Next-generation microelectronics and electrical power systems call for highenergy-density dielectric polymeric materials that can operate efficiently under elevated temperatures. However, the currently available polymer dielectrics are limited to relatively low working temperatures. Here, we report the solution-processable polymer nanocomposites consisting of readily prepared Al 2 O 3 fillers with systematically varied morphologies This article is protected by copyright. All rights reserved.including nanoparticles, nanowires and nanoplates. The field-dependent electrical conduction of the polymer nanocomposites at elevated temperatures has been investigated. A strong dependence of the conduction behavior and breakdown strength of the polymer composites on the filler morphology has been revealed experimentally and further rationalized via computations. The polymer composites containing Al 2 O 3 nanoplates display the record capacitive performance, e.g. a discharged energy density of 3.31 J/cm 3 and a chargedischarge efficiency of >90% measured at 450 MV/m and 150 o C, significantly outperforming the state-of-the-art dielectric polymers and nanocomposites that are typically prepared via tedious, low-yield approaches.Dielectric film capacitors have the highest power density (on the order of MW) and the best rate capability (on the order of µs) among the electrical energy storage devices and are the key components in advanced electronics and electrical power systems. 1,2 Polymer dielectrics are enabling materials for high-energy-density film capacitors owing to their unique features such as facile processability, high breakdown strength and great reliability under the applied electric fields. [3][4][5][6][7][8][9][10][11][12] For instance, biaxially oriented polypropylene (BOPP) is currently used in the power inverters of

of 140 °C or above. [4,5] While dielectric ceramics are traditional materials for high-temperature capacitors, [6] they are severely limited by scalability, weight, fracture toughness, and breakdown strength in comparison to their polymer counterparts. [7][8][9][10][11][12][13][14][15][16][17] Biaxially oriented polypropylene film (BOPP), the state-of-the-art commercially available polymer dielectric, however, shows largely degraded high-field dielectric properties when operating at temperatures above 100 °C. [18] To address these imperative needs, a variety of well-established engineering polymers, including polycarbonate, polyimide (PI), polyetherimides, and poly(ether ether ketone), have been exploited as hightemperature dielectric materials. [19][20][21][22][23][24][25] As these aromatic polymers have high glass transition temperatures (T g ) and excellent thermal stability, it is anticipated that the engineering polymers would retain electromechanical properties and thus dielectric stability at high temperatures. However, when subjected to high applied fields, the engineering polymers exhibit limited working temperatures that are much lower than their T g s. [19,20] More recently, inorganic fillers represented by boron nitride nanosheets (BNNSs) have been incorporated into crosslinked divinyltetramethyldisiloxane-bis(benzocyclobutene) (c-BCB) to yield the dielectric polymer composites capable of operating efficiently at high temperatures, e.g. 150 °C. [26][27][28][29] Herein, we describe the hightemperature dielectric properties and capacitive performance of the PI-based polymer nanocomposites prepared via in situ polycondensation. Compared with c-BCB, PI possesses the inherent advantages including much better processability, considerably lower cost, and greater mechanical strength and flexibility, which potentially offers a scalable route toward robust hightemperature dielectric materials. [30,31] The investigation of the polymer composites containing the inorganic nanofillers with systematically varied dielectric constants (K) and bandgap (ΔE), including aluminium oxide (Al 2 O 3 ) with a K of 9.5 and a ΔE of 8.6 eV, hafnium dioxide (HfO 2 ) with a K of 25 and a ΔE of 5.8 eV, titanium dioxide (TiO 2 ) with a K of 110 and a ΔE of 3.5 eV, and BNNS with a K of 4 and a ΔE of 5.97 eV, [26,[32][33][34] would provide experimental guidelines for the design of highperformance high-temperature dielectric polymer composites. Modern electronics and electrical systems demand efficient operation of dielectric polymer-based capacitors at high electric fields and elevated temperatures. Here, polyimide (PI) dielectric composites prepared from in situ polymerization in the presence of inorganic nanofillers are reported. The systematic manipulation of the dielectric constant and bandgap of the inorganic fillers, including Al 2 O 3 , HfO 2 , TiO 2 , and boron nitride nanosheets, reveals the dominant role of the bandgap of the fillers in determining and improving the high-temperature capacitive performance of the polymer compo...

The electrification of transport requires dielectric materials capable of operating efficiently at high temperatures to meet the increasing demand of electrical energy storage at extreme conditions. Current high-temperature dielectric polymers rely on the incorporation of wide bandgap inorganic fillers to restrain electrical conduction and achieve high efficiencies at elevated temperatures. Here, we report a new class of all-polymer based high-temperature dielectric materials prepared from crosslinking of melt-processable fluoropolymers. The crosslinked polymers exhibit larger discharged energy densities and greater charge-discharge efficiencies along with excellent breakdown strength and cyclic stability at elevated temperatures when compared to the current dielectric polymers. The origins of the marked improvement in the hightemperature capacitive performance are traced to efficient charge-trapping by a range of the molecular trapping centers resulted from the crosslinked structures. In addition, the implementation of melt-extrudable polymers would enable scalable processing that is compatible with the current fabrication techniques used for polymer dielectrics, which is in sharp contrast to the dielectric polymer composites with inorganic fillers.