Preparation And Energy Storage Performance Of High Dielectric PVDF-based Composite Films

Dielectric capacitors are widely used in many fields such as pulse power supply technology,energy collection,inverters and passive components for national defense and civil industry because of their advantages of ultra-high power density,fast charge-discharge and long cycle life.In order to meet the challenges of the rapid development of modern industry,higher requirements for dielectric materials are put forward,such as high discharge energy density,high energy efficiency,light weight and easy processing.Dielectric ceramics have a large dielectric constant,but the breakdown strength is small.Although the dielectric polymer has large breakdown strength,its dielectric constant is low,so it is difficult to significantly improve the energy storage performance of single-phase materials.The ceramic/polymer composites obtained by compounding the dielectric ceramic and the polymer can break the enthalpy which cannot be obtained simultaneously with the dielectric constant and the high breakdown strength,and provides a possibility for the substantial improvement of the energy storage performance.At the same time,the composites havr the lightweight and highly flexible properties also contribute to the miniaturization of the device.In this paper,polyvinylidene fluoride（PVDF）with relatively high dielectric constant was used as polymer matrix,and PZT,BT and KNN dielectric ceramic particles were used as functional phases to prepare composites and their properties were studied.The main research contents and conclusions are as follows:PZT existed as a crystal phase nucleating agent in the polymer could induce the increase ofβphase content in PVDF,and the dielectric constant of PZT/PVDF composites increased significantly with the increase of high dielectric constant PZT particle content.The dielectric constant of the composites increases to 40.8 with the PZT content of 50 vol%,and the dielectric loss of the composites was less than 0.037,because of the low dielectric loss of the PZT particles.The low content of PZT particles were uniformly dispersed in the PVDF matrix and tightly bound to the matrix.The composites had no obvious defects and had high quality.The electrical conductivity and breakdown strength of the composites with 3 vol%PZT content were 2.11×10^-5 S/m and 340 kV/mm,respectively,and the discharge energy density can reach 8.88 J/cm³.Although increasing the PZT content was beneficial to increase the dielectric constant of the composites,the obvious defects such as particle agglomeration and pores in the composites led to a significant increase in electrical conductivity and a significant decrease in the breakdown strength,resulted in a large decline of energy density of the composites.When the PZT particle content increased to 50 vol%,the electrical conductivity of the composites increased to 1.02×10^-4 S/m,and the breakdown strength decreased to 130kV/mm.At this time,the discharge energy density decreased to 3.29 J/cm³.Dopamine was used to fabricate Dopa@BT particles,the presence of the dopamine shell improved the dispersibility of the particles in the PVDF matrix and the interfacial compatibility between the particles and the matrix,and thereby the dielectric loss,electrical conductivity and leakage current of the Dopa@BT/PVDF composites were significantly suppressed.The maximum discharge energy density of the composites with 10 vol%Dopa@BT content reached 6.01 J/cm³,which was 43.78%higher than that of the BT/PVDF composites.Ni@BT particles with nuclear-satellite structure were synthesized by growing ultrafine Ni particles on the surface of BT particles.The Coulomb-blockade effect of ultrafine Ni particles in this structure could effectively suppress the transmission and aggregation of the space charge between particles and particles and matrix.Hence,the dielectric loss,electrical conductivity and leakage current of the Ni@BT/PVDF composites were significantly reduced,as results,the energy storage loss of the composites was reduced,and its breakdown strength and energy density were effectively improved.The maximum discharge energy density of the composites with 10 vol%Ni@BT content reached 7.64 J/cm³,which is 82.76%higher than that of the BT/PVDF composites.When the Ni@BT particle content was continuously reduced to 3 vol%,the breakdown strength of the composite film was further increased to 350kV/mm,and the discharge energy density of the composites was as high as 9.55 J/cm³.The Dopa@KNN particles were obtained by coating the KNN particles with dopamine.The Dopa@KNN/PVDF composites had low dielectric loss,electrical conductivity and leakage current.When the particle content of Dopa@KNN was less than 13 vol%,the conductivity values of the composites at 100 Hz were less than 5.7×10^-6 S/m,and the leakage current density values are all in the range of 10^-7-10^-5 A/cm².The composite with 3 vol%Dopa@KNN exhibited lower conductivity and greater breakdown strength than pure PVDF.The discharge energy density was as high as 14.7 J/cm³ at the electric field of 480 kV/mm.