Significantly enhanced energy storage density of sandwich-structured (Na0.5Bi0.5)0.93Ba0.07TiO3/P(VDF–HFP) composites induced by PVP-modified two-dimensional platelets
Two-dimensional (Na0.5Bi0.5)0.93Ba0.07TiO3 (NBBT) platelets with a size of up to ca. 5 μm and thickness of 0.2–0.5 μm were introduced as fillers into a polymer matrix to prepare energy storage composites for the first time. The NBBT platelets were treated with an aqueous solution of H2O2 and coated with polyvinylpyrrolidone (PVP) before mixing with poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF–HFP)). The final composite was denoted as NBBT@PVP/P(VDF–HFP). Composites were prepared with NBBT@PVP loadings from 1 to 30 vol%. The relative permittivity of the composites increased significantly with increasing NBBT@PVP loading, while the breakdown strength decreased. To improve the breakdown strength of the composites, a sandwich-structure of multilayer films was developed, which used NBBT@PVP/P(VDF–HFP) composites with 1 vol% NBBT loadings as central hard layers and the composites with 30 vol% NBBT loadings as neighboring soft layers. The five-layered film, which contained three central hard layers and neighboring soft layers, showed excellent energy storage properties. The breakdown strength and the maximum energy storage density of the film reached 258 kV mm−1 and 14.95 J cm−3, respectively. The energy efficiency remained 0.9 at an electric field of 200 kV mm−1. The findings provide a new approach to produce energy storage materials with high performance.