Ultrathin core-shell CaF₂@SiO₂ nanostructures for synergistic interfacial regulation in PEI toward high-temperature energy storage

Polymer dielectrics capable of operating at elevated temperatures are essential for advanced capacitive energy-storage systems. However, thermally activated charge injection and interfacial electric-field distortion severely degrade breakdown strength and energy density under high-temperature conditions. Here, we report a CaF₂@SiO₂ core-shell architecture to regulate the interfacial electronic structure in polyetherimide (PEI) dielectrics. The ultrathin SiO₂ shell simultaneously establishes a dielectric gradient and a stepwise energy-level barrier across the CaF₂-SiO₂-PEI interface. This dual-gradient (dielectric and energy-level) interfacial configuration suppresses thermally activated carrier transport, mitigates interfacial charge accumulation, and homogenizes local electric-field distribution. Consequently, the PEI/CaF₂@SiO₂ composite achieves a high recoverable energy density of 6.71 J cm⁻³ at 150 °C under 650 MV m⁻¹ with high efficiency and maintains 5.68 J cm⁻³ at 200 °C. The composite also exhibits excellent cycling stability over 50,000 cycles and ultrafast discharge characteristics. This work provides an effective strategy for integrating energy-level regulation, trap engineering, and electric-field homogenization in polymer dielectrics for reliable high-temperature energy storage.