Dielectric capacitors are the key components in advanced electronics and electrical systems owing to their highest power density among the electrical energy devices. [1][2][3][4][5][6][7] While ceramic dielectrics are of large dielectric constants and high thermal stability, [8][9][10][11][12] polymer dielectrics possess high tolerance to voltage, great reliability, scalability, and light weight, and therefore are preferred for high-energy-density high-power film capacitors. [13][14][15][16][17] However, the current polymer dielectrics are unable to match the temperature requirements of the emerging applications of electrical energy storage and conversion in harsh environments [18][19][20][21][22] because of their inherently poor thermal stability. For example, while the near-engine-temperature in electric vehicles can reach to above 120 °C, [23] the operating temperature of biaxially oriented polypropylene (BOPP), which is the best commercially available polymer dielectric and currently used in power inverters of electric vehicles, is below 105 °C. [24] The wide bandgap semiconductors like silicon carbide (SiC) and gallium nitride that are well positioned to replace traditional silicon power devices boost the operating temperatures of next-generation capacitors beyond 150 °C. [19] To address these urging needs, a variety of engineering polymers with high thermal stability, such as polyimides (PIs) and fluorene polyesters (FPEs), have been exploited as high-temperature dielectric materials. [20,25] Unfortunately, all the polymers show poor charge-discharge efficiencies under elevated temperatures and high applied fields, [26] which is due to sharply increased electrical conduction attributable to various temperature-and field-dependent conduction mechanisms, e.g., charge injection at the electrode/dielectric interface. [27,28] Ceramic dielectrics are relatively insensitive to temperature and able to maintain the energy-storage performance throughout a broad temperature range, [8][9][10][11][12] but they still suffer from considerable energy loss under high electric fields and elevated temperatures. [29] More recently, the addition of 2D wide bandgap nanostructures such as boron nitride nanosheets (BNNSs) into the polymer has been demonstrated to effectively reduce the conduction loss and largely improve the charge-discharge High-temperature capability is critical for polymer dielectrics in the nextgeneration capacitors demanded in harsh-environment electronics and electrical-power applications. It is well recognized that the energy-storage capabilities of dielectrics are degraded drastically with increasing temperature due to the exponential increase of conduction loss. Here, a general and scalable method to enable significant improvement of the high-temperature capacitive performance of the current polymer dielectrics is reported. The high-temperature capacitive properties in terms of discharged energy density and the charge-discharge efficiency of the polymer films coated with SiO 2 via plasma-enhanced chemical...

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.

The growing demand for advanced electronics requires dielectrics operating at high temperatures. The development of high-temperature dielectric polymers is reviewed from the perspective of structure design, dielectric and capacitive performance.

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.

Compared to electrochemical energy devices such as batteries and supercapacitors, dielectric film capacitors have greater power densities and faster charging and discharging rates and are the essential components in power electronics. [4][5][6] Dielectric polymers possess unique features in comparison to their ceramic counterparts, including high breakdown strength, low dielectric loss, facile preparation, and graceful failure mechanism, which make them the materials of choice for scalable high-energy-density capacitors. [7][8][9][10][11] More recently, there is an urgent demand for dielectric materials capable of operating efficiently at elevated temperatures, e.g., 150 °C, in advanced electronics, electrified vehicles, and aerospace power systems. However, dielectric polymers are limited to relatively low working temperatures. [11][12][13][14][15] For example, the operation temperature of biaxially oriented polypropylene (BOPP), the industrial benchmark dielectric polymer, is well below 105 °C under the applied electric fields. [15] A variety of innovative approaches, including the incorporation of wide bandgap inorganic fillers, [16][17][18] deposition of ceramic coatings onto polymer films, [19][20][21] addition of high-electronaffinity molecular semiconductors, [22] and utilization of multilayer-structured films, [23][24][25] have been developed to improve the high-temperature capacitive performance of dielectric polymers. While these approaches are effective in hindering electrical conduction and reducing energy loss at high fields and elevated temperatures, the energy densities of the current high-temperature dielectric composites are limited (below 4 J cm −3 in most cases) owing to relatively low dielectric constant (K) values of the fillers, such as ≈3.5-4 of SiO 2 and boron nitride nanosheets (BNNSs) [16,26] and ≈7.9-10 of Al 2 O 3 . [26] On the other hand, the direct introduction of high-K inorganic fillers, such as TiO 2 with a K of 110 (ref. [27]) and BaTiO 3 with a K of ≈3000 (ref. [28]), into dielectric polymers with the goal of increasing the energy density has yielded very high energy loss and largely reduced chargedischarge efficiency (η) with increasing applied field and temperature. [29,30] For instance, at an applied field of 400 MV m −1 , the η of the polyimide composites with 1 vol% BaTiO 3 nanofibers is only 55% at 150 °C versus 92% at 25 °C. [30] Herein, we present High-energy-density polymer dielectrics capable of high temperature operation are highly demanded in advanced electronics and power systems. Here, the polyetherimide (PEI) composites filled with the core-shell structured nanoparticles composed of ZrO 2 core and Al 2 O 3 shell are described. The establishment of a gradient of the dielectric constants from ZrO 2 core and Al 2 O 3 shell to PEI matrix gives rise to much less distortion of the electric field around the nanoparticles, and consequently, high breakdown strength at varied temperatures. The wide bandgap Al 2 O 3 shell creates deep traps in the composites and thus yields ...

