LITMIR.BIZ

Figure 1.1 Schematic of typical D‐E loop of dielectric materials.

      For capacitive energy storage in dielectric materials, the stored energy density cannot be fully discharged at the removal of the applied electric field because of various energy losses, such as polarization loss, conduction loss, and hysteresis loss [22]. In most cases, the energy loss would be converted into waste heat, which accelerates the dielectric aging and even causes thermal runaway of the dielectric materials [23, 24]. Therefore, the charge/discharge efficiency is another key parameter for dielectric materials. Since the energy loss is converted into waste heat, which is harmful to the operation of dielectric materials, it is more meaningful to concurrently improve the energy density and the charge/discharge efficiency of the dielectric materials, rather than simply increasing the energy density. In order to increase the charge/discharge efficiency, the energy loss, especially conduction loss under high electric field, should be suppressed. To achieve the inhibition of conduction loss, some highly insulated nanofillers have been introduced into the polymer dielectrics.

      Simultaneous enhancement of dielectric constant and breakdown strength is highly desired in the development of high performance polymer composites for electrical energy storage. However, in most cases, the increase in dielectric constant would result in the decrease of breakdown strength, which restricts the improvement of the energy density. So achieving high dielectric constant while maintaining high breakdown strength is still a great challenge. Because of the large mismatch of the electrical parameters between the inorganic nanofillers and polymer matrix, the electric field at the nanofiller/polymer interface is greatly enhanced, which results in the reduced breakdown strength. Moreover, the dispersion of inorganic nanofillers into polymer matrix is always a challenge in the development of polymer composites because of the high surface energy of nanofillers and the different surface physical and chemical properties between the nanofillers and polymer matrix. As a result, nanofiller aggregation is observed in polymer composites. The aggregation of nanofillers would introduce numerous defects in the polymer matrix, which would decrease the electrical breakdown strength and increase the conduction loss. To address these issues, various strategies have been proposed to develop high performance polymer composites for electrical energy storage, including controlling the nanofiller dimension and morphology, controlling the nanofiller distribution and orientation, modifying the nanofiller/polymer interface, introducing multiple nanofillers, and constructing multilayered composites.

1.3 Effect of Nanofiller Dimension

To increase the capacitive energy storage performance of the polymer composites, various inorganic nanofillers have been used to utilize the interfacial polarization and the high dielectric constant of the nanofillers, as well as the inhibition of conduction loss and electrical breakdown. Nanofillers with high dielectric constant, such as titanium oxide (TiO₂), barium titanate (BaTiO₃, BT), strontium titanate (SrTiO₃), barium strontium titanate (Ba_xSr_{1 − x}TiO₃, BST), lead zirconate titanate (PbZr_xTi_{1 − x}O₃, PZT), and calcium copper titanate (CaCu₃Ti₄O₁₂, CCTO), have been used to improve the dielectric constant and energy density of the polymer composites [25–30]. Moreover, nanofillers with moderate dielectric constant but highly insulating performance, such as magnesium oxide (MgO), aluminum oxide (Al₂O₃), and silicon oxide (SiO₂), have been used to enhance the breakdown strength and suppress the conduction loss [31–33].

      The dimension of the nanofillers can significantly influence the performance of the polymer composites. There are mainly three categories of the nanofillers: 0D fillers, 1D fillers, and 2D fillers. 0D fillers have nanoscale dimensions in all three directions, such as spherical nanoparticles. 1D fillers have nanoscale dimensions in two directions, suchas nanowires and nanofibers. 2D fillers have nanoscale dimension in only one direction, such as nanoplates and nanosheets.

      Compared with 0D nanofillers, 1D nanofillers are more efficient to increase the dielectric constant of polymer composites at relatively low nanoparticle content because of the lower percolation threshold of the high‐aspect‐ratio 1D nanofillers compared with 0D nanofillers [34]. For example, Tang et al. showed that the use of high‐aspect‐ratio BT nanowires can enhance the dielectric constant of poly(vinylidene fluoride‐trifluoroethylene‐chlorofluoroethylene) [P(VDF‐TrFE‐CFE)] terpolymer more efficiently than the BT nanoparticles [35]. The dielectric constant of P(VDF‐TrFE‐CFE)/BT nanowire composite can reach up to 69.5 at 17.5 vol% of BT nanowires, while the dielectric constant of P(VDF‐TrFE‐CFE)/BT nanoparticle composite is only around 52 at 30 vol% of BT nanoparticles. As the result of enhanced dielectric constant, the P(VDF‐TrFE‐CFE)/BT nanowire composites exhibit a high discharged energy density of 10.48 J/cm³, which is 45.3% higher compared with that of the 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.