Study On Morphology Regulation And Structural Design Of High Energy Storage PVDF-based Nanocomposites

The development of advanced energy storage technology is conducive to promoting the utilization of renewable energy,and alleviating the pressure caused by the exhaustion of fossil energy and environmental pollution.Compared with other electrochemical energy storage methods such as batteries and supercapacitors,dielectric energy storage possesses higher power density and faster charge-discharge speed,which makes it have broad application prospects in the field of pulse power devices and intermittent new energy storage.Flexible polymer dielectric materials have high electric breakdown strength,simple preparation process,but low dielectric constant.While ceramic-based dielectric materials have high dielectric constant,but low electric breakdown strength and difficult preparation process.Both limitations of polymer and ceramic make them have low energy storage density and cannot meet the requirements of modern microelectronics industry.In recent years,the research of polymer-based nanocomposites is devoted to the combination of high breakdown strength and high dielectric constant,to obtain a kind of dielectric energy storage material with high energy storage density and simple preparation process has attracted widespread attention.However,the introduction of ceramic fillers exist some problems,such as organic-inorganic interface compatibility,dielectric difference,conductivity difference and seepage threshold,which will seriously deteriorate the electric breakdown strength of composites.In this dissertation,polyvinylidene fluoride（PVDF）is used as the main polymer matrix.Combined with the macro multilayer structure design,the effects of ceramic fillers’length-diameter ratio,composition,and interfacial polarization on the dielectric properties,electric breakdown strength and energy storage performance of nanocomposite were studied.The mechanism of simultaneously improving the dielectric properties and electric breakdown strength of ceramic/polymer nanocomposites was studied through experimental characterization and finite element electric trees simulation,and high energy storage nanocomposite film with high dielectric constant and electric breakdown strength were prepared.Firstly,it is considered that the dielectric buffer shell can reduce the electrical performance difference between ceramic and polymer,prevent the local electric field distortion caused by strong interfacial polarization and inhibit the interfacial carrier transport.The 0.5(Ba_0.7Ca_0.3)Ti O₃-0.5Ba(Zr_0.2Ti_0.8)O₃（BCZT）nanofibers with high aspect ratio were prepared by electrostatic spinning process,and the boron nitride（BN）nanosheets were adhered to the surface of BCZT nanofibers by powder blending heat treatment.BCZT@BN nanofibers with one-dimensional heterojunction structure were constructed by using polydopamine surface modificatio.In the aspect of macroscopic structure design,an insulating layer is introduced to construct a double layer composite film.Results showed that when the filling content of BCZT@BN nanofiber is 1.6 wt%,the breakdown strength of the composite film is up to 718.54 MV m^-1,and the energy storage density is 24.3 J cm^-3.Finite element simulation further proved that the BN nanosheet shell effectively improves the distribution of electric field and current density at the ceramic/polymer interface,and inhibits the rapid deterioration of breakdown strength.After that,considering the low dielectric constant and high aspect ratio of ceramic fillers can avoid the local electric field distortion caused by the strong interfacial polarization,two-dimensional niobate strontium Sr₂Nb₂O₇（SNO）nanosheets with layered perovskite structure were selected to replace BCZT nanofibers.At the same time,BN nanosheets were introduced to build insulation network to further improve the breakdown resistance of composite film.The effects of SNO nanosheets on the energy storage performance of PVDF/PMMA（1:4）（BPM）composite films with sandwich and reverse sandwich structures were systematically studied.Results showed that compared with the polarized layer,the dielectric constant of sandwich composite film decreases slightly,but the breakdown strength is further improved:the energy storage density is 31.4 J cm^-3,and the breakdown strength is655.16 MV m^-1.Through the finite element electric trees simulation,a small amount of SNO introduction can induce the development of electric trees,which results in an increase in the number of branches and more energy consumption,and thus improves the breakdown strength of composite films.Next,considering the interfacial polarization and the micro-capacitive structure of ceramic fillers are beneficial to improve the dielectric constant of composite films.Moreover,the dielectric constant of SNO nanosheets is low,which means weak interfacial polarization in composite films,and limited increase of dielectric constant.As a result,strontium niobate@titanium dioxide（SNO@TO）and barium niobate@barium titanate（SNO@BTO）nanosheets with core-shell structure were prepared to enhance the interfacial polarization strength of SNO.In addition,considering that BN nanosheet can act as an insulation barrier in the composite film and hinder the movement of carriers,large size BN（L-BN）nanosheet was filled in BPM.Results showed that SNO@BTO nanosheet induces stronger interfacial polarization due to the high dielectric constant of BTO shell,and thus makes an increase in the dielectric constant of composite films at a large extent.The enhancement of interfacial polarization has an obvious inducement effect on the development of electrical trees,which improves the breakdown strength of composite films to a certain extent.Finally,a high polarization difference of 11.8μC cm^-2at 650MV m^-1is achieved from the 6-3-0 SNO@BTO reverse gradient composite film,and the discharge energy storage density is as high as 32.5 J cm^-3.Finally,considering the coulomb blocking effect of metal quantum dots,and the schottky contact between metal and ceramic,the electric charge carrier movement can be inhibited,which benefits to improve the breakdown strength of composite films.In addition,the modification of metal quantum on the surface of ceramic fillers can improve local capacitance,which is conducive to the improvement of dielectric constant.Therefore,Ag quantum dots was modified on the surface of SNO nanosheet by in-situ reduction reaction,and the prepared SNO@Ag nanosheet was introduced into BPM via a gradient composite structure to improve the energy storage performance of composite films.Results showed that the optimum ratio of SNO nanosheet and silver nitrate is 1 g:3 m L（SNO@3Ag）.When the filling ratio of SNO@3Ag nanosheet is 5-7-9,the breakdown strength of the composite film is585.13 MV m^-1,and the dielectric constant is 13.98,which contributs a high discharge energy storage density of 31.0 J cm^-3and charge and discharge efficiency of 66%.