The energy storage density of film capacitors

as the core energy storage components of power systems

mainly depends on the dielectric permittivity and breakdown strength of the dielectric material. However

commercial biaxially oriented polypropylene (BOPP) dielectric material with a relatively low dielectric permittivity severely restricts the enhancement of its energy storage density

which has become a key constraint for its application in advanced power capacitors. In this paper

a synergistic tuning strategy of solid-phase melt grafting and bi-directional stretching is proposed to prepare maleic anhydride (MAH) grafted polypropylene films (BO-(PP-g-MAH)) with different bi-directional stretching multiplicities

which achieves a significant enhancement of the energy storage density. The MAH polar groups enhance the polarization ability of polypropylene molecular chains to increase the dielectric permittivity

and the introduction of electron and hole traps can improve the breakdown strength. In addition

the biaxial stretching process induces oriented alignment of the polar groups to form a highly oriented crystal structure

which suppresses the carrier migration and reduces energy loss. The results show that the optimal BO-(PP-g-MAH) film exhibits a maximum discharged energy density of 7.33 J/cm³

a charge-discharge efficiency of 93.16%. This paper innovatively combines molecular graft modification technology with the biaxial stretching process

revealing the synergistic mechanism of graft modification and stretching ratio on the energy storage performance

providing a new approach for developing polymer dielectrics with high energy storage density.