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

Dielectric capacitors are essential components in advanced electronic and power systems due to their high power densities, fast charge-discharge rates, low losses, and excellent cycling stabilities. Polymer dielectrics, such as biaxially oriented polypropylene (BOPP), are preferred dielectric materials for high-voltage capacitors because of their high breakdown strength, flexibility, and easy processing. However, their relatively low thermal stability limits their applications in high-temperature environments, such as in electric vehicles and photovoltaic power generation systems. In this study, sandwich-structured dielectric films are prepared by using physical vapor deposition (PVD) to deposit aluminum oxide (Al2O3) layers onto thermoplastic polyimide (TPI) films to achieve high capacitive energy storage at high temperatures. The TPI films are chosen for their high glass transition temperature (Tg), while Al2O3 layers are deposited to enhance the Schottky barrier, thereby suppressing electrode charge injection, reducing leakage current, and improving breakdown strength at high temperatures. Various characterization techniques are employed to assess the microstructure, dielectric properties, and energy storage performance of the prepared Al2O3/TPI/Al2O3 sandwich-structured films. The results demonstrate that the Al2O3 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 film is 3.19×10–7 A/cm², whereas for Al2O3/TPI/Al2O3 sandwich-structured film, its leakage current density is 2.77×10–8 A/cm², a decrease of one order of magnitude. The suppression of charge injection and reduction of leakage current contribute to outstanding discharge energy density (Ud) and charge-discharge efficiency (η) at high temperatures. Specifically, at high temperatures of 150 and 200 °C, the Ud reaches 4.06 and 2.72 J/cm³, respectively, with η > 90%, i.e. increasing 98.0% and 349.4% compared with those of pure TPI films. Furthermore, the PVD process used for fabricating these sandwich-structured films is highly compatible with existing methods of producing metal electrodes in capacitors, offering significant advantages in production efficiency and cost control. This study suggests that the Al2O3/TPI/Al2O3 sandwich-structured films, prepared by 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.