Synergistic Linkage and Endcap Integration in Polyetherimides for High‐Performance Capacitive Energy Storage

High‐temperature polymers with excellent dielectric and intrinsic self‐healing capabilities are essential for advanced electrical and electronic systems. However, existing design approaches for high temperature polymers rely on the incorporation of bulky phenyl groups in the mainchain, resulting in compromised dielectric performance and self‐healing due to π–π stacking and elevated element ratio of ( C + N + S )/( H + O ). Here, a molecular design strategy is reported for polyetherimides that synergistically integrates alicyclic linkages with large endcap structures to concurrently enhance dielectric and self‐healing properties. Among the synthesized materials, a novel polyetherimide featuring an alicyclic linkage and a 2,4,6‐tri‐tert‐butylaniline (tTBA) endcap achieves an exceptional discharged energy density of 8.2 J cm − 3 at 808 MV m −1 and 150 °C, a 4X improvement over existing polyetherimides. Mechanism studies reveal that alicyclic linkages facilitate optimal volatilization pathways during pyrolysis, with reduced char residue formation, to ensure robust self‐healing. This research provides a molecular‐engineering design framework for tailoring high‐temperature polymers with concurrently enhanced dielectric and self‐healing properties.