(e) show the electric field distribution in corresponding polymer composites.
Source: Shen et al. [36]. Reproduced with permission of John Wiley & Sons.
Compared with 0D and 1D nanofillers, 2D nanofillers can impede the charge carrier transport and thus suppress the conduction loss because of the special lamellar structure [37–40]. Moreover, the 2D nanofillers can substantially improve the electrical breakdown strength of the polymer composites. Zhu et al. compared the electrical and capacitive energy storage performance of PVDF/TiO2 composites filled with 5 wt% of 0D, 1D, and 2D TiO2 nanofillers [41]. It is found that the 2D TiO2 nanofillers can not only increase the dielectric constant but also suppress the dielectric loss, which is desirable for electrical energy storage application (Figure 1.3). Because the 2D TiO2 nanofillers can effectively suppress the charge carrier transport, the leakage current of the composite with 2D TiO2 nanofillers is only a half of that of the composite with 0D TiO2 nanofillers. The composite with 2D TiO2 nanofillers also exhibits higher electrical breakdown strength (i.e. 566 MV/m) in comparison to that of the composite with 0D (i.e. 295 MV/m) and 1D (i.e. 382 MV/m) TiO2 nanofillers, representing an increase of 92 and 48%, respectively. As the result of increased breakdown strength and dielectric constant, as well as the suppressed conduction loss, the composite with 2D TiO2 nanofillers shows the highest discharged energy density of 13 J/cm3 at 570 MV/m, which is about 236 and 382% that of the composite with 0D (i.e. 5.5 J/cm3 at 400 MV/m) and 1D (i.e. 3.4 J/cm3 at 300 MV/m) TiO2 nanofillers, respectively. Moreover, the charge/discharge efficiency of the composite with 2D TiO2 nanofillers is also the highest among the three kinds of composites because of the suppressed conduction loss.
Figure 1.3 (a) TEM image of 2D TiO2 nanofillers, (b) atomic force microscopy (AFM) image of 2D TiO2 nanofillers and corresponding surface morphology of the selected line, (c) discharged energy density, (d) charge/discharge efficiency, (e) leakage current density, and (f) electrical breakdown strength of the PVDF/TiO2 composites with 5 wt% of 0D, 1D, and 2D TiO2 nanofillers.
Source: Zhu et al. [41]. Reproduced with permission of American Chemical Society.
Apart from the 2D nanofillers with high dielectric constant, such as TiO2 nanoplates, 2D nanofillers with high insulating performance, such as hexagonal boron nitride nanosheets (BNNSs), aluminum oxide (Al2O3) nanoplates, and montmorillonite (MMT) nanosheets, have been widely used in polymer composites for electrical energy storage because of their unique ability to improve the breakdown strength of the polymer composites [42–44]. For example, owing to the wide bandgap (~6 eV) and excellent breakdown strength (~800 MV/m) of BNNS, the P(VDF‐TrFE‐CFE)/BNNS composite can reach up to a high electrical breakdown strength of 610 MV/m at the BNNS filler content of 12 wt% (Figure 1.4), which represents an enhancement of 70% as compared with that of the pristine P(VDF‐TrFE‐CFE) polymer of 362 MV/m [23]. Moreover, the P(VDF‐TrFE‐CFE)/BNNS composite shows an order of magnitude improvement in the high‐field electrical resistivity over the pristine polymer. These results indicate that the highly insulated 2D nanofillers can function as an efficient barrier layer to impede the electrical conduction and breakdown. Because of the increased breakdown strength and electrical resistivity, the P(VDF‐TrFE‐CFE)/BNNS composite shows a maximum discharged energy density of 20.3 J/cm3, which is 121% over that of the P(VDF‐TrFE‐CFE) polymer, i.e. 9.3 J/cm3. In addition to the enhanced discharged energy density, the charge/discharge efficiency of P(VDF‐TrFE‐CFE)/BNNS composite is also significantly enhanced, i.e. 83% at 300 MV/m and 80% at 600 MV/m.
1.4 Orientation of Nanofillers
Considering the superiority of high‐aspect‐ratio nanofillers over 0D nanofillers, constructing anisotropic dielectric polymer composites with aligned nanofillers can fully utilize the high‐aspect‐ratio nanofillers to tailor the dielectric properties and electrical energy storage performance of the polymer composites. For example, it is shown that when the 1D nanofillers are aligned in parallel to the electric field direction, the enhancement of the dielectric constant is more obvious. While for increasing the electrical breakdown strength, nanofillers aligned perpendicular to the electric field direction are more effective [3]. These phenomena are closely related to the morphology of the nanofillers and the electric field distribution in the polymer composites. Considering that 2D nanofillers are usually used to increase the electrical breakdown strength of the polymer composites, it is obvious that the perpendicularly aligned 2D nanofillers are more effective to impede the charge transport and electrical breakdown propagation.
Using the high‐throughput phase‐field simulation, Shen et al. demonstrated the effect of 1D and 2D nanofiller alignment on the electrical breakdown strength of the polymer composites filled with 10 vol% nanofillers [36]. The breakdown phase growth is substantially different in the polymer composites with different nanofiller morphologies and their alignment (Figure 1.5). The polymer composite with vertically aligned nanofibers shows the lowest breakdown strength owing to the condensed electric field distribution that promotes the growth of the breakdown phase. Among the five simulated structures, only the polymer composite with parallel nanosheets exhibits the higher electrical breakdown strength than that of the pure polymer because of the dispersed electric field. These results indicate that the volume fraction of the nanofillers should not be higher than 10 vol% where high electrical breakdown strength is desired.
Figure 1.4 (a) TEM image of BNNSs, (b) cross‐section scanning electron microscopy (SEM) image of the P(VDF‐TrFE‐CFE)/BNNS composite, (c) Weibull breakdown strength and shape parameter as functions of BNNS fraction, (d) electrical resistivity of P(VDF‐TrFE‐CFE)/BNNS composite with different contents of BNNSs under 10 MV/m, (e) D‐E loops of P(VDF‐TrFE‐CFE)/BNNS composite with 12 wt% of BNNSs, and (f) discharged energy density and charge/discharge efficiency of pristine P(VDF‐TrFE‐CFE) and P(VDF‐TrFE‐CFE)/BNNS composite with 12 wt% of BNNSs.
Source: Li et al. [23]. Reproduced with permission of The Royal Society of Chemistry.
Figure 1.5 3D simulations of microstructure effects on breakdown, (a) breakdown phase morphology in the nanocomposites with different microstructures; (b) evolutions of the breakdown phase volume fraction under applied electric fields; (c) extracted breakdown strengths for corresponding nanocomposites.
Source: Shen et al. [36]. Reproduced with permission of John Wiley & Sons.
Various material processing methods have been proposed to align the nanofillers in polymer composites, including dielectrophoresis, film stretching assembly,