Preparation And Properties Of BOPP-based Composite Dielectric Films With High Energy Density

Polymer dielectric materials have the advantages of high breakdown strength,low dielectric loss,easy processing,and high flexibility,resulting in a wide range of applications in the field of electronics,energy and communication.Among the polymer dielectrics,biaxially-oriented polypropylene(BOPP)is the most successful polymer dielectric material in commercial application.BOPP has the advantages of high breakdown strength,excellent frequency stability,and low dielectric loss.However,there are still some challengs,such as the low dielectric constant of BOPP(～2.2),leading to the energy storage density of only 2～3 J/cm3.Adding high dielectric filler to the PP matrix is an intensively studied method,but processing,molding and then biaxial stretching will greatly reduce its mechanical properties which has a disadvantage on its energy storage properties.To address the low energy storage density of BOPP films,in this paper,sandwich composite films of different components and structures were designed and prepared.Chlorinated polypropylene(CPP)was used as the middle layer matrix to construct the sandwich composite films of blending,filling and organic/inorganic composite.The energy storage properties of BOPP composite films prepared by simple and easy industrial scaling method are significantly improved.The main results are as follows.(1)All organic sandwich-structured composite films were designed and prepared.The outer layer was BOPP film with high breakdown strength,and the middle layer was the blend of polyvinylidene fluoride(PVDF)with high dielectric constant and the binder CPP.The effects of the blending ratio of PVDF and CPP on the dielectric properties of sandwich films were studied.The dielectric constant of the composite film is the highest when CPP:PVDF=1:1(wt%),and it can form a good bonding with BOPP.The energy storage density of the composite film increases to 5.07 J/cm3 at the electric field of 450 MV/m,which is 186%of BOPP(2.71 J/cm3).The charge-discharge efficiency of the composite film decreased from 87.3%of BOPP to 82.5%,which still maintained a high level.(2)Methyl methacrylate(MMA),trifluoroethyl methacrylate(TFEMA),hexafluorobutyl methacrylate(HFBMA)and nona-fluorohexyl methacrylate(NFHMA)were grafted onto CPP chains by free radical polymerization,respectively.The above grafted CPP polymers were blended with PVDF and prepared into sandwich structure film with BOPP.Its energy storage characteristics were further improved.The energy storage density of sandwich film grafted with PMMA reached 5.89 J/cm3,and the charge-discharge efficiency was 80.9%.The discharged energy density of PNFHMA-grafted composite film is 5.70 J/cm3,and the charge-discharge efficiency is up to 88.1%,even higher than that of pure BOPP film.The electron cloud distribution of CPP and four monomers is simulated by first principle calculation.The results show that there is charge accumulation around Cl atom in CPP,followed by MMA.When F atoms are introduced into the monomer structure,the phenomenon of charge accumulation is greatly weakened,and the corresponding dielectric loss of the sandwich-structured film is inhibited and the charge-discharge efficiency is improved.(3)Inorganically filled sandwich structure films were constructed,in which two BOPP films were bonded by CPP,and high dielectric filler barium titanate nanoparticles(BT)were introduced through the CPP layer to enhance polarization.The BT particles were first surface-treated with n-octyltriethoxysilane to improve their dispersion in the substrate,and then the surface-modified BT particles were finely dispersed in CPP solution.The composite film with sandwich structure without defects between the layers was prepared by coating the BOPP surface with different thicknesses of CPP through solvent cast film formation,and then the coated surfaces of the two BOPP films were placed opposite to each other for hot pressing,which had an outer layer of BOPP and a middle layer of CPP/OBT.The energy storage density reached the highest when 40 wt%BT was loaded,and the discharged energy storage density was 7.17 J/cm3 under at 450 MV/m,which was 262%that of BOPP.The charge-discharge efficiency decreased slightly from 83.6%to 80.9%.(4)Organic/inorganic composite doped BOPP-based sandwich films were prepared.Polyaniline coated barium titanate particles(PANI@BT)were prepared by in-situ polymerization of aniline in the presence of hydroxyl modified BT particles,and PANI@BT composite dielectric fillers with different coating morphologies were prepared by adjusting the monomer addition.The typical "mulberry" and "core-shell" coated composite fillers were chosen to improve the dielectric constant while suppressing the dielectric loss through their synergistic effect.When the electric field is applied outside the composite material,the hybrid particles are less likely to form conductive pathways compared with the pure "core-shell" coating particles.Compared with the pure "mulberry" coated particles,the increased uniformity of the surface morphology of barium titanate effectively suppresses the electric field aggregation between fillers,thus suppressing dielectric loss.The sandwich composite film was prepared by using CPP/PVDF blend as the middle layer,filled with PANI@BT hybrid fillers and BOPP as the outer layer.The discharge energy density reached 7.31 J/cm3 at 30 wt%of hybrid filler addition,and the charge-discharge efficiency was 77.3%.(5)BOPP was used as the outer layer,CPP/PVDF as the middle layer,and two-dimensional boron nitride nanosheets(BNNS)were added to the CPP/PVDF blend to enhance its breakdown strength.The discharge energy of the sandwich-structured films had the highest discharged energy density of 5.17 J/cm3 at 3 vol%addition of BNNS in the middle layer,and the charge-discharge efficiency was maintained at a high level of 82.1%.