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Figure 1.11 (a) Schematic of the trilayer‐structured film composed of PVDF/BNNS as outer layers and PVDF/BST as the middle layer, (b) cross‐sectional SEM image of trilayer‐structured polymer composites, (c) discharged energy density, and (d) charge/discharge efficiency of PVDF‐based composites with various compositions and structures.

      Source: Liu et al. [87]. Reproduced with permission of John Wiley & Sons.

The multilayer‐structured polymer composites can also employ all‐inorganic layers. Zhu et al. fabricated a series of PVDF‐based multilayer‐structured films interlayered by assembled BNNS layer [94]. The BNNS in the middle layer is aligned in the in‐place direction by the layer‐by‐layer solution casting method (Figure 1.12). By controlling the BNNS concentration, the thickness of the middle BNNS layer can be adjusted from 100 to 700 nm. The simulations of the electric field distribution and electrical tree propagation indicate that the assembled BNNS interlayer can effectively mitigate the electric field distortion and impede the electrical breakdown. The composite film interlayered with BNNS exhibits remarkably suppressed leakage current, improved breakdown strength of 612 kV/mm, and enhanced discharged energy density of 14.3 J/cm³. Compared with the composite with randomly distributed BNNS nanofillers, the proposed interlayered BNNS can significantly reduce the content of the BNNS in the film, i.e. the BNNS content in the layer‐structured film is only 0.16 vol%, while high loading of 5–10 vol% BNNS nanofiller is needed for the conventional composite to achieve desired performance. Also, all‐inorganic layers can be coated onto the surface of the polymer films to construct multilayer‐structured polymer composites [95]. Zhou et al. proposed a roll‐to‐roll plasma‐enhanced chemical vapor deposition (roll‐to‐roll PECVD) method to deposit SiO₂ layer onto various polymer films [96]. The proposed roll‐to‐roll PECVD method enables the scalable, high‐throughput, and environmentally benign deposition of uniform and compact SiO₂ layer on the surface of polymer films. The deposited SiO₂ layer would act as the barrier at the electrode/dielectric interface, which impedes the charge injection and enables the substantial improvement in the electrical energy storage performance of the coated films.

1.8 Conclusion

      Advances in the nanomaterials and nanotechnology have promoted the development of polymer composites for electrical energy storage application. The field of dielectric polymer composites has witnessed great progress, and the progress is still accelerating continuously. The discharged energy density achieved in the polymer composites has already exceeded 20 J/cm³, which is comparable to other energy storage methods, such as electrochemical capacitors. Various innovative material structure designs and processing methods, such as nanofiller morphology control, nanofiller surface modification, nanofiller alignment, and multilayer‐structured composites, have been proposed to improve the performance of the dielectric polymer composites.

To further improve the performance of the dielectric polymer composites to meet the increasing demand from application, the following points should be addressed in the future studies. The majority of the high discharged energy density dielectrics are based on PVDF‐based polymers with high dielectric constant. However, these polymer composites suffer from the high energy loss, i.e. >20% in most PVDF‐based polymer composites at high electric field, which is not acceptable in many applications. So efforts should be made to improve the charge/discharge efficiency of the high‐energy‐density polymer composites. The charge/discharge efficiency should be comparable to the benchmark of biaxially oriented polypropylene (BOPP) film. It should be noted that the current high‐energy‐density polymer composites are mostly designed for room temperature application because of the relatively low thermal stability of the polymer matrix. However, the emerging applications under extreme conditions require polymer dielectrics capable of high temperature application. Although great achievements have been made in high temperature polymer dielectrics, the discharged energy density is still relatively low, i.e. <5 J/cm³ at 150 °C. To further improve the performance of the polymer composites, fundamental understanding of the interfacial properties should be advanced, which would assist the rational design of the material structure. Moreover, the application of big data and machine learning technologies should be explored in the material structure design and optimization to guide the development of dielectric polymer composites. Last but not least, the low‐cost, scale‐up film processing method should be developed to produce the high‐quality, large‐scale polymer composite films. All these efforts together would push the field of dielectric polymer composites to a new stage to meet the emerging demand for electrical energy storage in various industrial applications.