Polymer-based dielectrics play an important role in electrostatic capacitor by their high energy density ($U_e$) and flexibility. Herein, we designed a simple high $U_e$ polymer-based dielectrics by controlling the morphology and surface modification of inorganic fillers. To decrease the difference in dielectric properties between fillers and matrix of the nanocomposites, HfO₂ acting as the buffer layer with high insulation and appropriate permittivity coated onto the surface of TiO₂ nanosheets (TiO₂ Ns) to form a core–shell structure. The introduction of HfO₂@TiO₂ nanosheets (HfO₂@TiO₂ Ns) makes the nanocomposite with higher dielectric permittivity and lower dielectric loss than poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) matrix. In addition, the HfO₂@TiO₂ Ns can establish an efficient barrier to limit the space charge conduction, hamper the growing electric trees, and the HfO₂ layer with high insulation could hinder the mobility of charge carriers. The breakdown strength ($E_b$) of nanocomposite is superior to that of polymer matrix. A small addition of 3$$wt.% HfO₂@TiO₂ Ns into P(VDF-HFP) matrix can raise the $E_b$ to 480.7$$MV/m and present a maximum discharged $U_e$ of 13.9$$J/cm³. This work demonstrates that it is an effective strategy to improve the $U_e$ via designing the structure and surface state of inorganic filler simultaneously.