All-organic sandwich-structured BOPP/PVDF/BOPP dielectric films with significantly improved energy density and charge–discharge efficiency

Biaxially orientated polypropylene (BOPP) is widely used in film capacitors for its easy process, high breakdown strength and competitive cost. However, it is still a real challenge to further improve its energy storage density. Herein, we combined BOPP with polyvinylidene fluoride (PVDF), which has the best electroactive properties among conventional dielectric polymers, into an all-organic sandwich-structured composite to improve the energy storage performance. In the configuration, commercial BOPP films served as outer layers and the binder chlorinated polypropylene (CPP) was blended with PVDF as the middle layer to strengthen the interfacial adhesion. When PVDF and CPP were blended at a weight ratio of 1:1, an enhanced discharge energy density (Ue) of 5.07 J/cm3 was obtained, with charge–discharge efficiency (η) of 82.5 %, much higher than that of BOPP (2.71 J/cm3 with η of 87.3 %). Furthermore, CPP was grafted with four different methacrylates. With the polymer-grafted CPP in the middle layer, the Ue and η of the sandwich-structured composites were further improved. For example, with methyl methacrylate as grafting monomer, the Ue and η of the sandwich-structured film were up to 5.89 J/cm3 and 80.8 %, respectively, while with nonafluorohexyl methacrylate as grafting monomer, the corresponding value of Ue and η were 5.70 J/cm3 and 88.1 %, respectively. This work demonstrated a facile strategy to combine the advantages of BOPP and PVDF by a sandwich structure to raise the energy density significantly and keep the high efficiency simultaneously.