The Research On Structural Design,Fabrication And Energy Storage Characteristics Of Dielectric Capacitor Based On The P(VDF-HFP) Nanocomposite Membrane

In recent years,the development of green and low-carbon renewable energy is extremely urgentwith the dramatic increase in global energy demand,which stimulates the rapid growth of advanced energy storage technologies.Energy storage is an indispensable means to improve the efficiency and stability of electric energy utilization.Among the advanced energy storage devices,dielectric capacitors are suitable for many fields such as high pulsed power technology and hybrid electric vehicles due to their high power density and good reliability.However,the inherently low energy density of dielectric capacitors hinders their further applications compared to other energy storage devices such as batteries and supercapacitors,especially in small packaging devices with the requirement of high capacitance.Therefore,to meet the needs of device miniaturization and integration,increasing the energy storage density has become very key research hot spot in the dielectric energy storage fields.At present,dielectric capacitors with polymer-based membranes as functional layers have attracted widespread attention due to their advantages such as high breakdown strength and easy processing.However,polymer-based dielectric capacitors are limited by a small permittivity,so the energy storage density is not satisfactory.In this thesis,organic-inorganic composites are prepared by rational macro-structure design and the introduction of low-load inorganic nanofibers in the polymer matrix to obtain dielectric capacitors with high energy storage density.The main research contents are as follows:（1）Chapter 2 introduces the preparation,structure,and energy storage performance of P（VDF-HFP）-based sandwich-structured nanocomposites.Here,newly designed sandwich-structured nanocomposites are proposed,which includes the introduction of low-loading 0.4BiFeO₃-0.6SrTiO₃（BFSTO）nanofibers into the P（VDF-HFP）matrix as the polarization layer（B-layer）to offer high permittivity and the selection of PMMA/P（VDF-HFP）all-organic blend film as the insulation layer（P-layer）to improve breakdown strength（E_b）of the nanocomposites.The optimized sandwich-structured PBP nanocomposite exhibits significant enhancement in E_b（668.6 MV/m）,generating a discharged energy density of 17.2 J/cm³.The dielectric and Kelvin probe force microscope results corroborate that the outer P-layer has a low surface charge density,which can markedly impede the charge injection from the electrode/dielectric interface and thereby suppress the leakage current inside the nanocomposite.Furthermore,both the finite element simulations and capacitive series models demonstrate that the homogenized distribution of electric field in the PBP sandwich-structured nanocomposite favors the improvement of energy storage performance.This work not only provides insightful guidance into the in-depth understanding of the dielectric breakdown mechanism in sandwich-structured nanocomposites,but also offers a novel paradigm for the development of polymer-based nanocomposites with high E_band discharged energy density.（2）In Chapter 3,the preparation,structure,and energy storage performance of BaZr_0.2Ti_0.8O₃（BZT）@SiO₂/P（VDF-HFP）nanocomposites have been investigated.To further improve the polarization,BZT nanofibers with high permittivity are introduced into the P（VDF-HFP）matrix,and the surface of BZT nanofibers was coated with SiO₂shells to alleviate the dielectric mismatch between the filler and matrix.Finally,BZT@SiO₂/P（VDF-HFP）nanocomposites were prepared based on electrospinning and hot-pressing processes.Weibull breakdown distribution shows that the breakdown strength of nanocomposites first increases and then decreases with the increase of BZT@SiO₂nanofibers.3wt%BZT@SiO₂/P（VDF-HFP）achieves a discharge energy density of 11.7 J/cm³at an electric field of 452MV/m,which is 45%higher than that of pure P（VDF-HFP）.After the optimization of filler content,the gradient distribution（GD）of 3 wt%BZT@SiO₂nanofibers was realized in the polymer matrix by layer-by-layer electrospinning and hot-pressing.The leakage current test results show that 3 wt%GD BZT@SiO₂/P（VDF-HFP）is more conducive to suppressing the charge injected from the electrode,thereby increasing the breakdown strength of the nanocomposite.Under the electric field of 508 MV/m,the polarization difference of 3 wt%GD BZT@SiO₂/P（VDF-HFP）nanocomposite designed by filler gradient structure is 7.33μC/cm²,the discharge energy density is further increased to 15.4 J/cm³,while the energy storage efficiency remained at 69.7%.In addition,the 3 wt%GD BZT@SiO₂/P（VDF-HFP）has an ultra-fast charge-discharge rate and good cycle stability.This work is of great significance for promoting the development of P（VDF-HFP）-based high-performance dielectric materials towards high energy storage density.