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Polymer dielectrics have gained significant usage in electrical engineering due to their unique merits, such as good flexibility, easy processing, and low cost. However, it is a huge challenge that polymer dielectrics maintain high breakdown strength meanwhile high permittivity for enhanced energy density. Here, sandwiched polymer films are constructed for enhanced energy density. Increased permittivity has been achieved via blending BaTiO3@polyfluorosiloxane with a polymer as the top and bottom layer. Furthermore, the neat polymer as the middle layer is without critically sacrificing breakdown strength. The energy storage density increases from 7.8 J/cm3 of the neat polymer to 10.41 J/cm3 of the sandwich structure at 460 MV/m. This research offers a feasible route for fabricating capacitive films with enhanced energy density. [ABSTRACT FROM AUTHOR]

Enhanced dielectric constant and high breakdown strength offers immense promise for excellent energy storage performance, which is of critical significance in modern electronics and power systems. However, polymer nanocomposites with traditional routes have to balance between dielectric constant and breakdown strength, hence hindering substantive increases in energy density. Herein, a sandwiched polymer nanocomposite film has been constructed to take full advantage of the individual component layers. BaTiO 3 nanoparticles are coated with a fluoropolymer to form core-shell structures and then introduced into a polymer as the top and the bottom layers of a sandwich film for enhancing polarization. Moreover, boron nitride nanosheets (BNNSs) in the middle layer of the sandwich film exert positive effects on the inhibition of current leakage for high breakdown resistance. The breakdown strength increases from 480 MV m -1 of the neat polymer to 580 MV m -1 of the sandwiched film. Additionally, the film exhibits a higher dielectric constant in comparison with the neat polymer. The sandwiched film displays a superior energy density (15.75 J cm -3 ), which is about 1.9 times that of the neat polymer. This work proposes a feasible route to achieve excellent energy storage of polymer dielectrics by synergistically introducing insulating fillers and additional dipoles in a sandwiched polymer nanocomposite film.