LAUSR

High‐temperature polymer dielectrics have broad application prospects in next‐generation microelectronics and electrical power systems. However, the capacitive energy densities of dielectric polymers at elevated temperatures are severely limited by carrier excitation and transport. Herein, a molecular engineering strategy is presented to regulate the bulk‐limited conduction in the polymer by bonding amino polyhedral oligomeric silsesquioxane (NH2‐POSS) with the chain ends of polyimide (PI). Experimental studies and density functional theory (DFT) calculations demonstrate that the terminal group NH2‐POSS with a wide‐bandgap of Eg ≈ 6.6 eV increases the band energy levels of the PI and induces the formation of local deep traps in the hybrid films, which significantly restrains carrier transport. At 200 °C, the hybrid film exhibits concurrently an ultrahigh discharged energy density of 3.45 J cm−3 and a high gravimetric energy density of 2.74 J g−1, with the charge‐discharge efficiency >90%, far exceeding those achieved in the dielectric polymers and nearly all other polymer nanocomposites. Moreover, the NH2‐POSS terminated PI film exhibits excellent charge‐discharge cyclability (>50000) and power density (0.39 MW cm−3) at 200 °C, making it a promising candidate for high‐temperature high‐energy‐density capacitors. This work represents a novel strategy to scalable polymer dielectrics with superior capacitive performance operating in harsh environments.