Interface‐Strengthened Polymer Nanocomposites with Reduced Dielectric Relaxation Exhibit High Energy Density at Elevated Temperatures Utilizing a Facile Dual Crosslinked Network

High‐temperature ceramic/polymer nanocomposites with large energy density as the reinforcement exhibit great potential for energy storage applications in modern electronic and electrical power systems. Yet, a general drawback is that the increased dielectric constant is usually achieved at the cost of decreased breakdown strength, thus leading to moderate improvement of energy density and even displaying a marked deterioration under high temperatures and high electric fields. Herein, a new strategy is reported to simultaneously improve breakdown strength and discharged energy density by a step‐by‐step, controllable dual crosslinking process, which constructs a strengthened interface as well as reduces molecular chains relaxation under elevated temperatures. Great breakdown strength and discharged energy density is achieved in the dual crosslinked network BT‐BCB@DPAES nanocomposites at elevated temperatures when compared to noninterfacial‐strengthened, BT/DPAES composites, i.e., an enhanced breakdown strength and a discharged energy density of 442 MV m −1 and 3.1 J cm −3 , increasing by 66% and 162%, and a stable cyclic performance over 10 000 cycles is demonstrated at 150 °C. Moreover, the enhancement through the synergy of two crosslinked networks is rationalized via a comprehensive phase‐field model for the composites. This work offers a strategy to enhance the electric storage performances of composites at high temperatures.