Study On The Dielectric And Energy Storage Properties Of PVDF/Boron Nitride-Titania Nanosheets Composite Films

As advanced energy storage capacitor materials,polymer-based composite dielectrics have the advantages of high mechanical strength and fast charge and discharge,and are widely used in modern electrical and electronic fields.Two-dimensional nanofillers can greatly improve the energy storage performance of polymer-based dielectrics and have attracted extensive attention.In this paper,polyvinylidene fluoride(PVDF)is chosen as polymer matrix,using boron nitride nanosheets coated with polydopamine(BN@PDA),exfoliated titania nanosheets of different sizes(LTNSs and STNSs),and BN-STNSs as fillers to fabricate a series of new composite films with excellent energy storage performance.The composite films are prepared via simple physical blending and hot pressing annealing process.The effect of introducing 2D nanosheets on the microstructure and dielectric energy storage performance of composite films and the enhancement mechanism were studied.Through the dopamine self-polymerization reaction,a polydopamine shell layer was coated on the surface of BN nanosheets,which could improve the dispersion of BN nanosheets in PVDF matrix and the compatibility with the matrix.Due to the low permittivity of BN@PDA nanosheets,the permittivity of the composite films are all lower than that of pure PVDF.Since an appropriate amount of BN@PDA nanosheets can suppress the carrier injection,the breakdown strength of the composite films increases first and then decreases with the increase of contents.Doping a small amount of nanosheets can improve the discharge energy density(U_e)of the composite films.The 1 wt%PVDF/BN@PDA composite exhibits an outstanding U_eof 5.5 J/cm~3,which is higher than that of pure PVDF(3.2 J/cm~3).Large titania nanosheets(LTNSs;11-59μm)and small titania nanosheets(STNSs;0.9-2.8μm)and PVDF/LTNSs and PVDF/STNSs composite films were prepared by adjusting the the high-temperature growth time of the precursor.The study found that there is a correlation between the size effect of titania nanosheets and the width of the band gap.Theoretical calculations(DFT)and experimental tests(UV)confirmed that the smaller the size of titania nanosheets,the wider the band gap,the higher the surface hydroxyl content,and the enhanced interface of the composite film.The interaction can improve the permittivity and breakdown strength of the composite film,thereby improving the energy storage performance of the composite film.The surface hydroxyl content of STNSs is higher.Hydroxyl groups can form hydrogen bonds with PVDF molecular chains,improve the dispersion and compatibility of fillers in the matrix,and at the same time induce the crystallization of PVDF molecular chains to polar phaseβphase,improve the permittivity of the composite film,and effectively suppress the dielectric loss.The breakdown strength of PVDF/STNSs composite films is significantly improved,in addition to the good dispersion of fillers,the widened band gap with decreasing nanosheet size,and a more efficient electric field homogenization.Therefore,the PVDF/STNSs composite film has excellent energy storage performance.For example,The 0.5 wt%PVDF/STNSs composite exhibits an excellent U_eof 11.7 J/cm~3.In order to further improve the discharge energy density of PVDF/BN@PDA composite films,0.5 wt%of medium-permittivity STNSs nanosheets were added to PVDF/BN@PDA composite films with different compositions to prepare novel co-doped composite films.STNSs and BN@PDA nanosheets form a local interface overlap region,which significantly improves the permittivity and polarization of the co-doped composite films,and the breakdown field strength does not deteriorate significantly.Compared with the binary composite film,the co-doped composite film has a great improvement in the dielectric constant and discharge energy density.When the doping composition of BN@PDA nanosheets is 1 wt%,the discharge energy density of PVDF/BN-STNSs composite film is 12.1 J/cm~3,which is much higher than that of PVDF/BN@PDA composite film(5.5 J/cm~3).When the doping composition of BN@PDA nanosheets is 1 wt%,the discharge energy density of PVDF/BN-STNSs composite film is 12.1 J/cm~3,which is higher than that of PVDF/BN@PDA composite film(5.5 J/cm~3).Using the space charge distribution test,combined with the finite element simulation method,based on the multi-action zone model and the bipolar charge transport model,the core-shell structure,size factors and multiphase fillers reinforced composite thin film dielectric storage were systematically studied from different angles.The mechanism of energy characteristics provides a theoretical basis for expanding the application of 2D materials in the field of energy storage.