Dielectric polymers for electrostatic energy storage suffer from low energy density and poor efficiency at elevated temperatures, which constrains their use in the harsh-environment electronic devices, circuits, and systems. Although incorporating insulating, inorganic nanostructures into dielectric polymers promotes the temperature capability, scalable fabrication of high-quality nanocomposite films remains a formidable challenge. Here, we report an all-organic composite comprising dielectric polymers blended with high-electron-affinity molecular semiconductors that exhibits concurrent high energy density (3.0 J cm −3) and high discharge efficiency (90%) up to 200°C, far outperforming the existing dielectric polymers and polymer nanocomposites. We demonstrate that molecular semiconductors immobilize free electrons via strong electrostatic attraction and impede electric charge injection and transport in dielectric polymers, which leads to the substantial performance improvements. The all-organic composites can be fabricated into large-area and high-quality films with uniform dielectric and capacitive performance, which is crucially important for their successful commercialization and practical application in high-temperature electronics and energy storage devices.

terms of a graceful failure. [1] These devices have been applied in many high-tech fields such as hybrid electric vehicles, electrical defibrillators, pulsed power systems, and power grids. Nevertheless, the inferior energy density of the polymer dielectrics significantly restrains the development of film capacitors in the future applications. For instance, the commercial benchmark poly mer dielectric biaxially oriented polypropylene (BOPP) only possesses an energy density of <4 J cm −3 , [2] which leads to the cumbrous volume and cost in the practical applications. For example, film capacitors occupy ≈35% the volume and ≈40% the cost of the power inverters in hybrid electric vehicles. [3] The energy density (U e ) of dielectric materials can be derived from U e = ∫EdD , where E is the applied electric field and D denotes the electrical displacement. For linear dielectrics, the formula evolves into U e = 1/2ε 0 ε r E b 2 , where ε 0 is the vacuum permittivity, and ε r and E b are the dielectric constant and breakdown strength of dielectrics, respectively. Clearly, U e depends on the dielectric constant and breakdown strength of dielectrics, where E b is more important because of its quadratic relationship to U e .Boron nitride nanosheets (BNNSs), a 2D nanomaterial with a wide bandgap (≈6 eV), have proved to be an intriguing dopant in dielectric polymer nanocomposites for the interest of enhancing dielectric strength and energy efficiency, because they are of high intrinsic breakdown voltage, can serve as efficient scattering centers for charge carriers, and possess large electrical resistance, in addition to their great thermal conductivity and mechanical strength. [4] To make use of the exceptional effects of BNNSs on dielectric polymers, a straightforward approach is to homogeneously disperse such nanosheets in the polymer matrices to impede charge conduction. In princi ple, with increasing the population density of BNNSs in the polymer matrices, the transport of charge carriers becomes increasingly constrained by these 2D topological barriers, leading to better dielectric performance. A strain forced orientation (i.e., mechanical stretching) of the BNNSs along the film direction is usually applied as an extra step to further enlarge the coverage of the embedded nanofillers in the transverse plane direction, and hence improve the dielectric performance of the composite film. [5] However, with increasing the feed Polymer dielectrics such as poly(vinylidene fluoride) (PVDF) have drawn tremendous attention in high energy density capacitors because of their high dielectric constant and ease of processing. However, the discharged energy density attained with these materials is restrained by the inferior breakdown strength and electric resistivity. Herein, PVDF composite films with a nanosized interlayer of assembled boron nitride nanosheets (BNNSs) that is aligned along the in-plane direction are prepared through a simple layer-by-layer solution-casting process. Compared to the pristine PVDF, the composite films ...

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This paper reports an unpredictable insulation surface flashover phenomenon triggered by surface charge clusters under DC electric field, which is experimentally revealed by a donut-shaped dust cluster figure, presenting the outline of these surface charge clusters and showing a hollow donut-shaped structure interestingly. The charge cluster is randomly formed when ions originating from local micro-protrusions accumulate on the insulation surface, the edge of which produces a strong electrical field to induce local streamers. These local streamers provide an inducing factor, unpredictably triggering surface flashover. An in-depth illustration of how the dust phase transition depicts surface charge cluster evolution is proposed. This uncovered phenomenon has significant physical implications in revealing unpredictable insulation surface flashover and has great significance in further developing of HVDC gas insulated power transmission systems and other fields, such as microelectronics and space aircrafts.

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Grafting modification is an effective method to enhance the electrical characteristics of polymeric materials by establishing deep traps that prevent carriers from being injected and transmitted. However, grafted polymers for electrical insulation suffer from large leakage current at elevated temperatures, limiting their application in harsh environments. We report that an itaconic anhydride grafted polypropylene synthesized by the solution grafting method preserves excellent insulating properties up to 120 °C. It is demonstrated that the grafted itaconic anhydride restrains free electrons employing strong electrostatic attraction and obstructs charge injection and transport. Furthermore, we reveal that strong intramolecular forces, deep energy traps, and fragmentized spherulites are essential factors contributing to grafted polymers' superior high-temperature electrical properties. This work provides insights into the sophisticated charge transport mechanisms in grafted polymers and their effects on the insulation characteristics, which is critical for the suitable design of polymeric materials with exceptional electrical properties.

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In article number 1901826, Xingyi Huang, Qi Li and co‐workers develop novel composite films with a nanosized interlayer of assembled boron nitride nanosheets (BNNSs) that is aligned along the in‐plane direction, representing a new paradigm for high performance dielectric materials. This compact interlayer of assembled BNNSs can largely block the propagation of electrical treeing, giving rise to improved dielectric energy storage performance.