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

ABSTRACT 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 (CaF ₂ ) nanoparticles in a polyetherimide (PEI) matrix. A comparative study with strontium and barium fluorides (SrF ₂ , BaF ₂ ) 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 M ²⁺ 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/2CaF ₂ nanocomposite achieves an exceptional discharge energy density of 6.54 J cm ⁻³ at 150°C, which markedly outperforms composites with SrF ₂ or BaF ₂ , 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.