Excellent Energy Storage Properties with High-Temperature Stability in Sandwich-Structured Polyimide-Based Composite Films

In recent years, polymer-based dielectric capacitors have attracted much more attention due to the advantages of excellent flexibility, light weight, and high power density. However, most studies focus on energy storage performances of polymer-based dielectrics at room temperature, and there have been relatively fewer investigations on polymer-based dielectrics working under high-temperature conditions, which is much closer to the practical applications. Besides, dielectric capacitors operating in a high-temperature environment require excellent temperature stability of structure and performance. In this paper, high-temperature-resistant polyimide (PI) is selected as the matrix material, and 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 (BZT–BCT) nanofibers are used as the filling phase. By analyzing the energy storage behaviors of BZT–BCT/PI composites at different temperatures, it can be found that when the doping content of BZT–BCT nanofibers is more than 1 vol % the dielectric strength of the composites drops sharply when the temperature increases from 25 to 150 °C, resulting in serious deterioration of energy storage properties. On the basis of this, a composite film with a sandwich structure has been designed, where BZT–BCT/PI with different volume fractions is the intermediate layer and hexagonal boron nitride (h-BN) with good thermal and insulating properties is introduced in the top and bottom layers with a content of 5 vol %. Consequently, the results have shown that the energy storage properties of the constructed sandwiched dielectric composite films exhibit excellent temperature stability. The maximum field strength of the composite film with a BZT–BCT content of 1 vol % in the intermediate layer is 360 and 350 kV/mm under temperatures of 25 and 150 °C, and the storage density is 2.3 and 1.83 J/cm3 respectively.