Constructing Deep Traps to Achieve Excellent Dielectric Properties in Crystal-Based HfO₂/PEI Nanocomposite Films with Ultralow Filler Loadings

Mixing fillers featured with wide band gaps in polymers can effectively meet the requirement of higher energy storage densities. However, the fundamental relationship between the crystal structures of fillers and the dielectric properties of the corresponding nanocomposites is still unclear. Accordingly, we introduced ultralow contents of the synthesized cubic Hafnium dioxide (c-HfO 2 ) or monoclinic Hafnium dioxide (m-HfO 2 ) as deep traps into poly(ether imide) (PEI) to explore their effects on dielectric properties and the charge-blocking mechanism. m-HfO 2 /PEI showed better charge trapping due to the higher electron affinity and deeper trap energy. At room temperature, the 0.4 vol % m-HfO 2 /PEI maintains an ultralow dielectric loss of 0.008 while obtaining a dielectric constant twice that of pure PEI at 1 kHz, simultaneously outperforming in terms of leakage current density, breakdown strength (452 kV mm –1 ), discharge energy density ( U d, 5.03 J cm –3 ), charge–discharge efficiency (η, 92%), and dielectric thermal stability. At 125 °C, it exhibits a considerable U d of 2.48 J cm –3 and a high η of 85% at 300 kV mm –1, surpassing the properties of pure PEI. This promising work opens up a new path for studying HfO 2 -derived dielectrics with unique crystal structures.