Spacers are key components that are used to support high voltage conductors in gas-insulated substations or gas-insulated lines. The analysis of the surface charge patterns on spacers remains a difficult task, which requires a comprehensive understanding of the physical mechanism of the gas-solid interface charging phenomenon. In this letter, we reported a field dependent property of surface charge accumulation patterns on spacers under DC stress. We verified this finding through experiment, and further, we put forward a field-dependent charging model based on dominant charge transport behavior under different electric fields. It was found that the charging characteristics of the spacer are dominated by the Ohmic conduction from the volume below an electric field of 2.5 kV/mm. When the electric field stress is higher than 2.5 kV/mm, the charging property of spacers is dominated by the enhanced gas ionization according to Townsend's law. The correctness of this model was verified by surface charge measurement results in literature studies, and a method for determining the dominant mechanism of charge accumulation under different electric fields was proposed.

2 W kg −1 of batteries and 10 2 -10 6 W kg −1 of electrochemical capacitors, among the energy storage devices. [6,7] Polymers represented by biaxially oriented polypropylene (BOPP) are preferred dielectrics for high-energydensity capacitors because of their high breakdown strength (>700 MV m −1 ), low energy loss (0.02% at 25 °C), great reliability, and facile processability. [8] One of the critical challenges for technological implementation of polymer dielectrics is the largely deteriorated capacitive performance with increasing operation temperature. [9][10][11] While BOPP exhibits excellent charge-discharge efficiencies (η) at room temperature, its η decreases steeply with increasing temperature, e.g., from 96.2% at 25 °C to 68.5% at 120 °C at 400 MV m −1 , which limits the operation of BOPP at temperatures below 105 °C under the applied fields. [12,13] At temperatures above 85 °C, the operating voltage of BOPP film capacitors must be derated. On the other hand, the rising trend of transportation electrification and the growing demand for electronics used in harsh environment applications, such as those found in aerospace and underground oil and gas exploration systems, require polymer dielectrics to operate efficiently at high temperatures. [9,[14][15][16][17] For instance, in electric vehicles, BOPP film capacitors in the power converters are located near engines where the temperature is around 140-150 °C. [18] To reach the full potential of polymer dielectrics in advanced electronics and electrified transportation, it calls for efficient operation of high-energy-density dielectric polymers under high voltages over a wide temperature range. Here, the polymer composites consisting of the boron nitride nanosheet/polyetherimide and TiO 2 nanorod arrays/polyetherimide layers are reported. The layered composite exhibits a much higher dielectric constant than the current high-temperature dielectric polymers and composites, while simultaneously retaining low dielectric loss at elevated temperatures and high applied fields. Consequently, the layered polymer composite presents much improved capacitive performance than the current dielectric polymers and composites over a temperature range of 25-150 °C. Moreover, the excellent capacitive performance of the layered composite is achieved at an applied field that is about 40% lower than the typical field strength of the current polymer composites with the discharged energy densities of >3 J cm −3 at 150 °C. Remarkable cyclability and dielectric stability are established in the layered polymer nanocomposites. This work addresses the current challenge in the enhancement of the energy densities of high-temperature dielectric polymers and demonstrates an efficient route to dielectric polymeric materials with high energy densities and low loss over a broad temperature range.