High energy storage density achieved in polymer composites by hierarchical interface engineering design

• Unique hierarchical structure (Ba 5 Nb 4 O 15 @TiO 2 @Al 2 O 3 ) constructed on the surface of ultrathin nanosheets by interfacial engineering technique. • The division of labour effects at the hierarchical interface yields a discharge energy density of 8.12 J cm −3 at 640 MV m −1 under room temperature and 5.2 J cm −3 at 520 MV m −1 when subjected to a temperature of 150 ℃ for the polymer composites. • Finite element simulations provide an in-depth study of the rationality of the interfacial modulation strategy, which is shown to improve the energy storage performance of polymer composites significantly. The field of interfacial engineering, particularly improving polarization and managing the charge transfer route via sensible interface design, aiming to boost energy storage density and efficiency, has emerged as a major challenge. Through the utilization of the solvothermal method, ultra-thin two-dimensional Ba 5 Nb 4 O 15 nanosheets were successfully synthesized, and a hierarchical interface (Ba 5 Nb 4 O 15 @TiO 2 @Al 2 O 3 , abbreviated as BNO@TO@AO) was constructed on their surfaces via interfacial engineering techniques. The hierarchical interface’s multi-interfacial polarization properties significantly enhance the dielectric characteristics. Additionally, the energy level mismatch at these interfaces enables the generation of double heterojunction electric fields, which effectively counteract the majority of external electric fields, thereby improving the breakdown strength. In-depth electrical analysis has demonstrated that the PEI/BNO@TO@AO composite, which exhibits a division of labour effect, attains a superior discharge energy density ( U d ), and finite element simulations further validate this conclusion. Notably, the ultra-low content polymer composites exhibit an impressive discharged energy density of 8.12 J cm −3 at 640 MV m −1 and room temperature, as well as 5.2 J cm −3 at 520 MV m −1 and 150 °C. Employing a hierarchical interface design approach offers significant potential and could lead the way in developing high-energy density polymer dielectric capacitors.