Abstract
With the rapid advancement of new energy technologies such as hybrid vehicles, photovoltaic and wind power systems, high-voltage film capacitors face increasingly demanding energy storage requirements under high-temperature (≥150 °C) conditions. However, the widely used dielectric material—commercial biaxially oriented polypropylene (BOPP)—is limited to long-term operation below 70 °C, making it unsuitable for such applications. Elevated temperatures and strong electric fields promote charge injection and transport in polymers, leading to increased conduction loss and decreased breakdown strength, severely constraining energy storage performance.
This thesis reports the preparation of the dielectric polymer nanocomposites by combining wide-bandgap oxides with high-temperature polymers to suppress charge injection and carrier transport, thus enhancing breakdown strength (Eb) and energy storage density under harsh environments. Through structural design, band structure analysis, and multiscale characterization, this thesis systematically elucidates how interfacial barriers and charge traps regulate high-temperature energy storage performance. The findings offer both theoretical insights and strategic solutions for next-generation high-temperature dielectric materials.
Alumina (AO) nanosheets were synthesized via hydrothermal methods and in-situ incorporated into a polyimide (PI) matrix to form uniform nanocomposites. Energy band analysis indicated that AO nanosheets generate deep charge traps at the PI-AO interface to lower leakage current density. Kelvin probe force microscopy (KPFM) confirmed the presence of interfacial deep traps, while thermally stimulated discharge current (TSDC) measurements quantified the trap depth and trap density. At 150 °C, the optimized nanocomposite achieved a high discharged energy density (Ue) of 2.72 J cm-3 with over 90% charge–discharge efficiency (η)—approximately 4.5 times that of pristine PI.
To further suppress high-temperature charge injection at the electrode/polymer interface, wide-bandgap oxide nanocoatings were introduced between the electrode and polyetherimide (PEI) substrate via the atomic layer deposition (ALD) method. Compared with ZrO2 (ZO) and TiO2 (TO), AO nanolayer, with a wide bandgap (Eg=6.3 eV) and 30 nm thickness, formed an effective injection barrier at the electrode/polymer interface. A 30-nm Al2O3 nanocoating reduced leakage current by two orders of magnitude under 150 °C and 200 MV m-1, raising the Ue of PEI from 1.41 to 3.6 J cm-3 at >95% η.
To further enhance the suppression of charge injection by the oxide interfacial layers, this thesis introduced energy level mismatches within the wide-bandgap oxide layers to induce interlayer charge traps for inhibiting charge injection. Multilayer interfacial structures were fabricated by alternately depositing AO and ZO nanolayers between the electrode and the polymer substrate. Energy band analysis indicated that ultrathin ZO layers formed potential wells within the AO matrix to enhance charge trapping. TSDC and Pockels effect measurements confirmed the presence of deeper traps and the suppressed charges dissipation in the five-layered materials (AO-ZO-AO-ZO-AO, denoted as 5-PEI), while KPFM revealed the highest interfacial charge density at the electrode/polymer interface, indicating the excellent charge retention and blocking capabilities of the multilayer nanostructure, thereby weakening the charge transport and enhancing performance. The 5-PEI film maintained the lowest leakage current density from 10 MV m-1 to 300 MV m-1. At 150 °C and η>90%, its Ue increased from pristine PEI’s 1.86 to 4.52 J cm-3, demonstrating excellent reliability in the aging experiment. Furthermore, this strategy presented generality with different polymer matrices. Using polycarbonate (PC) as a matrix, the 5-PC composite achieved 8.5 J cm-3 at 150 °C with η above 95%, which is the best performance compared to the state-of-the-art. This thesis provides a new interfacial engineering strategy for developing high-temperature polymer dielectric materials with high energy storage performance.