BN/MgO Bilayer Heterostructures for High-Temperature Energy Storage in Poly(ether imide) Films

Polymer dielectric films are highly attractive for advanced capacitors owing to their lightweight, high-power density, and excellent processability. However, their energy-storage performance deteriorates severely at elevated temperatures due to enhanced charge injection and conduction loss. Herein, a bilayer heterostructured dielectric barrier composed of boron nitride (BN) and magnesium oxide (MgO) is constructed on both sides of poly(ether imide) (PEI) films via RF magnetron sputtering to overcome these limitations. The BN layer provides a moderate potential barrier with excellent insulation, while the MgO layer introduces an additional high-energy barrier owing to its wider bandgap, forming a stepwise barrier structure and an internal reverse electric field at the BN/MgO interface. This built-in field effectively suppresses thermally induced charge injection and conduction pathways, enhancing the insulation and mitigating dielectric loss. Benefiting from the synergistic bilayer barrier effect, the resultant BN/MgO/PEI/MgO/BN composite films achieve a high discharged energy density of 4.30 J cm–3 and a charge–discharge efficiency of ∼90% at 150 °C, together with excellent cycling stability over 5 × 104 cycles. This work provides a strategy for interfacial band engineering in polymer dielectrics and offers practical guidance for designing high-performance capacitors operating under harsh thermal and electrical conditions.