Semi-alicyclic fluorinated polyimide with ultrahigh energy density enabled by bandgap-topology co-engineering

Aromatic polyimide (PI) with high glass transition temperature (T_g) shows promise as a polymer dielectric for energy storage, but its rigid aromatic structure and electron delocalization cause significant conduction loss, degrading energy storage performance and breakdown strength (E_b) under high temperatures. Herein, we introduce a novel semi-alicyclic fluorinated polyimide (H-FPI) designed via a molecular engineering strategy that synergistically integrates bandgap and topological conformation modulation. Specifically, the alicyclic group elevates the lowest unoccupied molecular orbital (LUMO) while strong electron-withdrawing trifluoromethyl (—CF₃) substitution depresses the highest occupied molecular orbital (HOMO), creating a wide bandgap (4.2 eV). Concurrently, the chair-conformation alicyclic backbone and sterically bulky —CF₃ groups synergistically disrupt molecular planarity, reducing π-orbital overlap to suppress charge transfer while restricting chain mobility to yield a high T_g of 272 ℃. Remarkably, H-FPI film delivers a high energy density of 6.02 J/cm³ with a superior breakdown strength of 626 MV/m at 200 ℃, surpassing commercial PI and fluorinated polyimide (FPI) by 1261% and 55%, respectively. Furthermore, H-FPI film exhibits exceptional capacitor charge-discharge cyclability, enhanced mechanical robustness, and excellent thermal stability. This work establishes a new molecular design paradigm for organic capacitors in electrified transportation and smart grid systems requiring high-temperature working reliability.