Organic/inorganic nanocomposites (OINs) can be potentially used as high-performance capacitors due to their rapid charge-discharge capability along with respectable power density. The coupling effect of the filler/matrix interface plays a prominent role in the dielectric and electric properties of OINs. Along with a review of contemporary theoretical models, recent advances in interfacial optimization to improve energy density through careful interface control and design are also presented. Possible mechanisms that may improve energy density and potential applications for high-energy-density capacitors are also highlighted.

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Flexible electrostatic capacitors are potentially applicable in modern electrical and electric power systems. In this study, flexible nanocomposites containing newly structured one-dimensional (1D) BaTiO@AlO nanofibers (BT@AO NFs) and the ferroelectric polymer poly(vinylidene fluoride) (PVDF) matrix were prepared and systematically studied. The 1D BT@AO NFs, where BaTiO nanoparticles (BT NPs) were embedded and homogeneously dispersed into the AO nanofibers, were successfully synthesized via an improved electrospinning technique. The additional AO layer, which has moderating dielectric constant, was introduced between BT NPs and PVDF matrixes. To improve the compatibility and distributional homogeneity of the nanofiller/matrix, dopamine was coated onto the nanofiller. The results show that the energy density due to high dielectric polarization is about 10.58 J cm at 420 MV m and the fast charge-discharge time is 0.126 μs of 3.6 vol % BT@AO-DA NFs/PVDF nanocomposite. A finite element simulation of the electric-field and electric current density distribution revealed that the novel-structured 1D BT@AO-DA NFs significantly improved the dielectric performance of the nanocomposites. The large extractable energy density and high dielectric breakdown strength suggest the potential applications of the BT@AO-DA NFs/PVDF nanocomposite films in electrostatic capacitors and embedded devices.

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.