Suppressed High‐Temperature Conduction Losses for Energy Storage of Dielectric Composites by Fillers with Polymorphic Polar Nanoregions

Abstract Dielectrics with high service temperatures and improved energy storage density are urgently in the fields of new energy vehicles and power electronics. However, dielectrics usually suffer from increased losses and leakage currents at high temperatures, resulting in a rapid decline in energy density and efficiency. In this work, the polyetherimide (PEI) composites incorporated with ultra‐low loading of relaxor ferroelectric filler 0.85(0.8BaTiO 3 ‐0.2(Bi 0.5 Na 0.5 )TiO 3 )‐0.15CaZrO 3 (BT‐BNT‐CZ) with polymorphic polar nanoregions (PNRs) are prepared, of which the high‐temperature loss is effectively suppressed and the polarization is enhanced. The coexistence of R‐T phase PNRs of BT‐BNT‐CZ can effectively reduce residual polarization, and improve the temperature stability of the composites. Furthermore, the high electron affinity (3.8 eV) of BT‐BNT‐CZ acts as a charge trap, reducing carrier mobility and leakage current density in the composites. As result, 0.5 wt.% BT‐BNT‐CZ/PEI composite reduces the leakage current density by orders of magnitude compared to pure PEI, improving the energy density to 3.8 J cm − 3 with 90% efficiency at 200 °C. It also shows outstanding cycling stability, even after 10 6 charge–discharge cycles at 200 °C and 300 MV m −1 , the efficiency maintains over 97%. This work offers a scalable pathway for developing composite dielectrics with satisfactory capacitive energy storage performance at high temperatures.