High‐Temperature Fluorinated Polyimide Dielectric Captative Energy Storage Enabled by Domain‐Engineered KNbO ₃ ‐SrTiO ₃ @Al ₂ O ₃ Nanofillers

Polymer dielectrics exhibit remarkable advantages, including high power density, elevated operating voltage, and excellent processability. In dielectric films, the ferroelectric domain architecture and its flipping dynamics are pivotal for energy storage, as they govern electric displacement and charge–discharge efficiency. A widely adopted strategy to enhance polarization involves incorporating ferroelectric ceramics into the polymer matrix. However, this approach inevitably induces higher dielectric loss and compromises charge–discharge efficiency. Here a ferroelectric‐paraelectric KNbO 3 ‐SrTiO 3 nanofillers is introduced that effectively suppress ferroelectric domain volume, mitigate hysteresis, and reduces remnant polarization. Phase‐field simulations corroborate that domain polarization undergoes more facile reversal, substantially minimizing dielectric loss while preserving high polarization. To further enhance the breakdown strength ( E b ), the KNbO 3 ‐SrTiO 3 filler is encapsulated with an Al 2 O 3 coating. Consequently, the KNbO 3 ‐0.2 SrTiO 3 @Al 2 O 3 /FPI nanocomposite film achieves outstanding dielectric capacitor performance, featuring an impressive energy storage density of 6.09 J cm −3 , a higher displacement difference ( D max ‐ D r ) of 2.08 µC cm −3 , and an E b of 611 MV m −1 at 150 °C under 100 Hz. This work presents a forward‐thinking strategy for the scalable industrial production and deployment of high‐performance dielectric capacitors.