Synergistic Molecular Engineering of Crosslinked Polymer Dielectrics for High‐Temperature Capacitive Energy Storage

Abstract Polymer dielectric capacitors are critical for high‐temperature energy storage, yet current materials face a trade‐off between thermal stability and capacitive performance due to conduction loss or insufficient polarization. Here, a modular molecular engineering to simultaneously optimize molecular polarity, topological crosslinking, and free volume in alicyclic polymers is designed. By incorporating thermally crosslinkable benzocyclobutene (BCB) and sulfone‐methyl (─SO 2 CH 3 ) groups into norbornene‐based monomers via ring‐opening metathesis polymerization (ROMP), crosslinked networks with decoupled non‐conjugated backbones and polar moieties are constructed. The polymers exhibit a wide optical bandgap ( E g > 3.7 eV), high thermal stability ( T g > 350 °C), and suppressed dissipation ( D f ≈ 0.0006). Optimized P50‐B250 delivers an exceptional discharged energy density ( U d ) of 8.00 J cm −3 at 150 °C (≥90% efficiency), while fully crosslinked P0‐B300 retained U d of 7.34 J cm −3 at 200 °C and 4.65 J cm −3 at 250 °C, outperforming conventional dielectrics. Molecular dynamics (MD) simulations revealed that crosslinking increases free volume fraction by ≈40%, inhibiting interchain charge transfer complexes (CTCs). Density functional theory (DFT) calculations confirm that sulfonyl‐enhanced polarization and crosslinking collectively restrict charge migration. This work establishes a general framework for designing polymer dielectrics by integrating structural modularity and topological control, offering pathways for next‐generation energy storage applications under extreme conditions.