P(VDF‐TrFE‐CFE) terpolymer of 7.21 J/cm3.
Shen et al. compared the electrical breakdown propagation in poly(vinylidene fluoride) (PVDF)/BT nanoparticle and PVDF/BT nanofiber composites based on the phase‐field simulation [36]. As shown in Figure 1.2, the breakdown phase grows from the nucleation within the composites when the electric field increases beyond the threshold (i.e. 165 kV/mm). In the PVDF/BT nanoparticle composite, the breakdown phase tends to grow at the vulnerable filler/polymer interface and then passes through the fillers near the breakdown path. While in the PVDF/BT nanofiber composite, the breakdown phase propagation is rather different. It is found that the breakdown phase in the nanofiber‐based composite tends to penetrate through the nanofibers until the electric field reaches a higher threshold. As a result, the nanofiber‐based composite exhibits higher electrical breakdown strength compared with the nanoparticle‐based composite. The quantitative breakdown phase growth behavior indicates that the breakdown phase starts to increase when the electric field reaches 140 kV/mm. Compared with nanofiber‐based composite, nanoparticle‐based composite shows a higher increase rate of the breakdown phase. The breakdown phase in the nanoparticle‐based composite gets saturated when the electric field approaches 200 kV/mm, indicating that the nanoparticle‐based composite is totally breakdown. In stark contrast, the nanofiber‐based composite gets totally breakdown when the electric field reaches 225 kV/mm. The difference in the electrical breakdown behavior comes from the different electric field distribution. The electric field in the nanoparticle‐based composite concentrates at the two shoulders along the electric field direction. In the nanofiber‐based composite, the electric field concentrates at the vertices of the nanofibers. So the breakdown phase is easier to get through the nanoparticles. Therefore, the nanoparticle‐based composite exhibits lower breakdown strength.
Figure 1.2 The breakdown phase propagation simulation based on phase field model in composites filled with 10 vol% of (a, b) BT nanoparticles and (c, d) BT nanofibers, (e) the evolution of breakdown phase under applied electric field. Insets in (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
