Direct Fluorination Strategy to Enhance Energy Storage Performance of Biaxially‐Oriented Polypropylene Films for High‐Temperature Capacitor Applications

Polymer dielectric films are widely used in energy storage regions for high energy efficiency and energy density. However, traditional commercial dielectric films such as biaxially oriented polypropylene (BOPP) can hardly meet the growing demand for advanced capacitors at elevated temperatures. In this work, a facile two-process strategy of direct fluorination followed by the biaxial stretching on PP sheet is proposed to produce fluorinated BOPP (FBOPP) films with a special island-like structure formed on the surface. Due to the high electronegativity of fluorine, the introduction of fluorine groups onto PP chain effectively lowers the LUMO from 1.42 to 1.1 eV, thereby enhancing the electron affinity of the FBOPP films. Accordingly, the surface fluorinated layer can construct electron traps on the surface of FBOPP films to inhibit the injection and transport of electrons, resulting in the maximum discharge energy density at charged-discharged efficiency of 90% (Uη90) of the FBOPP films of 1.34 J cm-3 at 120 °C which is 197.8% higher than that of BOPP films. This two-step strategy through directed fluorination and biaxial stretching can be implemented using existing industrial processes, making it suitable for large-scale, continuous production of BOPP films for high-temperature applications.