Gradient-structure-enhanced dielectric energy storage performance of flexible nanocomposites containing controlled preparation of defective TiO₂ and ferroelectric KNbO₃ nanosheets

Next generation power system needs dielectrics with increased dielectric energy density. However, the low energy density of dielectrics limits their development. Here, an asymmetric trilayered nanocomposite, with a transition layer (TL), an insulation layer (IL), and a polarization layer (PL), is designed based on poly(vinylidene fluoride)-polymethyl methacrylate (PVDF-PMMA) matrix using KNbO₃ (KN) and TiO₂ (TO) as the nanofillers. The morphology and defect control of the two-dimensional nano KN and nano TO fillers are realized via a hydrothermal method to increase the composite breakdown strength (E_b) and the composite energy density (U_e). The asymmetric trilayered structure leads to a gradient electric field distribution, and the KN and TO nanosheets block charges transfer along z direction. As a result, the development path of the electrical trees is greatly curved, and E_b is effectively improved. And the U_e value of the nanocomposites reaches 17.79 J·cm⁻³ at 523 MV·m⁻¹. On the basis, the composite U_e is further improved by defect control in TO nanosheets. The nanocomposite KN/TO/PVDF-PMMA containing TO with less oxygen vacancy concentration (calcined at oxygen atmosphere) acquires a high U_e of 21.61 J·cm⁻³ at 548 MV·m⁻¹. This study provides an idea for improving the energy storage performance by combining the design of the composite dielectric structure and the control of nanofillers’ defect and morphology.