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(b) breakdown strength, and (c) leakage current density at 100 kV/mm of the polymer@BT‐based PVDF composites.

      Source: Zhu et al. [63]. Reproduced with permission of American Chemical Society.

In addition to improving the compatibility of the nanofillers, the shell layer of the core‐shell structured nanoparticles can also act as a functional layer to tailor the performance of the polymer composites. It is well documented that the deep traps introduced by chemical modification of grafting functional groups onto polymer chains can slow down the charge carrier transport, thus suppressing the conduction current in polymers [65–68]. So, the functional group grafted polymers can be used to act as the shell layer of the core‐shell structured nanoparticles. Zhou et al. modified the surface of MgO nanoparticles with polypropylene‐graft‐maleic anhydride (PP‐g‐mah) and blended the PP‐mah‐MgO core‐shell structured nanoparticles with polypropylene (PP) matrix [69]. PP‐g‐mah is used because the functional groups in PP‐g‐mah can offer deep traps to suppress the conduction current. Moreover, the similar physical and chemical characteristics of PP‐g‐mah and PP matrix make the PP‐mah‐MgO nanofillers highly miscible with the PP matrix (Figure 1.8). By using the proposed nano‐isothermal surface potential decay (nano‐ISPD) method, the local deep charge traps introduced by the shell layer is directly probed with nanoscale spatial resolution at the nanofiller/polymer interface. It is found that the surface potential decay at the interfaces in the PP‐mah‐MgO/PP composite is much slower than that of the un‐MgO/PP composite, indicating that the interfacial charge traps in PP‐mah‐MgO/PP composite are much higher than that of the un‐MgO/PP composite, which verifies the design of the shell layer. Benefited from the interfacial deep charge traps introduced by the shell layer, the PP‐mah‐MgO/PP composite exhibits superior electrical energy storage performance with excellent temperature stability.

      In addition to the polymer shell layers, inorganic materials including SiO₂, TiO₂, and Al₂O₃ have been employed as the shell layers of the core‐shell structured nanofillers [70–74]. The highly insulated shell layers, such as SiO₂ and Al₂O₃, can serve as a barrier layer to suppress the electrical conduction and breakdown, yielding increased electrical breakdown strength and charge/discharge efficiency. The shell layers with medium dielectric constant, such as TiO₂, can act as a buffer layer to mitigate the local electric field distortion in the high‐dielectric‐constant nanofiller/polymer composites. Moreover, the core‐shell structured strategy is also applicable to the high‐aspect‐ratio 1D and 2D nanofillers.

1.6 Polymer Composites with Multiple Nanofillers

As mentioned before, the discharged energy density is related to both the dielectric constant and electrical breakdown strength of the dielectrics. So both high dielectric constant and high electrical breakdown strength are required. However, high‐dielectric‐constant nanofillers can only increase the dielectric constant of the polymer composites but decrease the electrical breakdown strength because of the local electric field distortion. While the high‐insulating nanofillers usually exhibit low dielectric constant. Then, a strategy of introducing multiple nanofillers with different functionalities has been proposed to simultaneously increase the dielectric constant and the electrical breakdown strength. The benefit of the multiple nanofillers doping comes from the combined advantages of every single nanofiller, i.e. highly insulated nanofillers to increase the electrical breakdown strength and high‐dielectric‐constant nanofillers to promote the dielectric constant [75–78].

      

Figure 1.8 (a) schematic of the preparation of the core‐shell structured pp‐mah‐mgo nanoparticles, (b) tem image of the pp‐mah‐mgo nanoparticle, (c) local charge trap level distribution at the interfacial region obtained from nano‐ispd measurement, (d) frequency‐dependent dielectric constant and dissipation factor at room temperature, (e) temperature‐dependent breakdown strength, and (f) discharged energy density and charge/discharge efficiency at 120 °C.

      Source: zhou et al. [69]. Reproduced with permission of elsevier.

Liu et al. introduced the highly insulated BNNS and high‐dielectric‐constant BST nanowires into P(VDF‐TrFE‐CFE) terpolymer [79]. With the inclusion 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

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