High-energy density in Si-based layered nanoceramic/polymer composites based on gradient design of ceramic bandgaps

It is difficult to simultaneously achieve a high electrical breakdown strength, dielectric permittivity, discharged energy density and charge-discharge efficiency by simply blending electrically conductive nanoparticles with insulating polymers in large-scale preparations. In this work, a family of gradient sandwich-structured Si-based semiconducting nanoparticle/fluoropolymer nanocomposite films with a high breakdown and good energy storage performances was fabricated using a triple solution casting process by fine tuning the type and volume concentration of the Si-based nanofiller used in each layer of the sandwich films. Differing from traditional homogeneous composite films with mono-layered structures, novel inhomogeneous composite films with gradient sandwich structures were fabricated. This fabrication was achieved by implementing a gradient-design strategy used to produce high breakdown layer-by-layer composite dielectrics and the induced polarization tactics used to produce high-permittivity Si-based semiconductor/polymer composite dielectrics. Beta-Si3N4, alpha-SiC and monocrystalline Si nanoparticles (all ∼100 nm) were introduced into the upper, middle and bottom layers of the fluoropolymer matrices, respectively. The volume concentrations of the Si-based nanofillers in each of the three layers were controlled, and 1 vol% filler in each layer was found to be optimal. In this case, the preferred electric field applied to each layer due to the gradient design could result in the highest breakdown strength (360 MV m−1) of the composite. The interface blocking effect present between two adjacent layers might inhibit the electrical tree growth between the two electrodes, contributing to a significantly enhanced breakdown property. As a result, a discharged energy density of 13 J cm−3 and a charge-discharge efficiency of 67% at 350 MV m−1 were obtained. This work might pave the way for the large-scale preparation of high-performance nanocomposite dielectrics based on the interface blocking effect.