Dielectric materials exhibiting high power densities have garnered significant interest as crucial constituents for capacitors employed in energy storage systems and diverse electronic devices. Regrettably, the intrinsic low energy densities of dielectrics pose significant limitations to their practical application. Hence, the pursuit of enhanced energy storage properties and superior dielectric performance in dielectric materials necessitates the exploration of novel approaches to material design and processing. Here, a special design that introduces a negative-ε´ layer in a sandwich structure is explored for simultaneously realising high breakdown strength and permittivity, which are two critical factors influencing energy density. In a tri-layer composite, the outer positive-ε´ layer consisting of BN/PVDF offers high breakdown strength due to the presence of abundant BN sheets and micro-interfaces in the composite can effectively block the electric trees expansion. Simultaneously, the negative-ε´ layer of MWCNTs/PVDF provides elevated permittivity and enhanced energy density due to the synergistic effect between the positive and negative-ε´ layers, along with interfacial polarizations in the micro-interfaces connecting adjacent layers. As a result, the tri-layer composite, with a thickness ratio of 1:20:1, achieves an extraordinary permittivity of 432. Importantly, a high discharge energy density of approximately 27 J/cm3 is obtained by carefully balancing the dielectric properties of single layers. This investigation presents a novel insight on the design of high-energy-density dielectric materials through introducing a negative ε´ layer, offering significant benefits for energy storage applications.
Research Article
Boosted energy storage density of sandwich-structurecomposites: A unique strategy via introducing negative permittivity layer
https://doi.org/10.21203/rs.3.rs-3116018/v1
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Sandwich-structure composite
Dielectric properties
Energy density
Negative permittivity
Polymer dielectrics with a high dielectric constant (ε´), low dielectric loss, or high breakdown strength (BDS) are essential components in dielectric capacitors, which are prospective candidates for advanced electrical power systems, radars, and hybrid electric vehicles [1–4]. However, obtaining a polymer-based composite with high BDS and high ε´, as required for dielectric capacitors, is a major challenge.
To overcome the limitations of polymer dielectrics with intrinsically low energy density (mainly low permittivity and low breakdown strength), many efforts have been made. For example, high dielectric constant ceramics (BaTiO3, SiO2, etc.) [5–8] or conductive fillers (graphene, metal, etc.) were introduced in composites [9, 10]. A ceramic–polymer composite can enhance the dielectric properties to a certain extent owing to high permittivity of inorganic fillers and increased interfacial polarisation. For instance, Huang et al. reported PS nanocomposites with PMMA@BaTiO3 core–shell nanoparticles, simultaneously endowed with a high dielectric constant of ≈ 69 and a dielectric loss tangent of approximately 0.08 [11]. Yet, the permittivity of ceramic–polymer composites does not exceed 100, even with a high ceramic loading. At the same time, enhanced permittivity is realised at the expenses of not only destroying breakdown strength but also sacrificing the polymer matrix flexibility. Another strategy of further enhancing energy density is to fabricate a percolative composite by adding conductive filers. Shen et al. achieved an excellent discharge energy density of 27 J/cm3 as well as high permittivity and high breakdown strength by introducing Ag nanodots in a poly(vinylidene fluoride-hexafluoropropylene) substrate [12]. However, the dielectric loss of percolative composites is difficult to control within an acceptable range. Furthermore, the breakdown strength decreases, and the dielectric loss increases nonlinearly, being far from our expectations. Recently, multilayer high-ε´ composites have received extensive attention for balancing the advantages and disadvantages of the aforementioned technical routes. The additional enhanced dielectric constant and BDS as well as suppressed loss, which are key factors in multilayer composites, are mainly resulted from interface polarisations and charge accumulations at the adjacent-layer interface. Wang et al. prepared a gradient nanocomposite by increasing ceramic filler loadings layer-wise. If interfaces are present, the dielectric breakdown strength constant is mutated, and the electrical tree growth probability is inhibited. Consequently, the material is difficult to break electrically. In addition, gradient layered composites have high energy density owing to the interface polarisation of layers including a large content of ceramic fillers [13]. Shi et al. fabricated asymmetric tri-layer all-polymer dielectrics, of which a high efficiency of 89.9% along with an energy density of 12.15 J/cm3 were concurrently acquired. Furthermore, a permittivity of 6.4 and a low dielectric loss of 0.02 were achieved [14]. Jiang et al. [15] demonstrated a multilayer BaTiO3/P(VDF-HFP) composite, which showed an obviously increased permittivity (≈ 16 @1 kHz) and a substantially enhanced BDS (≈ 600 kV mm− 1). Furthermore, a high Ud of ≈ 30.15 J/cm3 and a high η of ≈ 78% were attained simultaneously.
