Gradient Dielectric Distributed Nanofiber Engineered Polyetherimide for Enhanced Energy Density at High Temperatures

Polymer dielectric composites are promising candidates for next‐generation pulse capacitors, yet it remains challenging to combine high energy density, high charge‐discharge efficiency, and reliable operation at elevated temperatures. Here, 1D core–shell SiO 2 @Hf 0.5 Zr 0.5 O 2 (SiO 2 @HZO) nanofibers are designed as dielectric‐gradient nanofillers and incorporated into a polyetherimide (PEI) matrix. The HZO nanofiber core enhances polarization and provides mechanical reinforcement, while the SiO 2 shell functions as an insulating dielectric buffer that improves interfacial compatibility and reduces dielectric mismatch between the ceramic filler and polymer matrix. Finite‐element analysis and electrical‐tree simulations reveal that the SiO 2 shell homogenizes the local electric‐field distribution and suppresses electrical‐tree propagation, thereby enhancing the breakdown field. As a result, the optimized composite containing 0.2 wt% SiO 2 @HZO delivers a discharged energy density of 7.96 J/cm 3 with an efficiency of 93.71% at room temperature. At elevated temperatures, it maintains discharged energy densities of 6.38 J/cm 3 at 150°C and 3.85 J/cm 3 at 200°C, with efficiencies above 90%. The composite also exhibits stable charge‐discharge cycling over 10 5 cycles at 200°C. This work demonstrates that dielectric‐gradient core–shell nanofibers offer an effective route to high‐temperature polymer dielectrics with enhanced breakdown strength, high energy density, and improved electrical reliability.