Double-gradients design of polymer nanocomposites with high energy density

Polymer dielectrics are encouraging contenders for high-density energy storage applications. The energy density of a polymer dielectric depends on the breakdown strength and dielectric constant. However, a polymer dielectric with a high breakdown strength usually has a low dielectric constant, or vice versa. Therefore, it is critical to perform microstructure design to achieve the optimal energy-storage performance. In this work, we propose to improve the energy density of a polymer nanocomposite by introducing double gradients into the microstructure. More specifically, employed a stochastic model to simulate the microstructure effect on the breakdown strength and revealed that the microstructure with opposite gradients for the two nanofillers can effectively enhance the breakdown strength. Then, we fabricated composites of polyetherimide (PEI) filled with 0.5Ba(Zr_0.2Ti_0.8)O₃–0.5(Ba_0.7Ca_0.3)TiO₃ fibers (BZCTFS) and boron nitride nanosheets (BNNS), where BZCT and BNNS have opposite gradients. The optimal microstructure was shown to have an enhanced breakdown strength of 580 kV/mm and dischargeable energy density of 4.87 J/cm³, which are 38% and 95% higher than those of the pure polymer (420 kV/mm and 2.5 J/cm³), respectively. It is expected that the double-gradients design strategy will be extensively used to engineer the microstructure of polymer nanocomposites to obtain high-energy-density storage.