Single‐Crystalline BaZr_0.2Ti_0.8O₃ Membranes Enabled High Energy Density in PEI‐Based Composites for High‐Temperature Electrostatic Capacitors

Abstract Dielectric capacitors are promising for high power energy storage, but their breakdown strength (E_b) and energy density (U_e) usually degrade rapidly at high temperatures. Adding boron nitride (BN) nanosheets can improve the E_b and high‐temperature endurance but with a limited U_e due to its low dielectric constant. Here, freestanding single‐crystalline BaZr_0.2Ti_0.8O₃ (BZT) membranes with high dielectric constant are fabricated, and introduced into BN doped polyetherimide (PEI) to obtain laminated PEI–BN/BZT/PEI–BN composites. At room temperature, the composite shows a maximum U_e of 17.94 J cm⁻³ at 730 MV m⁻¹, which is more than two times the pure PEI. Particularly, the composites exhibit excellent dielectric‐temperature stability between 25 and 150 °C. An outstanding U_e = 7.90 J cm⁻³ is obtained at a relatively large electric field of 650 MV m⁻¹ under 150 °C, which is superior to the most high‐temperature dielectric capacitors reported so far. Phase‐field simulation reveals that the depolarization electric field generated at the BZT/PEI–BN interfaces can effectively reduce carrier mobility, leading to the remarkable enhancement of the E_b and U_e over a wide temperature range. This work provides a promising and scalable route to develop sandwich‐structured composites with prominent energy storage performances for high‐temperature capacitive applications.