Preparation And Energy Storage Performances Of PVDF-based Composite Dielectric Films

With fast charging-discharging rate,high power density and long cycle life,dielectric capacitors have been widely used in electronic devices and play an important role in advanced electronics,hybrid vehicles and pulsed power systems.Dielectric capacitors with high discharge energy density are an urgent requirement to meet the miniaturization of electronic devices and ensure good performance.The discharge energy density of dielectric capacitors depends on the dielectric material with excellent dielectric properties(high dielectric constant and low dielectric loss)and breakdown strength.Single-phase high-dielectric ceramic or polymeric dielectric materials do not have high energy storage performance,and their dielectric constants are usually enhanced by filling polymeric substrates with high dielectric constant ceramic materials,but the problems of how well the high-dielectric ceramic fillers can be dispersed uniformly in the polymeric substrate material and how to improve the dielectric constants without unduly degrading the breakdown performance limit their applications.To this end,this paper takes poly(vinylidene fluoride-hexafluoropropylene)(P(VDF-HFP))as the research object,and firstly,the effect of quenching on its physical phase,microstructure and dielectric properties is investigated to obtain the quenching process with the best energy storage performance.Finally,the effects of BT@Mg O core-shell nanoparticles on the dielectric properties,breakdown strength and energy storage properties of the composite dielectric films were investigated.The main findings are as follows:(1)P(VDF-HFP)dielectric films were prepared by solution casting method,and the effect of quenching temperature on the physical phase and phase content of the films was investigated.It was found that the physical phase and phase content of P(VDF-HFP)dielectric films affect their dielectric properties and energy storage properties.The unquenched dielectric films contain?and?phases,and after quenching treatment,the?phase of the dielectric films disappears and a new weakly polar?phase appears,and the?phase content and breakdown strength of the quenched P(VDF-HFP)dielectric films have a large difference,the?-phase content and breakdown strength were 39.52%and 4166k V/cm at 200°C,respectively,which were significantly larger than those of the quenched dielectric films at other temperatures.The quenched P(VDF-HFP)dielectric films have higher discharge energy density and charge-discharge efficiency,and the best energy storage performance of the quenched P(VDF-HFP)dielectric films at 200°C is 4.1 J/cm~3and 52.9%at 3972 k V/cm electric field,which are higher than that of the unquenched P(VDF-HFP)films at 3612 k V/cm electric field.film under 3612 k V/cm electric field by65.70%and 58.26%,respectively.(2)Surface modification of BT nanoparticles using F127.Due to the enhanced interface polarization and the reduction of defects,the BT composite dielectric film modified by F127 shows higher dielectric constant and lower dielectric loss.The dielectric constant and loss of 1 vol%BT/P(VDF-HFP)are 9.6 and 0.037,respectively,Compared with 1 vol%BT/P(VDF-HFP)composite dielectric film,the increase and decrease of 7.3%and 2.7%.F127 enhances the compatibility of BT nanoparticles with the P(VDF-HFP)matrix,resulting in a uniform dispersion of BT in the composite dielectric film,and thus the breakdown strength is improved.The modified BT nanoparticle-filled composite dielectric films have better energy storage performance:1 vol%BT@F127nanoparticle-filled P(VDF-HFP)composite dielectric films reach a discharge energy density of 5.0 J/cm~3at 3740 k V/cm,with a charge-discharge efficiency of 58.1%.(3)BT@Mg O nanoparticles with core-shell structure were prepared by coating highly insulating magnesium oxide(Mg O)on the BT surface by chemical precipitation method,and BT@Mg O/(P(VDF-HFP)composite dielectric films were prepared by using solution casting method.Compared with BT/P(VDF-HFP)composite dielectric films,the breakdown strength and discharge energy density of BT@Mg O/P(VDF-HFP)composite dielectric films were significantly improved.1 vol%BT@Mg O nanoparticle-filled P(VDF-HFP)composite dielectric films achieved a discharge energy density of 5.6 J/cm~3at an electric field strength of 4235 k V/cm and a charge-discharge efficiency of 51.3%.The core-shell structure BT@Mg O filler provides an effective way to improve the energy storage performance of polymer-based dielectrics.