Unfortunately, weather layer-structured composites or polymer-based composites usually have enhanced BDS or other excellent dielectric performances but relatively low permittivity, which is harmful to high energy storage density. In principle, a tri-layer composite can be equivalent to a capacitor C composing of three series connected capacitors, which can be shown as \(\text{C}=1/(1/{C}_{1}+1/{C}_{2}+1/{C}_{3})\). In general, C1, C2, and C3 are all positive values. Therefore, they all exceed C. If the value of C2 becomes negative and others remain positive, the capacitor C will be enhanced. According to C = εS/4πkd, enhanced capacitance will contribute to the permittivity improvement. Therefore, introducing a negative-ε layer into a layer-structured composite will improve permittivity. In other words, this is beneficial for the high energy density of the composite. Wang et al. prepared a Ba(Fe0.5Ta0.5)O3/PVDF-Ni/PVDF sandwich polymer composite. Its permittivity ε´ ≈ 130 @1 kHz along with low loss tangent tanδ ≈ 0.14 @ 1 kHz were compared with those of a PVDF matrix (ε´ ≈ 10 and tanδ ≈ 0.020). These improved values were attributed to the introduction of a negative permittivity layer [16]. Shi et al. experimentally discovered that a tri-layered structure composite could balance the dielectric permittivity and the dielectric loss tangent with a higher ε´ ~ 90 @10 kHz and low tanδ ~ 0.025 @10 kHz in comparison with those of pure PVDF [17]. Although the introduction of a negative-ε layer is beneficial for energy storage performance and dielectric properties of layer structure composites, they should be further improved, and their mechanisms need to be clarified.
In this study, a tri-layer composite consisting of negative-ε´ and positive-ε´ layers is explored. For a positive-ε´ layer, there are many candidates for composites with various fillers (dielectrics and ceramics) [18, 19]. However, for a negative-ε layer, positive-ε layers are difficult to match because of larger absolute values of negative permittivity. Fortunately, our previous works prepared numerous negative-ε composites, the negative dielectric constant of which could be tuned via content or morphology of conductive fillers. Based on this, BN/PVDF polymer-based composites were chosen as a positive permittivity layer. As a result, BN sheets contributed to the BDS enhancement of the tri-layer composite because of its intrinsic high BDS properties.
Meanwhile, multi-wall carbon nanotubes/polyvinylidene fluoride (MWCNTs/PVDF) composites were chosen to obtain a matched negative dielectric constant according to our former study [20, 21], leading to good dielectric properties and discharged energy density. Accordingly, tri-layer composites of BN/PVDF-MWCNTs/PVDF-BN/PVDF were fabricated utilising the method of layer-wise hot-pressing, and the permittivity, BDS, and energy storage property were investigated comprehensively.
2.1 Preparation of a single-layer composite
PVDF powders, BN sheets (purity: ＞95%, diameter: 50 μm), and MWCNTs (purity: ≥95%, length: 50 μm, external diameter: 8–15 nm, and internal diameter: 3–5 nm) were provided by Shanghai Aladdin Biochemical Technology Co. Ltd, and all materials were utilised with no further purification.
BN/PVDF and MWCNTs/PVDF mixture were separately acquired by the method of ball-milling with 300 r/min for 20 min. A single-layer BN/PVDF composite was formed after the homogeneous BN/PVDF powders were hot-pressed at a pressure of 15 MPa at 160 ℃ for 10 min and cooled down to ambient temperature. The BN mass fractions were ranged from 5 wt% to 25 wt%. Following the idea above, MWCNTs-PVDF composites were fabricated with an MWCNTs content of 2–40 wt%. All composites were circular with a diameter of 25 mm and a thickness of 0.5 ± 0.02 mm.
2.2 Preparation of a tri-layer composite
Tri-layer composites of BN/PVDF-MWCNTs/PVDF-BN/PVDF were prepared using the method of layer-wise hot-pressing. The middle MWCNTs/PVDF layer was sandwiched by a BN/PVDF composite. First, as the bottom layer, homogeneous BN/PVDF powders were added into hot-pressed die to hot-pressing under a pressure of 2 MPa for 2 min at ambient temperature. Subsequently, the MWCNTs/PVDF powders were added into the die above the BN/PVDF layer with 2 MPa for 2 min as the middle layer. Furthermore, the top layer was acquired via copying the first step, that is, adding the BN/PVDF mixture on the middle layer. Finally, a tri-layer composite was formed under hot-pressing with 20 MPa pressure for 10 min at 160 ℃ and then cooled down to room temperature. Here, composite X-Y-X means that the outer layers include X wt% BN and that the middle layer includes Y wt% MWCNTs. Further, 1:Z:1 indicates the thickness ratio of BN/PVDF-MWCNTs/PVDF-BN/PVDF composites.
