Excellent energy storage in polymer composites achieved by surface coating and bulk regulation of multiscale boron nitride

Polymer dielectric films play a critical role in power electronic devices, but the low energy storage density (U_d) limits their potential for integration into emerging energy storage equipment. Ferroelectric polymer dielectrics are candidates for high U_d materials due to high dielectric constant. However, their high dielectric loss results in low charge-discharge efficiency (η) and degradation of insulation properties due to thermal breakdown. Hence, this work employs multiscale boron nitride (BN) to synergistically regulate the surface and bulk structures of polyvinylidene fluoride (PVDF), achieving simultaneous optimization of U_d and thermal conductivity (κ). The constructed BN/PVDF composites incorporate internally micron-sized BN flakes, with a surface coating of boron nitride nanosheets (BNNSs). The combination of the bulk BN and the surface BNNSs coating effectively suppresses charge injection and prevents the propagation of conductive pathways during breakdown. Consequently, the coated composite containing 10 wt% BN yields a remarkable U_d of 22.99 J cm⁻³ at 560 MV m⁻¹ (2.2 times that of PVDF) with a high η of 80.7%, while the κ is elevated by 29.2% relative to PVDF. Surface-bulk synergistic regulation offers a novel research approach for developing high energy storage dielectrics.