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Dielectric nanocomposites with an enhanced energy density are highly desired in modern electronic and electrical power systems. In this study, the authors developed a new flexible dielectric nanocomposite thin film using Y-doped barium strontium titanate (Y:BST) nanoparticles as fillers and polyvinylidene fluoride as a polymer matrix. The nanocomposites exhibited a high energy density of 11.07 J/cm^3 at 4300 kV/cm with a low filler content. Through the analysis of defect chemistry with X-ray photoelectron spectroscopy, the dopant Y^3+ occupies A-site of the perovskite structure, which is associated with the generation of Ba&Sr vacancies and oxygen vacancies for charge compensation. As a result, TiO(6)^2– octahedron distortion occurs, which is responsible for the enhanced dielectric permittivity. Meanwhile, the dielectric loss maintains a low level and the breakdown strength increases at a low filler loading, resulting in improved energy density.