A paper recently published in the journal Applied Surface Science demonstrated a novel strategy to improve energy density (Ue) in dielectric materials.
Study: Significantly enhanced energy storage properties in sandwich-structured polymer composites with self-assembled boron nitride layers. Image Credit: ogwen/Shutterstock.com
Background
The demand for dielectrics with low loss and high Ue has increased significantly in the high electric field energy storage applications with the rapid development of modern electrical systems.
Among the different dielectric materials, polymeric materials have attracted considerable attention in commercial applications due to their extraordinary properties, such as good flexibility and easy processing. However, the Ue of polymer film capacitors with insufficient permittivity (εr) is extremely low as it depends on the special self-healing performance and high Eb of the capacitors.
The low Ue of metalized polymer film capacitors restricts their use in several applications, which necessitated the development of dielectrics with a high Ue. For instance, biaxially-oriented polypropylene (BOPP), a common commercial polymer dielectrics capacitor, displays a low Ue of 2-3 J/cm3 at a high electric field.
Although the Ue of a capacitor primarily depends on both Eb and εr, the Eb is a more significant factor among them due to its quadratic relationship with the Ue. However, high Eb materials such as polymers typically have a low εr, while high εr materials usually have a low Eb. This factor increases the challenges of improving the Ue as maintaining a balance between the Eb and εr remains extremely difficult.
High εr conductive fillers or ceramics can be introduced into the polymeric matrix to enhance the Ue. Poly (vinylidene fluoride) (PVDF), a commonly used polymeric matrix, possesses a high εr due to the presence of a highly polar carbon-fluorine bond with a robust dipole moment.
Additionally, several ceramics, such as lead zirconate titanate (PZT) and barium titanate (BT), used in the synthesis of composite dielectrics also possess a substantially high εr compared to polymers. Composite dielectrics can be fabricated using these ceramics and PVDF-like polymers in 2-2, 1-3, and 0-3 modes based on the dispersion of ceramic particles/fillers in the polymeric matrix.
However, the large difference in conductivity and εr in these composites can cause an uneven electric field, leading to the low Eb and high conduction loss. Although surface modification of ceramic particles/fillers can enhance the Eb to a certain extent, the overall energy storage performance of conventional composite dielectrics fabricated in 0-3 mode remains unsatisfactory for commercial applications.
Several inorganic materials with a high bandgap, such as hexagonal boron nitride (h-BN) and aluminum oxide, can be introduced in composite dielectrics to overcome the problem of low Eb. These high bandgap materials can be blended with the polymer matrix either in a layer-by-layer structure or a 0-3 mode to extend the conductive route of charges in the composite to increase the Eb.
Among the high bandgap inorganic materials, the exfoliated h-BN with a two-dimensional (2D) graphene-like layer structure has demonstrated a great barrier effect owing to its excellent Eb, low εr, and large forbidden bandgap. Thus, the addition of h-BN in form of BN nanosheets (BNNs) into composite dielectrics such as PVDF/BT with optimized composition can effectively enhance the Eb.
However, the generation of different interfaces among the three materials can complicate the electric field distribution and cause undesired effects, leading to unpredictability in the energy storage performance. This issue can be resolved by adding the BN with a layer-by-layer structure in the composite dielectrics.
The Study
In this study, researchers fabricated boron nitride-poly (vinylidene fluoride-co-hexafluoropropylene)/barium titanate-boron nitride (BN-P(VDF-HFP)/BT-BN) composite and evaluated their effectiveness for dielectric energy storage applications. The P(VDF-HFP)/BT composite film was coated by a BN layer to obtain a sandwich-structured dielectric composite.
The BN thin film was synthesized using the oil-water interfacial self-assembly process. Initially, BN powder was mixed with n-hexane under ultrasonication for 10 min, and the obtained suspension was again subjected to tip-sonication for 6 h to collect BNNs. Subsequently, the obtained BNNs were gradually trapped at the water/hexane interface, which resulted in the formation of a uniform and continuous BN film. Eventually, the obtained BN was transferred to a glass slide and dehydrated at 70 oC.
P(VDF-HFP) and BT nanoparticles were dissolved in N, N-dimethylformamide (DMF) under ultrasonication and stirred vigorously to obtain a stable suspension. Subsequently, the suspension was cast on BN film-coated glasses to fabricate bilayer P(VDF-HFP)/BT-BN composites. The bilayer composites were then heated along with the glass slides at 70oC to volatilize the solvent completely before peeling off the films from the slides.
The sandwich-structured dielectric composites were obtained using the hot-pressing method, where BN layers were placed as the bottom and top surfaces of the P(VDF-HFP)/BT. Single-layer P(VDF-HFP)/BT composites were also fabricated and used as a reference in the study. The BT mass content was set at 5 wt%, 3wt%, and 1 wt% in BN-P(VDF-HFP)/BT-BN, P(VDF-HFP)/BT-BN, and P(VDF-HFP)/BT composites, respectively.
Transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) method were used to characterize the samples. A Novocontrol Concept 80, an auto voltage withstanding tester, and a Premier II ferroelectric test system were used to characterize dielectric properties and measure Eb and the current density, respectively.
Observations
BN-P(VDF-HFP)/BT-BN composites with a novel sandwich-structured design were fabricated successfully, with the middle layer and outer layer of the composites containing BT and self-assembled BN films, respectively. The fabricated composite demonstrated an exceptional Ue due to simultaneous improvements in Eb and εr.
The outer BN layers with high insulation resisted charge injection, while the middle BT layer with high εr increased the electrical polarization of the composites. The BN layer played a crucial role in significantly improving the Ue and Eb of the BN-P(VDF-HFP)/BT-BN composites by reducing the leakage current density and allowing redistribution of the electric field.
The optimized BN-P(VDF-HFP)/BT-BN composite with 3 wt% of BT demonstrated a high Ue of 19.4 J/cm3 at 550 MV/m, which was almost 1.9 times and 2.3 times higher than single-layer P(VDF-HFP) film and pristine P(VDF-HFP). Finite element results indicated that the sandwich-structure-like design of the composites played a significant role in improving the Ue and Eb.
Taken together, the findings of this study demonstrated the potential of BN-P(VDF-HFP)/BT-BN composites as a high-performance electrostatic capacitor. Additionally, the study also showed an efficient and novel strategy to prepare and design advanced multilayered dielectrics with exceptional energy storage properties.
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Source
Wei, X., Xie, Y., Li, J. et al. Significantly enhanced energy storage properties in sandwich-structured polymer composites with self-assembled boron nitride layers. Applied Surface Science 2022. https://www.sciencedirect.com/science/article/pii/S0169433222012235?via%3Dihub
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