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• A high-performance low-cost energy storage halogenated polymer material is developed. • Increased permittivity caused by fluorophenyl leads to high energy storage density. • High charge trap density caused by halogen groups leads to high cycling efficiency. • Moderate crosslinking structure further deepen the charge traps in dielectrics. Capacitor dielectric films exhibiting high energy storage density and efficiency within a wide operating temperature range are crucial for advancing electrical and electronic devices. The low energy storage density and working temperature as well as the high manufacturing costs of the state-of-the-art BOPP films limit their use as an energy storage unit for developing smart grids or the internet of things, while most of the polymer-based dielectric films reported currently are facing the issues of the rapid efficiency deterioration with the increasing of temperature. In this work, the high energy storage density and efficiency are achieved in the low-cost flexible epoxy films innovatively modified by trifluoro-phenyl group functionalization. Introducing strongly electronegative fluorinated phenyl groups greatly improves the energy storage density due to the increase of the total molecular dipole moment of the polymer film. Meanwhile, achieving the high energy storage efficiency is accomplished via gradually increasing the charge trapping site density of epoxy resins functionalized with different densities of fluorinated phenyl groups, which can be demonstrated through both thermally stimulated current (TSC) spectroscopy and the first-principles calculations results. Finally, it is experimentally demonstrated that deeper charge traps can be obtained in slightly crosslinked trifluoro-phenyl functionalized epoxy films, resulting in an energy storage density of 3.31 J/cm3 and a high energy storage efficiency of 95% at 100 °C. [ABSTRACT FROM AUTHOR]