal phases, on the breakdown strength and dielectric and energy storage properties of the films were studied. Under low electric field, the dielectric constant was governed by the crystallinities of the films, and the dielectric loss was more related to the polarity of their crystal phases. Under high electric field, the high polarity of the crystal phases favored high-maximum, remnant, and irreversible polarization of the films. The lower crystallinity of the films with the same crystal phases led to a higher maximum and remnant polarization but a lower irreversible polarization. Under direct-current electric field, the discharged energy efficiency was mainly dominated by the polar nature of crystal phases. Under an electric field below 300 MV/m, the discharged energy density and energy loss of the three kinds of films were rather close, regardless of the phase transition. When the electric field was over 300 MV/m, the overall discharged energy density was dominated by the practical breakdown strength. γ-PVDF with a proper crystallinity and crystal grain size is expected to realize an energy density over 10 J/cm ³ under an electric field over 400 MV/m.