of BST nanowires, the dielectric constant of the ternary P(VDF‐TrFE‐CFE)/BNNS/BST nanowires composite reaches 52.7, which is 15% improvement over the pristine P(VDF‐TrFE‐CFE) polymer (Figure 1.9). The electrical breakdown strength of the ternary composite with 5 wt% BST nanowires and 12 wt% BNNS is 589 kV/mm, which is 62% higher than that of the pristine P(VDF‐TrFE‐CFE) of 363 kV/mm. Owing to the concurrently increased dielectric constant and breakdown strength, the ternary composite with 5 wt% BST nanowires and 12 wt% BNNS exhibits a super discharged energy density of 24.4 J/cm3 at an electric field of 625 MV/m, which is 295% that of the pristine polymer matrix.
Figure 1.9 (a) Large‐scale cross‐section SEM image of the ternary nanocomposite, the dashed lines point out the location of BST nanowires while the ellipses indicate the existence of the BNNS. (b) Cross‐section SEM image of the ternary nanocomposite, (c) discharged energy density, and (d) charge/discharge efficiency of the ternary nanocomposites.
Source: Liu et al. [79]. Reproduced with permission of Elsevier.
In addition to directly adding multiple nanofillers in the polymer matrix, Luo et al. prepared a hybrid nanofiller with BT nanoparticles tightly embedded in BN by calcining the BT nanoparticles and BN at high temperature [80]. By employing the BN as the host of the BT nanoparticles, not only the dispersion of the BT nanoparticles is promoted but also the local electric field distortion caused by the BT nanoparticles is mitigated. Introducing the hybrid nanofillers into PVDF can achieve a high electrical breakdown strength of 580 kV/mm and a high discharged energy density of 17.6 J/cm3 (Figure 1.10). Some other methods such as covalent bonding can also be used to prepare the hybrid nanofillers [81].
1.7 Multilayer‐structured Polymer Composites
In addition to incorporating multiple nanofillers strategy, one can also combine the advantages of each kind of nanofiller by constructing multilayer‐structured polymer composites with different functional layers. For example, polymer composite with high‐dielectric‐constant nanofillers can act as the high electric displacement layer, which promotes the polarization and achieves high dielectric constant. While the polymer composite with highly insulating nanofillers can act as the charge barrier layer, which impedes the charge carrier transport and electrical breakdown propagation. The multilayer‐structured polymer composites can integrate the advantages of each functional layer by rationally designing every single layer, as well as the spatial arrangement of the different layers. With the rationally designed multilayer structure, the polymer composite can simultaneously achieve the high discharged energy density and high charge/discharge efficiency [82–86]. Moreover, the multilayer‐structured strategy is also compatible with other strategies, such as nanofiller surface modification and nanofiller orientation, which can further improve the electrical energy storage performance of the polymer composites.
Figure 1.10 (a) Schematic of the preparation process of the BT@BN hybrid nanofillers, (b) TEM image of the prepared BT@BN hybrid nanofillers, (c) electrical breakdown strength, and (d) electrical resistivity and leakage current density of PVDF‐based composites with different type of nanofillers.
Source: Luo et al. [80]. Reproduced with permission of John Wiley & Sons.
Liu et al. prepared a trilayer‐structured polymer composite composed of two outer layers of PVDF/BNNS composite and the middle layer of PVDF/BST nanowire composite [87]. The outer PVDF/BNNS layers functionalized with highly insulating BNNS provide the trilayer‐structured film with high electrical breakdown strength, while the middle PVDF/BST nanowire layer improves the polarization of the trilayer‐structured film. By carefully controlling the nanofiller content in each layer, a high breakdown strength of 588 MV/m can be obtained by the redistribution of the electric field and the suppression of electrical tree development by the outer PVDF/BNNS layers (Figure 1.11). With the high dielectric constant and high breakdown strength of the trilayer structure, the polymer composite exhibits a discharged energy density of 20.5 J/cm3. In addition to multilayer‐structured composites with different nanofillers in different layers, polymer composites with same nanofiller but different filler contents in different layers can also improve the performance of the multilayer‐structured composites [88–90]. Furthermore, the multilayer‐structured composites with the gradient distribution of different nanofillers can be designed [91–93].
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
