Introduction The innovative development of advanced energy storage devices is imminent due to the continuous depletion of non-renewable energy sources and the increase in energy demand. Compared with other energy storage devices, film capacitors have some advantages of high power density, low polarity, high insulation impedance and wide frequency response, which are a recent research hotspot. Meanwhile, film capacitors have the functions of bypass, decoupling, wave filtering and energy storage, etc., which are widely used in communication, electronics, aerospace, medical equipment, new energy and other industries. Recent investigation on new dielectric materials with a low dielectric loss, a high breakdown strength and a high energy density becomes popular. In this paper, poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) was modified via irradiation using a xenon heavy ion beam, and the effect of irradiation injection on the microstructure, dielectric properties and energy storage properties of the materials were investigated. The mechanism of intrinsic structure evolution by novel irradiation modification technology was discussed. Methods P(VDF-HFP) was first dissolved in N,N-Dimethylformamide (DMF) and stirred in a water bath for 24 h to obtain a homogeneous solution. The solution was then flowed onto a quartz glass substrate and cast with a spatula to form a P(VDF-HFP) liquid film. It was dried in a vacuum drying oven at 0.08 MPa and 80 ℃ for 24 h to evaporate the solvent. The films were heated at 120 ℃ for 12 h to completely remove the solvent residue. The cast film was peeled off from the glass substrate in alcohol and dried to obtain an untreated flexible polymer dielectric film. The pure film was slit and then vertically irradiated with a beam of xenon ions (129Xe27+) of a mononuclear energy of 19.5 MeV/u at an irradiation injection of 5×106, 5×107, 5×108, 5×109, and 5×1010 ions/cm2, respectively. The six irradiation gradient samples were washed, dried and stored for subsequent analysis. The crystal form and orientation of the samples with different irradiation injections were determined by X-ray diffractometer (XRD). The phase evolution of the samples was analyzed by Fourier transform infrared spectroscopy (FTIR), and the content of each phase was quantified in combination with XRD. The microstructural morphology of the films was characterized by thermal field emission scanning electron microscopy (FSEM). The electrical properties were tested by sputtering gold electrodes on the both sides of the film. The dielectric constant and dielectric loss were measured by an LCR precision meter. The hysteresis loops and breakdown strength of the films were tested by a ferroelectric analyzer.Results and discussion Irradiation produces a cross-linking structure, effectively fixing the free charge and hinder the formation of electrical pathways. Also, the cross-linking structure effectively fills the gaps on the surface caused via solvent evaporation and crystallization, which prevents the breakdown of a local high voltage. The phase αin P(VDF-HFP) decreases with increasing irradiation injection, from 62.65% for a pristine phase to 43.72% for an irradiation injection of 5×1010 ions/cm2. The polar phases β and γincrease monotonically with increasing the phase β to 19.97%, which is twice greater than that of the pristine phase, and the phase γ increases to 36.31%. The fast heavy ion irradiation will cause the polymer chains to break bonds and produce more polar dipoles, improving the dielectric constant of the material. Moreover, the defects introduced by high energy irradiation provide a larger turning space for the dipoles due to the reduction of material losses. At 1 kHz, the dielectric constant of 5×107 ions/cm2 irradiated injection samples increases from 8.43 to 9.97, and the loss decreases from 0.033 to 0.027. A suitable irradiation dose can effectively increase the breakdown strength of the material. The breakdown strength of the material increases and then decreases with the increase of irradiation dose. At 5×107 ions/cm2, the breakdown strength of the material increases from 440 MV/m to 540 MV/m for the untreated samples, which is related to the dominant phase transition of the γ-phase and the cross-linking structure generated via irradiation. The energy storage performance of this work is compared with representative P(VDF-HFP) modifiers reported perviously. Most of the reported P(VDF-HFP) modifiers have discharge energy density values of <10 J/cm3 and breakdown field strengths of < 500 MV/m. In contrast, this work achieves greater discharge energy density and breakdown strength. Conclusions The 129Xe27+ fast heavy ion irradiation modification promoted advantageous phase transitions and the formation of surface cross-linking structures, thus improving the energy storage efficiency and breakdown strength of the material. Meanwhile, the broken bonds caused via irradiation increased the dielectric constant of the material and the effective polarization. With the synergy of multiple effects, irradiation-modified polymer materials with a high breakdown field strength (i.e., 540 MV/m) and a high discharge energy density (i.e., 16.3 J/cm3) were prepared. This work indicated that irradiation modification could realize the preparation of polymer materials with a high energy storage performance, providing a theoretical and experimental basis for the development of dielectric capacitors.