Polymer nanocomposite dielectrics for capacitive energy storage.

Owing to their excellent discharged energy density over a broad temperature range, polymer nanocomposites offer immense potential as dielectric materials in advanced electrical and electronic systems, such as intelligent electric vehicles, smart grids and renewable energy generation. In recent years, various nanoscale approaches have been developed to induce appreciable enhancement in discharged energy density. In this Review, we discuss the state-of-the-art polymer nanocomposites with improved energy density from three key aspects: dipole activity, breakdown resistance and heat tolerance. We also describe the physical properties of polymer nanocomposite interfaces, showing how the electrical, mechanical and thermal characteristics impact energy storage performances and how they are interrelated. Further, we discuss multi-level nanotechnologies including monomer design, crosslinking, polymer blending, nanofiller incorporation and multilayer fabrication. We conclude by presenting the current challenges and future opportunities in this field.

Polyimides Physically Crosslinked by Aromatic Molecules Exhibit Ultrahigh Energy Density at 200 °C.

Polymer dielectrics possess significant advantages in electrostatic energy storage applications, such as high breakdown strength (Eb ) and efficiency (η), while their discharged energy density (Ud ) at high temperature is limited by the decrease in Eb and η. Several strategies including introducing inorganic components and crosslinking have been investigated to improve the Ud of polymer dielectrics, but new issues will be encountered, e.g., the sacrifice of flexibility, the degradation of the interfacial insulating property and the complex preparation process. In this work, 3D rigid aromatic molecules are introduced into aromatic polyimides to form physical crosslinking networks through electrostatic interactions between their oppositely charged phenyl groups. The dense physical crosslinking networks strengthen the polyimides to boost the Eb , and the aromatic molecules trap the charge carriers to suppress the loss, allowing the strategy to combine the advantages of inorganic incorporation and crosslinking. This study demonstrates that this strategy is well applicable to a number of representative aromatic polyimides, and ultrahigh Ud of 8.05 J cm-3 (150 °C) and 5.12 J cm-3 (200 °C) is achieved. Furthermore, the all-organic composites exhibit stable performances during ultralong 105 charge-discharge cycles in harsh environments (500 MV m-1 and 200 °C) and prospects for large-scale preparation.

Regulating the Mechanical and Optical Properties of Polymer-based Nanocomposites by Sub-Nanowires.

Sub-nanowires (SNWs) exhibit great potential applications in nanocomposites owing to their high specific surface area, high flexibility, and similarity to polymer chains in dimension, which are a good entry point to bridge inorganic materials and polymer materials. Herein, we synthesized hydroxyapatite sub-nanowires (HAP SNWs) and engineered hydroxyapatite sub-nanowires/polyimide (HSP) gels and films by simple mixing of HAP SNWs and polyimide (PI). Benefiting from the interactions between HAP SNWs and PI, these nanocomposites were a continuous hybrid network. As the increase of HAP SNWs contents, the viscosity and modulus of HSP gels were greatly improved by one or two orders of magnitude compared with PI gel. HSP films not only maintained high transparency but also gained high haze, as well as exhibited enhanced Young's modulus. Thus, both HSP gels and films developed in this work are promising for various applications in coatings and high-performance films.

High-Energy-Density and High Efficiency Polymer Dielectrics for High Temperature Electrostatic Energy Storage: A Review.

Polymer dielectrics are key components for electrostatic capacitors in energy, transportation, military, and aerospace fields, where their operation temperature can be boosted beyond 125 °C. While most polymers bear poor thermal stability and severe dielectric loss at elevated temperatures, numerous linear polymers with linear D-E loops and low dielectric permittivity exhibit low loss and high thermal stability. Therefore, the broad prospect of electrostatic capacitors under extreme conditions is anticipated for linear polymers, along with intensive efforts to enhance their energy density with high efficiency in recent years. In this article, an overview of recent progress in linear polymers and their composites for high-energy-density electrostatic capacitors at elevated temperatures is presented. Three key factors determining energy storage performance, including polarization, breakdown strength, and thermal stability, and their couplings are discussed. Strategies including chain modulation, filler selection, and design of topological structure are summarized. Key parameters for electrical and thermal evaluations of polymer dielectrics are also introduced. At the end of this review, research challenges and future opportunities for better performance and industrialization of dielectrics based on linear polymers are concluded.

Scalable Ultrathin All-Organic Polymer Dielectric Films for High-Temperature Capacitive Energy Storage.

The miniaturization of electronic devices and power systems for capacitive energy storage under harsh environments requires scalable high-quality ultrathin high-temperature dielectric films. To meet the need, ultrasonic spray-coating (USC) can be used. Novel polyimides with a dipolar group, CF3 (F-PI), and all-organic composites with trace organic semiconductor can serve as models. These scalable high-quality ultrathin films (≈2.6 and ≈5.2 µm) are successfully fabricated via USC. The high quality of the films is evaluated from the micro-millimeter scale to the sub-millimeter and above. The high glass transition temperature Tg (≈340 °C) and concurrent large bandgap Eg (≈3.53 eV) induced by weak conjugation from considerable interchain distance (≈6.2 Å) enable F-PI to be an excellent matrix delivering a discharge energy density with 90% discharge efficiency Uη90 of 2.85 J cm-3 at 200 °C. Further, the incorporation of a trace organic semiconductor leads to a record Uη90 of ≈4.39 J cm-3 at 200 °C due to the markedly enhanced breakdown strength caused by deep charge traps of ≈2 eV. Also, a USC-fabricated multilayer F-PI foil capacitor with ≈85 nF (six layers) has good performance at 150 °C. These results confirm that USC is an excellent technology to fabricate high-quality ultrathin dielectric films and capacitors.