Design, fabrication, and high-temperature dielectric energy storage performance of thermoplastic polyimide/aluminum oxide sandwich-structured flexible dielectric films

Abstract

Dielectric capacitors are essential components in advanced electronic and power systems due to their high power density, fast charge-discharge rates, low losses, and excellent cycling stability. Polymer dielectrics, such as biaxially oriented polypropylene (BOPP), are preferred dielectric materials for high-voltage capacitors because of their high breakdown strength, flexibility, and ease of processing. However, their relatively low thermal stability limits their application in high-temperature environments, such as those found in electric vehicles and photovoltaic power generation systems. In this study, sandwich-structured dielectric films were prepared using physical vapor deposition (PVD) to deposit aluminum oxide (Al<sub>2</sub>O<sub>3</sub>) layers onto thermoplastic polyimide (TPI) films to achieve high capacitive energy storage at high temperatures. TPI films were chosen for their high glass transition temperature (<i>T</i><sub>g</sub>), while Al<sub>2</sub>O<sub>3</sub> layers were deposited to enhance the Schottky barrier, thereby suppressing electrode charge injection, reducing leakage current, and improving breakdown strength at high temperatures. Various characterization techniques were employed to assess the microstructure, dielectric properties, and energy storage performance of the prepared Al<sub>2</sub>O<sub>3</sub>/TPI/Al<sub>2</sub>O<sub>3</sub> sandwich-structured films. The results demonstrate that the Al<sub>2</sub>O<sub>3</sub> coating exhibits excellent interfacial adhesion with TPI films, successfully inhibiting charge injection and thereby reducing leakage current. For instance, at 150°C and 250 MV/m, the leakage current density of TPI films is 3.19×10<sup>-7</sup> A/cm<sup>2</sup>, whereas for Al<sub>2</sub>O<sub>3</sub>/TPI/Al<sub>2</sub>O<sub>3</sub> sandwich-structured films, it was 2.77×10<sup>-8</sup> A/cm<sup>2</sup>, a reduction of one order of magnitude. The suppression of charge injection and reduction of leakage current contributed to outstanding discharge energy density (<i>U</i><sub>d</sub>) and charge-discharge efficiency (<i>η</i>) at high temperatures. Specifically, at elevated temperatures of 150°C and 200°C, the <i>U</i><sub>d</sub> reached 4.06 J/cm<sup>3</sup> and 2.72 J/cm<sup>3</sup>, respectively, with an η > 90%, representing increases of 98.0% and 349.4% compared to pure TPI films. Furthermore, the PVD process used for fabricating these sandwich-structured films is highly compatible with existing methods for producing metal electrodes in capacitors, offering significant advantages in production efficiency and cost control. This study suggests that the Al<sub>2</sub>O<sub>3</sub>/TPI/Al<sub>2</sub>O<sub>3</sub> sandwich-structured films, prepared using the PVD process and exhibiting exceptional high-temperature capacitive energy storage performance, are highly promising for applications in environments with high temperatures and high electric fields.