Preparation And Energy Storage Properties Of Polyvinylidene Fluoride Based Composites

Driven by the urgent need of high power pulse capacitor,more and more attention has been paid to the research of high energy storage performance dielectric materials.The ideal dielectric materials have high breakdown strength,low dielectric loss,high dielectric constant,and have good processing properties.Although ceramic materials have high dielectric constant,they have low breakdown strength and high dielectric loss.In addition,the poor processing performance and lack of flexibility of ceramic materials greatly limit their application.Polymers are easy to process and have high breakdown strength,however their dielectric constant is low.In general,it is difficult to find a single dielectric material with all the desired properties.Introduction of ceramic fillers with high dielectric constant into polymer matrix with high breakdown strength is a promising method and has been widely studied,but there are still some problems to be solved,including the dispersion of nanofillers,the compatibility between polymer and nanofillers,and the high loading of the nanofillers.The results show that the properties of polymer matrix have great influence on the dielectric and energy storage properties of the nanocomposites.Due to the high electronegativity and small size of fluorine atom,the dielectric constant of PVDF and its copolymer is high.And PVDF and its copolymer have high breakdown strength,good flexibility and easy processing.In this paper,PVDF and its copolymer are used as the matrix.The nanocomposites with high energy density and good mechanical properties are obtained by simultaneously increasing the breakdown strength and dielectric constant of the materials through two methods of improving the aspect ratio and the compatibility of the fillers.The main results of this paper are as follows:1.Ferroelectric PbTiO3 nanosheets were synthesized by hydrothermal method and modified by dopamine.Then Poly(vinylidene fluoride-hexafluoropropylene)(P(VDF-HFP))based nanocomposites containing PbTiO3@PDA nanosheets were prepared by solution casting method.The crystallinity of P(VDF-HFP)first increases and then decreases due to the competition between the heterogeneous nucleation effect and the blocking of polymer segment movement of PbTiO3 nanosheets.Moreover,the introduction of PbTiO3 nanosheets does not change the crystallization phase of the P(VDF-HFP).The dielectric constant of the nanocomposites increases with the increase of PbTiO3@PDA nanosheets.However,the breakdown strength of the nanocomposites first increases and then decreases as the PbTiO3@PDA content increases.The P(VDF-HFP)based nanocomposite film containing 5 wt%PbTiO3@PDA nanosheets exhibits a discharge energy density with 7.09 J/cm3,which outperforms the pure P(VDF-HFP)film(5.83 J/cm3).2.Paraelectric cubic SrTiO3 nanosheets were synthesized by hydrothermal method,and a layer of polydopamine was coated on the surface of SrTiO3 nanosheets by oxidative self-polymerization of dopamine.The SrTiO3@PDA nanosheets were then introduced into the P(VDF-HFP)matrix through solution blending.The dielectric constant of the nanocomposites increases as the SrTiO3@PDA nanosheets content increases.Due to the shear effect,the majority of SrTiO3@PDA nanosheets tend to be oriented along the direction of the polymer plane.The oriented SrTiO3@PDA nanosheets obstruct the growth of electric tree in the breakdown process,which is conducive to the increase of breakdown strength.However,the higher content of SrTiO3@PDA nanosheets inevitably causes defects,so the breakdown strength of the nanocomposites first increases and then decreases as the SrTiO3@PDA nanosheets content increases.The largest value of the discharge energy density with 8.99 J/cm3 is obtained for the nanocomposite with 9 wt%SrTiO3@PDA nanosheets.3.Anatase TiO2 nanosheets with high aspect ratio were synthesized by hydrothermal method and modified by dopamine.Then,TiO2@PDA nanosheets filled polyvinylidene fluoride(PVDF)based nanocomposite films were prepared by solution blending.Due to the surface coating of insulating polydopamine,the dispersibility and compatibility of TiO2@PDA nanosheets increase.Therefore,compared with TiO2/PVDF,TiO2@PDA/PVDF nanocomposite films show an increase in dielectric constant,maximum electrical displacement,breakdown strength and a decrease in dielectric loss.When the addition content of TiO2@PDA nanosheets was 9 wt%,the breakdown strength and electrical displacement of the nanocomposite film increase simultaneously,and the discharge energy density reaches the maximum value(10.53 J/cm3),higher than the corresponding value of 9 wt%TiO2/PVDF nanocomposite film(7.19 J/cm3).4.The core-shell structured TiO2@SrTiO3@polydamine nanowires(TiO2@SrTiO3@PDA NWs)were synthesized via a combination of surface conversion reaction and in-situ polymerization method.Then,TiO2@SrTiO3@PDA NWs/PVDF nanocomposite films were prepared by solution blending.Due to the interface polarization induced in the TiO2/SrTiO3 interface of TiO2@SrTiO3 NWs,TiO2@SrTiO3@PDANWs/PVDF nanocomposites have higher dielectric constant and electrical displacement compared with TiO2@PDA NWs/PVDF and SrTiO3@PDA NWs/PVDF.And most of the charges in the nanocomposites containing TiO2@SrTiO3 NWs can only transfer in the interfacial region of core-shell structured nanofillers,leading to reduced electric percolation pathway and enhanced breakdown strength.The maximum discharge energy density of 10.34 J/cm3 can be achieved in 5 wt%TiO2@SrTiO3@PDA NWs/PVDF nanocomposite at 198 MV/m.