Filler-Gradient Composites Engineered by Varying Ni(OH)₂ Nanosheet loading for Enhanced Energy Storage Performances

Dielectric capacitors offer fast charge/discharge rates and high-power density but are limited by low energy density, requiring bulky stacking. Although single-layer composites attempt to combine both high dielectric constant and breakdown strength, a trade-off often limits their effectiveness. To resolve this conflict, sandwich-structured composites spatially separate polarization and insulation functions into different layers. However, the abrupt dielectric contrast at interlayer interfaces often results in localized electric field distortion, undermining breakdown reliability. In this work, we develop filler-gradient composites by varying the loading of Ni(OH)₂ nanosheets to construct transitional interlayers. This design mitigates electric field imbalance in conventional sandwich structures, enhancing dielectric constant, breakdown strength, and energy density. The results demonstrate that the filler-gradient structure facilitates uniform electric field distribution and enhanced interfacial polarization, resulting in a high energy density of 21.1 J/cm3 at 685.5 MV/m, and discharge efficiency of 70.1%.