Energy Storage Performance and Dielectric Properties of Surface Fluorinated BOPP Films

Various composite films with high energy storage performance and/or dielectric strength have been fabricated. However, due to some reasons, few of them have been used in practice, and biaxially oriented polypropylene (BOPP) film is still regarded as the state-of-the-art capacitor dielectric film. Previous work has indicated that direct fluorination can improve the dielectric strength of BOPP films. Herein, as a continuation of previous work, the energy storage performance and basic dielectric properties of the surface fluorinated BOPP film are investigated. The charge–discharge experimental results show that the surface fluorinated BOPP sample has the maximum energy storage or release density of 5.27 or 4.43 J/cm3, which is 59.8% or 44.6% higher than that of the virgin BOPP sample. The dielectric measurement results indicate that the surface fluorinated BOPP sample has slightly higher dielectric constant and dielectric loss than the virgin BOPP sample. The analyses of physicochemical characteristics reveal that the 0.75-
$\mu \text{m}$
-thick fluorinated surface layer has a high degree of fluorination. The results of the calculation based on the three-layer dielectric model and the dielectric measurements further show that the fluorinated surface layer has high dielectric constant and loss factor, similar to nonperfluoropolymers. The enhancement of energy storage performance and dielectric strength is mainly attributed to the blocking of the fluorinated layer to electron injection from the cathode due to a 60% reduction of the interfacial electric field and a decrease in free volume of the surface layer by the fluorination.