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

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.