2.3 Characterisation and measurements
The XRD patterns of specimens were acquired by X-ray diffraction (XRD, D/Max2550VB+/PC, Japan) with Cu-Kα radiation (λ = 1.5406 Å). A scanning electron microscope (SEM, SU-70, Hitachi) was utilised to analyse cross-sectional morphologies in samples. Furthermore, dielectric properties were detected by an LCR metre (Keysight E4980AL) in the frequency range of 20 Hz–MHz at ambient temperature. In addition, the BDS was examined by a breakdown voltage instrument (HCDJC-50KV, Beijing Huace Instrument Co. Ltd., China). The energy storage performance was acquired from P-E hysteresis loops obtained using a TF Analyser 2000 series (aixACT, Germany).
3.1 Composition and morphology of composite
Figure 1(c, d) respectively illustrate the cross-sectional morphology and N elemental distribution of composite 15-12-15 with thickness ratio of 1:1:1. In Fig. 1(c), the single BN/PVDF positive-ε´ and MWCNTs/PVDF negative-ε´ layers form a whole layer. The etter interfacial adhesion observed in the tri-layer composite contributes to an increased breakdown strength and dielectric constant, as reported in previous studies [22, 23]. Furthermore, Fig. 1(d) demonstrates the homogeneous dispersion of BN sheets in the outer layers, indicating that reduced accumulation can contribute to the enhancement of the BDS. To gain a deeper understanding of the morphological changes in the composites with different fillers, SEM images of the single-layer composites (BN/PVDF and MWCNTs/PVDF) are presented in S1(a, b). As the BN content increases from 5 wt% to 25 wt%, the BN sheets gradually stack and increase in size. Similarly, the MWCNTs establish 3D conductive networks as they come into contact, with the filler fraction increasing from 2 wt% to 40 wt%, as shown in S1(c, d).
To investigate the phase compositions of the tri-layer BN/PVDF-MWCNTs/PVDF-BN/PVDF composite after hot-pressing, the XRD patterns and FT-IR spectra of PVDF substrates with varying contents of MWCNTs or BN fillers are presented in Fig. 2. In Fig. 2(a), two distinct peaks at approximately 2θ = 26° and 43° are observed, corresponding to the crystallographic planes (002) and (100) assigned to MWCNTs [24].
As for the 2 wt% MWCNTs/PVDF composite, the characteristic peaks of MWCNTs are indistinctive due to either the lower MWCNTs content or significantly diminished peak intensity in comparison to the PVDF matrix. However, with an increase in MWCNTs loading, the peaks attributed to MWCNTs become more prominent. Concurrently, the BN/PVDF composite exhibits two groups of diffraction peaks: one originating from PVDF and the other corresponding to BN. Furthermore, the FT-IR spectra of the PVDF matrix with different fillers are presented in Fig. 2(b). The observed absorption peaks at 615, 762, 838, 879, 1072, 1185, 1280, and 1402 cm− 1 can be attributed to the characteristic α and β-phases of PVDF. Notably, the absorption peaks at 3430 cm− 1, denoted by a green dashed line, correspond to the presence of MWCNTs in the composite, while the blue dashed line at 1380 cm− 1 signifies the stretching vibration mode of h-BN. In conclusion, the PVDF-based composites were successfully fabricated, and no unexpected interactions were observed throughout the hot-pressing process.
3.2 Dielectric properties of the single-layer composites
The dielectric properties of the single-layer composite play a crucial role in determining the performance of the tri-layer composite. Therefore, we conducted a comprehensive investigation into the frequency-dependent behaviour of the complex dielectric constant and loss tangent of the single-layer composites, spanning a frequency range of 20 Hz to 1 MHz. Figure 3(a) illustrates the reduction in permittivity of the BN/PVDF composite with adding BN fillers, compared to the pure PVDF matrix. An initial factor contributing to this phenomenon is the accumulation of h-BN sheets within the composite, exceeding the critical value for filler-matrix compatibility. Furthermore, while h-BN sheets possess remarkable breakdown strength and insulating properties, their low permittivity makes the reduction of permittivity in the positive-ε´ layer composite. Fortunately, as depicted in Fig. 2(b), the dielectric loss tangent remains below 0.20 within the frequency range of 20 Hz to 1 MHz.
