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 SiO₂@Hf_0.5Zr_0.5O₂ (SiO₂@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 SiO₂ 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 SiO₂ 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% SiO₂@HZO delivers a discharged energy density of 7.96 J/cm³ with an efficiency of 93.71% at room temperature. At elevated temperatures, it maintains discharged energy densities of 6.38 J/cm³ at 150°C and 3.85 J/cm³ at 200°C, with efficiencies above 90%. The composite also exhibits stable charge-discharge cycling over 10⁵ 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.