Unprecedented Energy Density of Polyimide Dielectrics at Elevated Temperatures Utilizing Atomic Engineering to Decouple π‐Conjugation

Abstract Emerging demands for polymer dielectrics in high‐power electronics and harsh environments call for polymers with simultaneously high thermal stability and electrical performance. However, most polymers suffer from rapid conduction loss at elevated temperatures. Here it is shown that atomic‐level backbone chlorination of polyimide (Cl‐PI) imposes π‐electron localization and out‐of‐plane steric barriers that, together, suppress through‐plane hopping arising from both intrachain π‐conjugation and interchain π‐π stacking, dismantling long‐range conduction pathways. This cooperative mechanism markedly curtails high‐temperature conduction loss and yields outstanding capacitive performance, delivering discharge energy densities (U d ) of 9.52 J cm − 3 at 150 °C and 7.22 J cm −3 at 200 °C, with efficiencies exceeding 90%. Even at 250°C, an unprecedented U d of 6.79 J cm −3 is retained, outperforming reported high‐temperature dielectric polymers. Moreover, Cl‐PI exhibits excellent self‐cleaning behavior and cycling durability, sustaining over 10 6 cycles at 200 °C and 400 MV m −1 with minimal degradation. This work underscores the potential of atomic‐level backbone engineering to enable next‐generation polymer dielectrics for reliable, high‐temperature capacitive energy storage.