Structural Design And Electrical Properties Study Of Polymer-based Composites With High Energy Storage Density

Dielectric capacitors are widely concerned because of their high power density.In particular,polymer-based composites,which have good flexibility,easy to macro preparation and excellent electrical performance,can be used as the core components of electrical equipment such as pulse power supply,inverter and high-power energy storage system.However,due to the low energy storage density of polymers,large volume capacitors are needed to meet the demand of energy storage and conversion,which greatly limits the application field of polymer capacitors,and does not conform to the development trend of miniaturization and high power density of devices.Therefore,it is of great significance to find a suitable way to improve the energy storage density of dielectric materials.In recent years,ferroelectric polyvinylidene fluoride?PVDF?has attracted much attention due to its high relative dielectric constant.Although PVDF exhibits a high polarization intensity in a low electric field,the hysteresis effect will induce a large remnant polarization,resulting in serious energy loss and difficulty to obtain high energy storage density and efficiency.In order to solve the problems of PVDF,this paper designs the structure of polymer matrix,inorganic fillers and polymer matrix composites to optimize the energy storage performance.Firstly,given the disadvantage that PVDF itself has large remnant polarization,this paper chooses the linear polymer polymethyl methacrylate?PMMA?which has good compatibility with PVDF to blend with it.The effects of PMMA content on the structure and electrical properties of PMMA/PVDF blended films were studied.It was found that with the increase of PMMA content,the remnant polarization of PMMA/PVDF blended films decreased significantly,which effectively improved the charge discharge efficiency.At the same time,the breakdown strength of PMMA/PVDF blended films was also improved.However,due to the low dielectric constant of PMMA,when the PMMA content is too high,the dielectric constant of PMMA/PVDF blended films sharply decreased,which is not conducive to the increase of energy storage density.When the PMMA content is 40 wt.%,the energy storage density of the blended film is 8.70 J/cm³ and the charge discharge efficiency is 79.5%at 440 k V/mm electric field.Secondly,aiming at the problem of the low dielectric constant of the polymer matrix,this paper improves it by doping inorganic fillers and structure design of it.Barium calcium zirconate titanate fiber(0.5Ba(Ti_0.8Zr_0.2)O₃-0.5(Ba_0.7Ca_0.3)Ti O₂,referred to as BCZT),adding silver particles to BCZT fiber?BCZT+Ag?,and coaxial fibers with different thickness of Al₂O₃ shell on the surface of BCZT+Ag?BCZT+Ag@Al₂O₃-H and BCZT+Ag@Al₂O₃@Al₂O₃-L?were selected and prepared,and the above four fibers were respectively filled into the polymer matrix to obtain a single-layer composite film doped with inorganic filler was obtained.The influence of the doping amount of the fillers on the microstructure and electrical properties of the single-layer composite films was studied.It is found that when the doping amount is 1 vol.%and 3 vol.%,the comprehensive properties of the single-layer composite films are better.In addition,through the comparative analysis of the influence of the type of fillers on the properties of the single-layer composite film,it is found that BCZT+Ag as the filler presents the highest dielectric constant,but its breakdown field strength is the lowest;BCZT+Ag@Al₂O₃-H and BCZT+Ag@Al₂O₃-L two fillers introduce Al₂O₃ shell,which significantly improves the breakdown field strength of the composite film and its energy storage characteristics.For example,when 1%BCZT+Ag@Al₂O₃-H/40%PMMA/PVDF is applied to 360 k V/mm electric field,its energy storage density and charge discharge efficiency are respectively 4.86 J/cm³ and 75.7%;when 1%BCZT+Ag@Al₂O₃-L/40%PMMA/PVDF is applied to 350 k V/mm electric field,its energy storage density and charge discharge efficiency are respectively 6.05 J/cm³and 81.4%.In order to further improve the energy storage performance of the composite film,a sandwich structure composite film was designed on the basis of the above research.Among them,the polymer is the outer layer,and the single-layer composite film doped with inorganic phase is the middle layer.The microstructure and electrical properties of the composite films with different sandwich structures are systematically studied.It is found that the composite films with 1%BCZT+Ag@Al₂O₃-H/40%PMMA/PVDF as the middle layer and 40%PMMA/PVDF as the outer layer have excellent energy storage properties.When 540k V/mm electric field is applied,the energy storage density and charge discharge efficiency are 16.89 J/cm³ and 66.2%,respectively.The above research shows that the energy storage performance of the composite film can be improved by doping inorganic phase or sandwich structure construction,but it is difficult to further optimize the energy storage performance by improving the electric field strength of the composite film due to the sharp increase of conductivity loss or even breakdown of the composite film when a higher electric field is applied.Therefore,in this paper,we designed a new structure of PMMA and PVDF composite organic film.The coaxial spinning film with PMMA shell and PVDF core layer were prepared by coaxial spinning technology,and then the linear/ferroelectric PMMA/PVDF all organic film with PMMA continuous distribution and PVDF fiber was obtained by hot pressing.It is found that the PMMA/PVDF film has higher breakdown strength and excellent energy storage performance.When 640 k V/mm electric field is applied,the energy storage density of 45%PMMA/PVDF organic film is 17.70 J/cm³,and the efficiency is 73.0%;when 630 k V/mm electric field is applied,the energy storage density of 51%PMMA/PVDF is 20.70 J/cm³,and the energy storage efficiency is 63.0%.