Energy storage properties of polyimide/BaTiO₃ nanocomposite films and their breakdown mechanism in a wide content range

Polyimide (PI) has excellent dielectric properties with superior thermal stability, and it is considered as a promising polymer dielectric for energy storage capacitors. Here, we studied the energy storage properties of PI composite films with BaTiO₃ (BT) nanoparticles in a wide content range. Benefiting from the high breakdown strength (520 kV/mm), the nanocomposite filled with 0.05 wt. % of BT exhibits an increased energy density of 4.51 J/cm³ at room temperature and keeps a good thermal stability (3.22 J/cm³ at 100 °C). Additionally, trap properties of the nanocomposites and their effect on breakdown strength were studied by thermally stimulated depolarization current. It was found that the dominant trap parameter on breakdown strength variation, with the increase in the BT content, is correlated with deep traps, deep and shallow traps especially their depths, and shallow trap density. Then, a physical quantity of average trap depth was introduced, the value of which decreases from 0.81 eV to 0.44 eV as the BT content increases to 50 wt. %, and a clear correspondence was discovered between the average trap depth and breakdown strength.