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Multilayer structural design (including sandwich structure) has received wide attention due to its complementary advantages of different layers in the field of high energy density dielectric materials. Although multilayer polymer composites with improved breakdown strength and energy density have been prepared in some studies, the key mechanism is not clear. In this work, different kinds of multilayer films consisting of various pure polymer [polyimide, poly(vinylidene fluoride) (PVDF), and polypropylene (PP)] layers were prepared to investigate the underlying mechanism of improved performances by multilayer structural design. Experimental results indicated that the relative position of the multilayer film and electrode had an effect on the result of breakdown strength. Meanwhile, when the layer with high relative permittivity (high-k layer) came in contact with the negative electrode, the thinner the high-k layer was, the more remarkable the enhancement of the multilayer materials breakdown strength was. Finally, a double-layer film of PP and PVDF with a Weibull breakdown strength of 649.31 MV/m, which was 60.83 MV/m higher than that of the commercial PP film, was prepared. The simulation result showed that the coating high-k layer could effectively restrain the local electric field distortion around the defect. This study reveals an effective way to improve the breakdown strength of films, which is of great significance for the improvement of capacitor performance and enhancement of energy storage.

Recent advances in the electrospinning technique offer significant opportunities for improved fiber reinforced polymer composites of interest for capacitor applications. Classical effective medium approximation may fail to reach exact solutions due to the intricately inhomogeneous distribution of the depolarizing field in fibers with arbitrary orientation degrees. In this work, a three-dimensional finite element method (FEM) model has been developed to in-depth study the effects of fiber alignment on the dielectric response in fiber/polymer composites. A quantitative relationship between the effective permittivity of composites and the alignment degree of fiber has been initially established. Furthermore, we define a parameter of the effective angle which has the equivalent contribution on the effective permittivity with the actually distributed multiple alignment angles of fibers in practical composite materials. The calculation results with FEM show good agreement with the experiment measurements delivered recently.