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.

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 ...

offer a high power density to meet the demand in rapid charge-discharge applications. [6][7][8][9] Supercapacitors exhibit higher power densities and long cycling lifespans, but are limited by the chemical and electrochemical stability of the electrolytes, as well as a relatively low operating voltage. [10][11][12][13] Polymer film capacitors possess the advantages of low cost, facile fabrication, excellent flexibility, and high operating voltage, display the highest power densities in comparison with batteries and supercapacitors, and are widely used in electronic devices and power systems. [14][15][16][17][18] Although the state-of-the-art capacitor film represented by biaxially oriented poly propylene (BOPP) exhibits ultrahigh charge-discharge efficiency, the energy density has been significantly limited by its low dielectric constant (K), which is only about 1-2 J cm −3 . [19] To address this issue, ferroelectric polymers represented by poly(vinylidene fluoride) (PVDF) and its copolymers and terpolymers with relatively high K (≥10) have been regarded as the most promising polymeric materials for high-energy-density film capacitors. [16][17][18][19][20][21][22][23] More importantly, since capacitors can contribute more than 25% of the volume and weight to the electric power systems, the dramatic improvement of energy density of film capacitors will help to reduce the volume, weight, and cost of electronic devices, hybrid electric vehicles, etc. [24,25] The K value of ferroelectric polymers, however, is still considerably low in comparison with those of ceramic dielectrics (e.g., K of 10 4 -10 5 ) for capacitive energy storage, though these ceramics suffer from low dielectric-breakdown strength (E b ) and poor scalability. [26][27][28][29] Thus, a composite approach has been developed to improve energy-storage capability via introducing high-K inorganic fillers into ferroelectric polymers with high E b and facile processability. For dielectric polymer nanocomposites, the total stored energy densities, which are the sum of the energy densities of the ceramic filler and the polymer phases, are derived from, where U d is the energy density, f 1 is the volume fraction of the ceramic filler, f 2 is the volume fraction of the polymer matrix, and g is the interfacial area between the filler and the polymer. As ferroelectric polymers have the highest energy densities among the known dielectric polymers, they have been considered as the material of choice as polymer-matrix candidates for dielectric polymer nanocomposites. Moreover, the relatively high K values of ferroelectric polymers help to alleviate local field distortion in the The introduction of inorganic components into a polymer matrix to form polymer composites is an emerging and promising approach to dielectric materials for capacitive energy storage. Ferroelectric polymers are particularly attractive as matrices for dielectric polymer composites owing to their highest dielectric constant (≥10) among the known polymers. Here, the important aspects and recent ...

Ferroelectric polymers have been regarded as the preferred matrix for high‐energy‐density dielectric polymer nanocomposites because of their highest dielectric constants among the known polymers. Despite a library of ferroelectric polymer‐based composites having been demonstrated as highly efficient in enhancing the energy density, the charge–discharge efficiency remains moderate because of the high intrinsic loss of ferroelectric polymers. Herein, a systematic study of the oxide nanofillers is presented with varied dielectric constants and the vital role of the dielectric match between the filler and the polymer matrix on the capacitive performance of the ferroelectric polymer composites is revealed. A combined experimental and simulation study is further performed to specifically investigate the effect of the nanofiller morphology on the electrica properties of the polymer nanocomposites. The solution‐processed ferroelectric polymer nanocomposite embedded with Al2O3 nanoplates exhibits markedly improved breakdown strength and discharged energy density along with an exceptional charge–discharge efficiency of 83.4% at 700 MV m−1, which outperforms the ferroelectric polymers and nanocomposites reported to date. This work establishes a facile approach to high‐performance ferroelectric polymer composites through capitalizing on the synergistic effect of the dielectric properties and morphology of the oxide fillers.

The exploration of high‐energy‐density electrostatic capacitors capable of operating both efficiently and reliably at elevated temperatures is of great significance in order to meet advanced power electronic applications. The energy density of a capacitor is strongly dependent on dielectric constant and breakdown strength of a dielectric material. Here, we demonstrate a class of solution‐processable polymer nanocomposites exhibiting a concurrent improvement in dielectric constant and breakdown strength, which typically show a negative correlation in conventional dielectric materials, along with a reduction in dielectric loss. The excellent performance is enabled by the elegant combination of nanostructured barium titanate and boron nitride fillers with complementary functionalities. The ternary polymer nanocomposite with the optimized filler compositions delivers a discharged energy density of 2.92 J cm−3 and a Weibull breakdown strength of 547 MV m−1 at 150°C, which are 83% and 25%, respectively, greater than those of the pristine polymer. The conduction behaviors including interfacial barrier and carrier transport process have been investigated to rationalize the energy storage performance of ternary polymer nanocomposite. This contribution provides a new design paradigm for scalable high‐temperature polymer film capacitors.

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