PEI-based all-organic composite films with simultaneous excellent energy storage density and high efficiency†

All-organic composite films have attracted the attention of researchers due to their excellent properties such as high breakdown strength, flexibility, and self-healing ability. However, they are facing a major challenge of not being able to simultaneously increase the energy storage density (U_e) and efficiency (η). Linear dielectric polyetherimide (PEI) with high η is currently the focus of research in the field of energy storage. Nevertheless, its low dielectric constant (ε_r) often does not result in satisfactory U_e. In this paper, PEI is selected as the matrix, and all-organic bilayer composite dielectric films with pure PEI as the bottom layer and a PVDF/PEI blended layer as the top layer are designed and prepared by the solution casting method. The results demonstrate that the high dielectric constant of PVDF and the microscopic interfacial polarization within the blended layer enhance the polarization strength of the bilayer composite film, while the low dielectric loss of PEI ensures the high η of the bilayer composite film, which significantly improves the U_e of the bilayer composite film. Ultimately, the bilayer composite film achieves an excellent U_e of up to 20.16 J cm⁻³ and a high η of 94.6% under an electric field of 640 MV m⁻¹ with a PVDF blend of 50 vol% components. The simulation of the electric breakdown process by finite element analysis provides support for the breakdown mechanism of the bilayer composite films. The U_e of the designed bilayer all-organic composite films exceeds that of the most advanced all-organic dielectric films reported to date, with η greater than 92%. Consequently, this paper provides a novel strategy for the design of all-organic dielectrics with high U_e and high η.