Single‐Crystalline BaZr0.2Ti0.8O3 Membranes Enabled High Energy Density in PEI‐Based Composites for High‐Temperature Electrostatic Capacitors

Dielectric capacitors are promising for high power energy storage, but their breakdown strength (Eb) and energy density (Ue) usually degrade rapidly at high temperatures. Adding boron nitride (BN) nanosheets can improve the Eb and high‐temperature endurance but with a limited Ue due to its low dielectric constant. Here, freestanding single‐crystalline BaZr0.2Ti0.8O3 (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 Ue 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 Ue = 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 Eb and Ue 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. The sandwich structured BaZr0.2Ti0.8O3 (BZT)/polyetherimide–boron nitride composites are prepared through layer‐by‐layer technique with water‐dissolvent transfer method and hot‐pressing process. By inserting the single‐crystalline BZT membranes as the inner layer, the composites exhibit good dielectric‐temperature stability and ultra‐high energy‐density. This study paves a scalable way to develop novel high‐temperature dielectric materials and capacitors with superior energy storage performance.