Structural Design And Energy Storage Performance Of PVDFBased Composites Incorporated With Low-Dimensional Oxides

Dielectric capacitor,as an important energy storage device,has the great advantages of extremely fast charging-discharging rate,ultrahigh power density,long lifetime and etc.It’s widely used in the applications of advanced electronic and electrical fields,such as new energy vehicles,pulse power equipment,photovoltaic power generation system and others.At present,the fundamental problem of restricting the developments is that the low energy density of dielectric materials originated from its l ow breakdown strength and unmatched dielectric constant.In this paper,the single-layered and multi-layered PVDF-based composites are successfully prepared,the mechanisms of improving energy storage performance of flexible polymer composites based on the design of multiple interfacial structures,including electrode-composites barrier layer,fillers-matrix incorporated transition layer,artificial heterogeneous layer inside inorganic fillers and interfacial layers are elaborated,the interfacial polarization and external electric field are adjusted,and thus,to achieve the purpose of enhancing breakdown strength and optimizing energy storage performance.Based on the design of fillers-matrix incorporated transition layer and matching principle of dielectric constant,Bi Fe O₃（BFO）and high-entropy(Eu_0.2Bi_0.2Y_0.2La_0.2Cr_0.2)₂O₃（EBYLCO）nanofibers are introduced into P（VDF-HFP）/PMMA matrix.Entanglements of long polymer chains and nanofibers help to form the highly cross-linked network and improve the quality of interfacial bonding.BFO has high ferroelectric polarization and low dielectric constant,which can bear high voltage and exhibit large polarization.High-entropy system possesses the feature of single-phase multi-structured polymorphic distortion,which contributes to the easier transition of electrons,accumulates large amount of electrons and holes at interfacial area.The more stable self-polarization forms in the composites,resulting in a higher interfacial polarization,which is produced by the high-entropy nanofibers with narrow bandgap and impurity energy level.0.6 wt.%BFO-P（VDF-HFP）/PMMA reaches a discharged energy density of 6.9 J/cm³ at the breakdown strength of 429.3 k V/mm,while the 0.6wt.%EBYLCO-P（VDF-HFP）/PMMA achieves the breakdown strength of 509.4k V/mm and the discharged energy density of 15.1 J/cm³,which has an enhancement of 119%.According to the regulation of artificial heterogeneous layer in one-dimensional nanofibers,the core-shell structured BFO@AO nanofibers with Al₂O₃（AO）core and BFO shell are fabricated,and then incorporated into P（VDF-HFP）/PMMA matrix.By utilizing the discrepancy of AO and BFO,the artificial heterogeneous interfaces construct,the built-in electric field inside nanofibers enhances,the external electric field is modulated and further,the breakdown strength and energy storage performance are significantly improved.When the thickness of BFO shell is around 15 nm,0.2 m BFO@AO-P（VDF-HFP）/PMMA obtains an ultrahigh breakdown strength of 759.0 k V/mm,high discharged energy density of 22.7 J/cm³ and energy efficiency of 72%.By constructing artificial heterogeneous layer in two-dimensional fillers,the lamellar-structured BFO/TO/BFO with opposite double heterojunction are prepared,which have the trilayered structure with BFO outer layers and Ti O₂（TO）middle layer.According to the discrepancy of Fermi level and energy bandgap between BFO and TO,the transition of electrons generate s and the heterojunction structures form at interfaces.Owing to the direction of heterojunction electric field on adjacent interfaces is opposite in the trilayered structure,the opposite double heterojunction produces the corresponding electric field.Combining with the advantages of large interfaces produced by lamellar-structured fillers,the counteract effect on external electric field maximizes,and thus,the breakdown strength and energy storage performance are significantly improved.The discharged energy density of 19.3 J/cm³ and energy efficiency of 61%are achieved i n 0.6 wt.%BFO/TO/BFO-P（VDF-HFP）/PMMA at the breakdown strength of 549.2 k V/mm.By designing the multi-dimensional interfacial structures,the improvements of energy storage performance are achieved by comprehensively effect of electrode-composites barrier layer,fillers-matrix incorporated transition layer and artificial heterogeneous layer,and the combined macroscopic interlayer interfaces.The differences in functional performance of layers,as well as the coupling effect and barrier effect of multi layered interfaces contribute to a higher breakdown strength and energy storage performance.The excellent breakdown strength and discharged energy density are achieved in the multilayered composites,where the outer layers are polymer-based composites and the inner layers are pristine matrix.BFO@AO-P（VDF-HFP）/PMMA/P（VDF-HFP）/PMMA/BFO@AO-P（VDF-HFP）/PMMA（AP-P-AP）reaches the breakdown strength of 647.6 k V/mm,the energy density of 20.2 J/cm³ and energy efficiency of 73%.