Superior high-temperature capacitive energy storage performance enabled by in situ grown nanodots in polymer nanocomposites

High-temperature electrostatic capacitors are essential for advanced power electronics and energy systems, yet most polymer dielectrics suffer from severe conduction loss and premature failure under coupled thermal and electrical stress. Here, we report a nanoconfinement strategy that enables the in situ growth of ultrasmall inorganic nanodots uniformly embedded within high-temperature polymers. These nanodots simultaneously enhance the dielectric constant and breakdown strength, while introducing abundant interfacial deep traps that suppress charge transport by shortening the hopping distance and increasing the activation energy. As a result, the optimized nanocomposite achieves state-of-the-art discharged energy densities of 7.03 J cm⁻³ at 200 °C and 3.40 J cm⁻³ at 250 °C with efficiencies exceeding 90%, and maintains stable operation over 50 000 charge–discharge cycles at 200 °C. Moreover, the scalable fabrication of large-area, defect-free films underscores its strong potential for practical applications. Overall, this work establishes a robust design paradigm for polymer dielectrics with suppressed high-temperature conduction and ultrastable capacitive energy storage performance, offering a pathway toward compact and reliable capacitors for extreme-environment deployment.