Study On Energy Storage Characteristics Of PVDF-Based Composite Dielectric Film

Energy storage capacitors,especially electrostatic film capacitors,have very broad application prospects in the military and civilian fields because of their ultra-high power density,fast charge and discharge efficiency,and high operating voltage.High-performance dielectric polymer materials are the key to electrostatic film capacitors,which require high dielectric comprehensive performance,high stability,and excellent film formation characteristics.However,the existing polymer materials have a lower energy storage density and cannot meet the needs of miniaturized,light-weight and high-performance film capacitors in hybrid electric vehicles,weaponry,aerospace and other fields,which limits the development and application of electrostatic film capacitors to a certain extent.Polymer materials represented by poly?vinylidene fluoride??PVDF?polymers have the advantages of high flexibility,high breakdown field strength,and high dielectric constant,which can be used as the preferred dielectric material to meet the huge demand in the field of energy storage.In recent years,in order to further improve the energy storage characteristics of polymers,researchers have done a lot of work,and it is found that compounding a polymer with a high breakdown field strength and a ceramic dielectric with a high dielectric constant to prepare a flexible nanocomposite with excellent properties is an effective means to improve the energy storage characteristics of dielectric materials.Based on this,in this dissertation,poly?vinylidene fluoride-hexafluoropropylene??P?VDF-HFP??based composite material doped with barium titanate?BaTiO₃?was selected as the main research object,and the following work was carried out:?1?In this paper,film preparation process of P?VDF-HFP?was systematically studied.The effects of different film preparation processes on the energy storage characteristics of polymer films were studied from three aspects:organic solvents,solution concentrations,and temperature of solvent drying.The study found that the selection of organic solvents with a higher solvent vapor pressure is more conducive to the preparation of dense and smooth P?VDF-HFP?films,and the prepared films also have more excellent energy storage characteristics.Further research found that the film prepared from 25 wt.%High-concentration polymer solution had better breakdown properties and ferroelectric properties.However,considering the effect of solution viscosity on processing,the solution concentration of 10 wt.%is selected for subsequent casting film preparation without affecting the film forming performance and electrical performance.The effect of temperature on the breakdown characteristics of P?VDF-HFP?films is huge.Experiments have shown that the film crystallized at high temperature has a more complete crystal structure and can improve the pressure resistance of the film.?2?Using P?VDF-HFP?as the matrix and dopamine-modified BaTiO₃ nanoparticles as fillers,composite films with different doping contents were prepared.The experiment found that the higher the doping amount,the higher the dielectric constant of the composite film.Adding a small amount of ceramic particles is helpful to slow down the tendency of the breakdown field of the composite film to decrease.This is mainly because a small number of ceramic particles increases the number of traps in the film,which is conducive to capturing unbound free charge.In this paper,the natural sedimentation method is used to control the distribution of BaTiO₃ through gravity,and a composite film with a gradient distribution of BaTiO₃ is prepared.The study found that the gradient-changed ceramic particle transition layer prepared by this process reduces the macro physical interface,thus avoiding the problem of too low breakdown voltage caused by the large difference in dielectric properties between the materials of each layer,while reducing the probability of introducing impurities in the preparation process is shown.The results show that this process can effectively improve the energy storage performance of dielectric composites.?3?In order to further improve the energy storage characteristics of the composite film,boron nitride nanosheets?BNNSs?were selected as the third phase filler,and a sandwich-structured composite film with an interlayer of BNNSs was prepared by a casting method.The added BNNSs content was only 0.75 wt.%,which could significantly improve the dielectric properties of the composite film and suppress the decrease in breakdown field strength due to the introduction of ceramic particles.Compared with the film only doped with BaTiO₃,the breakdown field strength of the sandwich-structured composite film was increased by 148.5%,and the energy storage density was increased by 235%.This reliable sandwich structure provides a new idea for improving both the dielectric constant and the breakdown field strength.