Polymer film capacitors confront critical challenges under harsh conditions of high temperatures and electric fields due to high conduction loss and degraded breakdown strength. Here, we report a novel polymer composite design: polypropylene (PP) reinforced with boron nitride nanosheet@barium titanate nanocrystal (BNNS@BTNC), achieving superior high-temperature energy storage performance. The optimized BNNS@BTNC/PP composite delivers a discharge energy density of 2.96 J/cm³ (5.9 times that of commercial high-temperature polymers, ~ 0.5 J/cm³) and maintains 92.4% efficiency under 550 MV/m and 120 °C. The wide-bandgap BNNS substrate is a physical barrier that inhibits electrical breakdown propagation paths. It captures charge carriers through deep traps, reducing leakage current and conduction loss. The BTNC uniformly distributed on high-aspect-ratio BNNS enhances interfacial polarization and phase compatibility. The oriented BNNS network significantly boosts in-plane thermal conductivity by 50%, effectively preventing thermal runaway. Finite element simulations validate the composite’s thermal stability, aligning with experimental results. Notably, the composite film maintains stable capacitive performance over extended charge–discharge cycles (50,000 cycles) in harsh environments (400 MV/m and 120 °C). These remarkable performances, combined with the scalable fabrication using commercially available raw materials, highlight the practical viability of the composite film for high-temperature capacitive energy storage applications.

Dielectric materials with enhanced energy storage performances are urgently demanded owing to the development of advanced capacitor equipment. However, low energy density and weak self-healing capabilities of current dielectric materials still limit the practical applications. Here, a biaxially oriented (polypropylene/two-dimensional (2D) Al₂O₃ nanosheets/grafted polypropylene) nanocomposite was proposed. The biaxial orientation enabled the directional arrangement of nanosheets in the polymer matrix. The oriented 2D nanosheets played a dominate role in the restriction of charge transportation and the tradeoff of energy consumption during breakdown and self-healing. Therefore, on one hand, the discharge energy density reached a considerable value of 9.64 J/cm³. On the other hand, the self-healing area of the metalized films was a 36% smaller than that of biaxially oriented polypropylene (BOPP) at the comparable self-healing energy, which was related to the long-term reliability of capacitor. The further experiments and simulations indicated that the oriented γ-A₂O₃ nanosheets (AONs) arrangement suppressed electric field distortion and hindered the charge transportation, which greatly enhanced the breakdown strength and ultimately improved the energy storage performance. This strategy presented a potential solution for improving the energy storage performance of capacitor films, which is suitable for current industrial production.