Large-aspect-ratio composite nanofibers with interior hierarchical interfaces are employed to break the adverse coupling of electric displacement and breakdown strength in flexible poly(vinylidene fluoridehexafl uoropropylene) nanocomposite films, a small loading of 3 vol% BaTiO3@ TiO2 nanofibers gives rise to the highest energy density (approximate to 31.2 J cm(-3)) ever achieved in polymer nanocomposites dielectrics.