Construction And Properties Of P(VDF-HFP)-Based Multilayer Composite Nanodielectric Materials

Existing polymer dielectric materials usually have too low energy density to meet the requirements of high energy storage density for high power systems and energy systems.Therefore,it is crucial to develop new dielectric materials with high energy storage density.At present,there are many research methods to improve the energy storage density of materials.Among them,the combination of a conductive layer with a high dielectric constant and an insulating layer with a high breakdown strength through the design of a multi-layer structure is a common and effective solution.Based on this,several（insulating layer-conducting layer-insulating layer）multilayer composites were prepared in this thesis,and the dielectric properties and energy storage properties of multilayer composite films were studied by adjusting the distribution of fillers in the matrix.Conductive fillers can achieve high dielectric constants at low levels compared to ceramic fillers.Therefore,silver nanowires（AgNWs）were selected as the conductive layer filler in this thesis.And with polytetrafluoroethylene-hexafluoropropylene P（VDF-HFP）as the outer layer and AgNWs@PDA/P（VDF-HFP）as the middle layer,a multilayer structure composite film（labeled as P-Ag-P）was prepared.The dielectric properties of the composite films with different filler contents were studied.The research shows that when the content of AgNWs@PDA is low,the increase of the dielectric constant of the composite film is mainly due to the interface polarization between the filler and the polymer matrix.When the content of the filler is high,based on the percolation theory,the dielectric constant of the composite film increases.The improvement mainly stems from the formation of the microcapacitor structure within the composite film.At the same time,the dielectric properties of the single-layer composite film and the multi-layer structure composite film with AgNWs@PDA content of 34 wt%were compared.The AC conductivity(2.55×10^-8 S/m)of the multi-layer composite film is significantly lower than that of the single-layer composite film(3.29×10^-8 S/m),which indicates that the construction of the multi-layer structure is beneficial to suppress the leakage current.In addition,in order to further improve the energy storage density,boron nitride nanosheets BNNSs with excellent electrical insulation and wide bandgap were selected as the insulating layer filler.A multilayer structure composite film（labeled as B-Ag-B）with the outer layer of BNNSs/P（VDF-HFP）and the middle layer of AgNWs@PDA/P（VDF-HFP）was prepared to explore the dielectric properties of the composite film by the introduction of BNNSs.performance and energy storage performance.The research shows that the energy storage density of the multi-layer composite film with BNNSs is increased by 137%compared with that without the addition of BNNS,which increases the tortuosity of the breakdown path,and the wide band gap of BNNSs can form deep traps to capture carriers and reduce leakage current,thereby improving the breakdown strength of the composite film.Thus,the energy storage density of the multilayer composite film is improved.The finite element simulation of the multilayer structure composite film was carried out by COMSOL.Combined with the experimental results,it is further proved that the improvement of the dielectric constant of the composite film mainly originates from the enhancement of the interface polarization and is related to the formation of the microcapacitor structure.The construction of the multi-layer structure is beneficial to suppress the generation of leakage current and the development of electrical branches,and the BNNSs will bear more local electric field strength,thereby improving the breakdown strength of the multi-layer structure composite film and improving the energy storage density of the composite film.