Simultaneously improved permittivity and breakdown strength, accompanied with reduced dielectric loss and conductivity, are difficult to achieve in polymer nanocomposites with conductive fillers by blending to construct a homogeneous structure. In this work, an MXene/fluoropolymer nanocomposite with a gradient sandwich structure has been prepared by casting various polymer solutions thrice, followed by annealing. Unfunctionalized Ti₃C₂T_x MXene nanosheets with lower conductivity and higher Young's modulus compared to graphene nanosheets have been utilized as fillers. Polyvinylidene fluoride (PVDF) has been employed as the polymer matrix. High interface compatibility between MXene and PVDF, as well as between the adjacent two sub-layers in the sandwich composite, has been obtained due to effects of p–π conjugation, hydrogen bond and similarity compatibility, leading to enhanced interface polarization and improved permittivity. The interface barrier effect between the neighboring two sub-layers restrains electric tree growth, resulting in elevated breakdown strength. Compared with mono-layered composites and PVDF, the sandwich composite shows improved comprehensive electrical properties. It has a permittivity of ∼26 at 100 Hz, a breakdown strength of ∼350 MV m⁻¹ and an energy density (efficiency) of ∼12.5 J cm⁻³ (∼64%) at breakdown strength. This work might open the way for large-scale fabrication of promising nanocomposites with a high energy storage density.

A high relative permittivity and breakdown strength are important to the high energy storage of dielectrics. However, in homogeneous composites, electric tree growth leads to premature breakdown. The thermal breakdown of a polymer matrix from excessive heat accumulation results in a low breakdown strength of the composites. In this work, polymer/2D MXene/2D BN nanocomposites were fabricated in a sandwich construction. Ti₃C₂ nanosheets with a high electric conductivity and stiffness, as well as hexagonal BN (h-BN) nanosheets with an ultrahigh thermal conductivity and breakdown strength, were employed as fillers. Polyvinylidene fluoride (PVDF) was used as the matrix. A mild interface polarization between MXene and PVDF endows the composite with the desirable dielectric properties. Charge injection from the electrodes can be depressed owing to BN in two outer sub-layers, and electric tree growth across the thickness can be impeded due to the interface barrier effect between two adjacent sub-layers. A high breakdown property was obtained. An acceptable temperature-dependence of relative permittivity and breakdown strength was gained and is ascribed to heat evacuation from MXene and BN. The inhomogeneous composite is superior to the homogeneous counterpart. The optimized sandwich composite has a breakdown strength of 235 MV m⁻¹ and relative permittivity of 24@1 kHz at 140 °C. This work might open up the way for the large-scale preparation of promising composite dielectrics.

Using V ₂ C-CNT switchboard-like hybrid particles as filler for preparing promising composite dielectrics based on fluorine-migration effect.