Roll-to-roll solution casting equipment. Credit: Yang et al.
Engineers and materials scientists are trying to develop more and more advanced devices to meet the growing needs of the electronics industry. These devices include electrostatic capacitors, devices capable of storing electrical energy in a dielectric between a pair of electrodes through the accumulation of electrical charges on the dielectric surfaces.
These capacitors are essential components of various technologies, including electric vehicles and photovoltaics (PV). They are often made using polymers as dielectric materials, synthetic substances made of large organic molecules with good intrinsic flexibility and good insulating properties.
Researchers from Tsinghua University and other institutes in China recently introduced a new strategy to fabricate sub-nanosheet-filled polymer composites with highly advantageous properties. The proposed method, described in a Natural energy paper, allowed them to manufacture a 100 meter long roll of a polymer-based subnanocomposite film.
“We have been paying attention to polymer-based subnanocomposites for years, in cooperation with Professor Xun Wang, Department of Chemistry, Tsinghua University,” Yang Shen, co-author of the paper, told Phys .org. “Our research (focuses) on capacitive energy storage of polymer dielectrics, which requires high polarization, fracture toughness, and suppression of charge migration, especially at high temperatures.”
Subnanomaterials are materials with at least one dimension less than 1 nm in length. These materials can take various forms, such as sub-nanowires, sub-nanosheets and sub-nanobelts. Previous studies have shown that subnanomaterials can exhibit various novel characteristics and properties, making them promising fillers for composite dielectrics.
“First, thanks to their sizes comparable to polymer chains, i.e. 1 nm, sub-nanomaterials exhibit high flexibility, meaning they can adjust their shapes to eliminate interfacial voids and merge dense interface in composites,” Shen explained. “Moreover, subnanomaterials have an atomic surface area ratio of almost 100% and an ultra-large specific surface area, which gives rise to much more remarkable interfacial phenomena than nanofillers, such as charge trapping and l ‘impediment to the path of decomposition.’
Polyoxometalate (POM)-based subnanomaterials are typically fabricated by assembling POM clusters in one or two dimensions. The unique structure resulting from this process allows these materials to capture and store many electrons via a reaction known as metal cation reduction, providing a promising alternative approach to converting electrical energy in dielectric devices.
“In recent years, we have attempted to use sub-nanomaterials as fillers and studied polymer-based sub-nanocomposites,” Shen said. “Initially, we focused on sub-nanowires and discovered an unexpected improvement in polarization. In this recent work, we shifted our attention to sub-nanowires and a substantial improvement in fracture toughness was found. underlined.”
Researchers have been trying to make high-quality polymer sub-nanocomposites for some time now, as they first had to overcome several technical hurdles. First, they had to identify the appropriate solvents to synthesize the materials.
Large polymer subnanocomposite dielectric film. Credit: Yang et al.
“Suitable solvents for polymers and subnanomaterials are totally different, namely N,N-dimethylformamide (DMF) or N-methylpyrrolidone (NMP) for the former and chloroform or cyclohexane for the latter, respectively” , Shen said.
“At first we chose chloroform as the solvent, but its low boiling point and rapid evaporation made the process of solution casting a composite film very difficult. We then turned to DMF/NMP and We encountered a poor distribution of the sub-nanomaterials they contain.”
To overcome the challenges encountered when using DMF/NMP solvents, researchers have used various dispersion processes, such as vigorous stirring and ultrasonic treatment of the materials. This ultimately allowed them to ensure that the subnanomaterials were evenly dispersed throughout their films.
Ultimately, Shen and his colleagues were able to make high-quality subnanocomposites with a filler content of less than 1 wt% and found that this was enough to significantly improve the dielectric performance of their materials, enabling a Ultra high U.d 7.2 J cm−3 with 90% charge-discharge efficiency and charge-discharge cycle stability up to 5 × 105 cycles at 200°C.
“Different from traditional nanocomposites, our subnanocomposites still have excellent flexibility, which suggests broad prospects for industrial roll-to-roll manufacturing and application with multiple configurations,” Shen said. “Furthermore, sub-nanomaterials have been proven to have enhanced effects on many common heat-resistant polymers, confirming their generality in capacitive energy storage.”
As part of their study, the researchers were able to fabricate a 100-meter-long roll of subnanocomposite film using solution casting equipment built in their laboratory. Remarkably, their manufacturing method appears easy to scale and could thus enable continuous roll-to-roll fabrication of subnanocomposites.
“As for traditional nanocomposite dielectrics, due to the high content of rigid inorganic nanofillers, there are several defects and voids at the interface,” Shen said.
“During roll-to-roll manufacturing, these interfacial defects will form microcracks, which will largely deteriorate the flexibility and hamper the industrial manufacturing of these nanocomposite films. In contrast, our subnanocomposites retain high flexibility and have a dense interface due to intrinsic flexibility and good interfacial compatibility with sub-nanomaterial polymers.
Shen and his colleagues found that the 100-meter-long polymer-inorganic sub-nanocomposite they produced exhibited stable energy storage performance and reliable properties. In the future, they hope that the proposed methods will enable large-scale fabrication of subnanocomposite dielectric materials, which could facilitate their integration into various devices.
In their next studies, the researchers plan to continue exploring the fabrication of polymer-inorganic subnanocomposite materials for energy storage capacitors. In addition to further improving the performance of subnanocomposites, they hope to further simplify their production.
“On the one hand, we will continue to explore the interaction between polymers and inorganic fillers at the subnano scale and demonstrate its impact on capacitive energy storage,” Shen added.
“Subnano-inorganics were found to exhibit excellent structural compatibility and similar scaling to polymer chains, which prompted us to introduce chemical bonds between them and form interface-free hybrid dielectrics. Besides, we also hope to promote their mass production and application in film capacitors.
“Although sub-nanocomposite has shown promise for continuous, roll-to-roll manufacturing of dielectric films, we still have many hurdles to overcome, such as high cost of raw materials and tedious synthesis of sub-nanomaterials. “
More information:
Minzheng Yang et al, Roll-to-roll fabricated polymer composites filled with sub-nanosheets exhibiting high energy density and cyclic stability at 200 °C, Natural energy (2024). DOI: 10.1038/s41560-023-01416-3
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