Multilevel Heterointerface Engineering Breaks the Trap‐Barrier Trade‐Off in High‐Energy‐Density Polymer Dielectrics

ABSTRACT The low energy density, inefficient operation, and thermal instability of polymer dielectrics hinder the deployment of film capacitors under harsh environmental conditions. Interface engineering has emerged as a powerful strategy to introduce charge traps or construct interfacial barriers, thereby regulating carrier dynamics and enhancing energy storage. Here, we propose a multilevel heterointerface engineering strategy that integrates boron nitride and barium niobate nanosheets through lattice interlocking. The large work‐function offset and bandgap contrast induce interfacial band bending and a built‐in electric field, forming a complementary trap‐barrier network that guides, blocks, and confines charge carriers. This design effectively suppresses charge injection and mobility, enhances interfacial polarization, and mitigates the propagation of breakdown pathways. Consequently, BNO@BN/PEI composites achieve exceptional energy storage performance, delivering 9.02 J cm −3 ( η = 92%) at room temperature and sustaining 6.1 J cm −3 ( η ≈ 90%) at 150°C, while still preserving 4.6 J cm −3 at 200°C. First‐principles calculations and finite element simulations further validate the structural and functional superiority of the multilevel heterointerface. This work establishes multilevel heterointerface engineering as a generalizable paradigm for breaking the trap‐barrier trade‐off in conventional dielectric design and paves the way for next‐generation high‐energy‐density and thermally robust polymer capacitors.