High‐Temperature Flexible Nanocomposites with Ultra‐High Energy Storage Density by Nanostructured MgO Fillers

High‐temperature performance is critical to the dielectric polymer capacitors used in environment electronic and high‐power applications. Here, the authors report a composite comprising polyimide (PI) dielectric polymers blended with high‐insulation magnesium oxide (MgO) nano‐filler that exhibits high breakdown strength, wide temperature range, and low dielectric loss. However, most dielectric polymers possess excellent energy storage properties at room temperature and cannot be used at high temperatures above 100 °C. Polyimide dielectric nanocomposites prepared by in situ polymerization, which are composed of easily prepared MgO fillers, have different morphologies, such as nanoparticles (0D), nanowires (1D), and nanoplates (2D). The experimental results show that the insulation behavior and breakdown strength of polymer composites are closely related to the morphology of MgO fillers, and the rationality of this conclusion is further proved by finite‐element simulation results. The polymer composites containing ultra‐low content (0.5 vol%) MgO nanosheets produce an ultra‐high capacitance performance, that is, the discharge energy density is 4.78 J cm−3 at 150 °C, which is significantly better than the most reported dielectric polymers and nanocomposites.