Janus composite film comprising metal–organic framework/polyimide and BNNSs/polyimide as high energy storage dielectrics

Abstract In order to prepare polymer dielectric materials with high energy storage density, a series of polyimide (PI) based Janus composite films were prepared by metal–organic frameworks (MOFs) material (UiO‐66) with high dielectric constant (ɛ_r) and boron nitride nanosheets (BNNSs) with high breakdown strength (E_b). As reported, the UiO‐66 can significantly improve the ɛ_r of the composites. The ɛ_r of 20 wt% UiO‐66/PI film is 10.51 at 10³ Hz, which is 3.1 times that of pure PI film (3.39 at 10³ Hz). However, the E_b of 20 wt% UiO‐66/PI is only 58 MV m⁻¹, which is only 16.51% of pure PI (352 MV m⁻¹). The sharp decline in E_b reduces the energy storage capacity of the composites. To solve this problem, first, BNNSs with an average diameter of 2 μm and a thickness of 1.36 nm (about 4–5 layer) were obtained by hydrothermal reflux stripping. BNNSs/PI//UiO‐66/PI Janus composite films with double‐layer structure were prepared by directly casting BNNSs/PI on the surface of UiO‐66/PI. For 0.3 wt% BNNSs/PI//10 wt% UiO‐66/PI, the calculated energy storage density (U_e) is 5.75 J cm⁻³ at 140 MV m⁻¹, which is 3.57 times that of pure PI film (1.61 J cm⁻³ at 140 MV m⁻¹). Meanwhile, the charge discharge efficiency is 82.55% (at 140 MV m⁻¹). Highlights BNNSs prepared by hydrothermal reflux stripping have fewer layers. BNNSs/PI composites improve the E_b of UiO‐66/PI composites by Janus structure. The U_e of BP‐0.3//UP‐10 composites with Janus structure was 5.75 J cm⁻³.