Enhanced Electrostatic Energy Storage of a Sandwich-Structured Polymer Composite Film Incorporating Surface-Functionalized ZnO Nanosheets

Film capacitors are known for their high power density, but their low energy densities are a major drawback. In this study, sandwich-structured polymer films incorporating dopamine-functionalized ZnO (ZnO@DA) nanosheets in the middle layer are developed to enhance the capacitive energy storage density. The middle layer is based on a poly(vinylidene fluoride-co-hexafluoropropylene) [P(VDF-HFP)] nanocomposite, and the outer layers are a blend of poly(methyl methacrylate) (PMMA) and P(VDF-HFP). Although the ZnO@DA nanosheet filler helps enhance the dielectric constant, this multilayered configuration, with the optimal ZnO@DA nanofiller content of 5 wt %, results in an enhancement of breakdown strength, i.e., from 175 to 452 MV m–1. Under the synergistic effect of enhanced interfacial polarization by ZnO@DA nanosheets and increased breakdown strength through a multilayered linear dielectric blend structure, the sandwich-structured film achieves a remarkable discharge energy density of 13.88 J cm–3 and an efficiency of 77% at a moderate electric field intensity of 375 MV m–1. Furthermore, the synergistic effects of ZnO@DA nanosheet fillers and the sandwich structure of the polymer film are verified by phase-field simulation. This research offers a cost-effective approach for creating polymer nanocomposite films for long-life, high-energy-density capacitors under moderate voltage conditions.