Superior High-Temperature Capacitive Energy Storage Enabled by Interfacial Ion-Matrix Synergy in Fluoride-Composites

High-temperature capacitive energy storage is critical for next-generation electronics and power systems, yet it is hindered by the severe performance degradation of polymer dielectrics at elevated temperatures. Conventional strategies using high-permittivity ceramic fillers often exacerbate leakage conduction, leading to failure above 100°C. Here, we demonstrate a paradigm shift through a dual-functional interfacial design exemplified by calcium fluoride (CaF₂) nanoparticles in a polyetherimide (PEI) matrix. A comparative study with strontium and barium fluorides (SrF₂, BaF₂) was conducted to underscore the uniqueness of this approach. Contrary to the conventional filler role, fluorides exhibit unique interfacial activities: F^- anions effectively neutralize protonated amine groups to mitigate mobile ions, while M²⁺ cations coordinate with carbonyl groups to create deep energy traps. This synergy redistributes charge and counterintuitively enhances breakdown strength and insulation at 150°C. Consequently, the optimal PEI/2CaF₂ nanocomposite achieves an exceptional discharge energy density of 6.54 J cm^-3 at 150°C, which markedly outperforms composites with SrF₂ or BaF₂, as well as most reported polymer composites. This work unveils the previously overlooked multifunctional role of fluorides and provides a novel materials design strategy for high-performance dielectric polymers under extreme conditions.