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 (Eb) and the composite energy density (Ue). 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 Eb is effectively improved. And the Ue value of the nanocomposites reaches 17.79 J·cm−3 at 523 MV·m−1. On the basis, the composite Ue 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 Ue 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.