Substantially enhanced high-temperature capacitive performance in BOPP films via coating with a magnetic inorganic nanolayer

Abstract

The utilization of biaxially oriented polypropylene (BOPP) in commercial film capacitors has gained increasing prominence in recent years, primarily due to its advanced ultra-low dielectric loss and cost-effectiveness. In order to mitigate the degradation of the capacitive performance of BOPP films induced by the metal-electrode charge injection under extreme operational conditions, this study introduces a simple, efficient, and environmentally benign modification method for growing CoFe₂O₄ nanolayers onto the surface of BOPP films via magnetron sputtering. The wide bandgap CoFe₂O₄ nanolayers can increase the potential barrier height between the metal electrode and dielectric films of the composite dielectrics. In particular, CoFe₂O₄ exhibits weak magnetic properties, generating a Lorentz force within the film plane under an applied electric field, which facilitates the lateral dissipation of electrode injected charges and suppresses the localized accumulation. On this basis, an intermediate charge-blocking layer of CoFe₂O₄ is also incorporated into the composite structure, leveraging the synergistic effects of the 'surface' and 'bulk' to effectively prevent carrier injection and transport. Furthermore, the high dielectric constant of the CoFe₂O₄ nanolayers and interfacial polarization effects with the polymers result in composite films showcasing a synergistic enhancement of the dielectric properties and insulation strength. Finally, the composite film demonstrated a record max discharge energy density (U_emax) of 3.06 J cm⁻³ with a charge/discharge efficiency of 87.1% at 120 °C. The proposed modification method offers a promising approach with excellent compatibility for large-scale manufacturing, such as roll-to-roll processing.