Significantly improved high-temperature charge-discharge efficiency of all-organic polyimide composites by suppressing space charges

In order to satisfy the miniaturization trends of power electronic devices and safe operation in harsh environments, high-temperature dielectric polymers with excellent energy density and great reliability are desired in advanced electronic and power systems as electrostatic capacitors. Herein, the all-polymer dielectric composite films, consisting of ferroelectric polymer poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) as filler and linear polymer polyimide (PI) as matrix, were fabricated by in situ polymerization method for electrostatic energy storage scenarios. Scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) results indicated that the P(VDF-HFP) was successfully compounded into the PI matrix. The dielectric permittivity of PI/P(VDF-HFP) energy storage composites was enhanced, while the breakdown strength and electrical resistivity values were wrecked slightly with enhancive mass fraction of P(VDF-HFP). Consequently, the discharged energy density (Ud) of PI-10, PI-20 and PI-30 were 2.60, 2.90, and 3.16 J/cm3 at 30 ℃ and 400 MV/m, respectively, which were 1.05, 1.17 and 1.27 times than those of neat PI film (2.48 J/cm3). Notably, an interesting phenomenon was observed that the charge-discharge efficiency (η) of all-organic composites was dramatically improved situated in high-temperature conditions. When the mass ratio of P(VDF-HFP) to PI was 1:4, the corresponding Ud and η of PI-20 could be significantly improved to 2.25 J/cm3 and 76.9% at 382 MV/m and 150 ℃, which were 6.7 and 10.5 times than those of neat PI film (0.336 J/cm3 and 7.3%) at 300 MV/m, respectively. Impressively, PI-30 film delivers an enhanced Ud of 1.81 J/cm3 and η of 71% at 200 ℃ and 308 MV/m. The significant improvement in the efficiency of composites under high-temperature environment is attributed to the suppression of space charges, which was verified by the TSDC results. In addition, the all-polymer composites exhibited outstanding cycling stability under harsh conditions of high temperature and high electric field. Our fabricated PI/P(VDF-HFP) composite films present potential energy storage applications in the high temperature environments with demand for flexible dielectric materials with upgraded permittivity, excellent energy density as well as reliable cycling stability.