For the MWCNTs/PVDF composite, the enlarged image in Fig. 2(c) presents the positive permittivity of the single-layer composite with 2 wt% MWCNTs loading. Upon further increasing the MWCNTs loading to 8 wt%, a sudden change in permittivity to negative values is observed, indicating a percolation phenomenon. This intriguing behaviour is attributed to the low-frequency plasma oscillation of free electrons [25]. By increasing the MWCNTs content to 40 wt%, the absolute value of the negative permittivity experiences a substantial increase, owing to the corresponding rise in effective electron concentrations, as evidenced by the Drude model equations Eq. (1, 2). Additionally, the micro-morphology of the MWCNTs/PVDF composite undergoes a transition from isolated distribution to interconnected dispersion, as depicted in Figure S1(c, d), accompanying the MWCNTs content increment from 2 wt% to 40 wt%. This transition leads to a significant change in the dielectric constant, shifting it from positive to negative values. In principle, the plasma-like negative dielectric constant can be investigated by the Drude model[26]:
$${\epsilon }^{＇}=1-\frac{{\omega }_{p}^{2}}{{\omega }^{2}+{\omega }_{\tau }^{2}}$$
1
$${\omega }_{p}=2\pi {f}_{p}=\sqrt{\frac{{n}_{eff}{e}^{2}}{{m}_{eff}{\epsilon }_{0}}}$$
2
Here, ωp and ωτ are the plasma and collision frequencies, respectively. The meff and neff are the effective mass of delocalised electrons and the average concentration of charges, respectively.
As shown in Fig. 2(c), the experimental data of the negative permittivity values are consistent with the results fitted by the Drude model. In comparison to the BN/PVDF composite, the MWCNTs/PVDF composite exhibits a slight increase in dielectric loss, which subsequently increases significantly with higher MWCNTs contents. This elevated loss tangent stems from interfacial polarization and dipole orientation within the percolative MWCNTs/PVDF composite. Concurrently, as the MWCNTs content approaches the percolation threshold, the dominant conduction mechanism shifts from hopping conduction to metal-like conduction. In summary, we have successfully fabricated single-layer composites with tailored dielectric performance. Subsequently, we investigate their impact on the dielectric properties of tri-layer composites.
3.3 Dielectric properties of the tri-layer composites
As can be seen in Fig. 4 and 5, extensive research conducted to elucidate the correlation between thickness ratio and dielectric performance of the single-layer composite with the properties of the tri-layer composite. Initially, we investigated the influence of the positive-ε´ layer on the properties of the tri-layer composite. Fig. 4(a, b, and c) presents a series of dielectric spectra for the tri-layer composites containing varying amounts of BN. For the sake of clear comparison, the permittivity and dielectric loss tangent (@ 10 kHz) are summarized in Fig. 4(d). At the same thickness ratio, the dielectric constant of the tri-layer composite increases as the BN content is increased from 5 wt% to 15 wt%, as shown in Fig. 4(a). The obtained dielectric constant of the tri-layer composite is higher than that of the single-layer composite in Fig. 3(a). However, beyond 25 wt% of BN content, the dielectric constant starts to decrease. This decline can be attributed to the adverse influence stemming from the BN sheets, which gradually outweighs the positive effect of interfacial polarizations in the tri-layer composite. Interestingly, the tri-layer composite exhibits a dielectric loss tangent ranging from 0.01 to 0.06, equivalent to that of the PVDF substrate.
Subsequently, Fig. 5 presents the permittivity spectra of the BN/PVDF-MWCNTs/PVDF-BN/PVDF composites with varying MWCNTs contents. As depicted in Fig. 5(d), the permittivity of the tri-layer composite exhibits an increase as the permittivity of the middle layer transitions from positive to negative values. Specifically, this phenomenon occurs with an increase in MWCNTs loading content from 2 wt% to 40 wt%, attributable to the synergistic effect between the positive-ε´ and negative-ε´ layers. This observation underscores the viability of our present tri-layer composite design consisting of the negative-ε´ layer. Furthermore, the micro-interface area between the MWCNTs fillers and the PVDF matrix increases as the MWCNTs content rises, thereby intensifying the interfacial polarization effect and contributing to the enhancement in permittivity. Moreover, the current sandwich structure design furnishes numerous macro-interfaces between layers, thereby inducing robust interfacial polarization that further amplifies the permittivity of the tri-layer composite [27, 28]. Remarkably, the tri-layer composite exhibits unexpectedly low dielectric loss tangent, as illustrated in Figure 5(d). This intriguing phenomenon can be attributed to the hinderance of electron mobility within the tri-layer composite, caused by substantial resistance variations between adjacent layers. Simultaneously, the blocking effect efficiently curtails the propagation of conduction channels, leading to a significant reduction in conductive flow and leakage dielectric loss [29].
In addition to the influence of single-layer permittivity, the thickness ratio between adjacent layers also plays a crucial role in determining the dielectric properties of the tri-layer composite. Figure 4 and Figure 5 present the thickness ratios of various tri-layer composites, ranging from 1:1:1 to 1:20:1. Evidently, the permittivity of all tri-layer composites increases as the thickness ratio decreases (Figure 4(d) and Figure 5(d)). For instance, when the thickness ratio is reduced to 1:20:1, the 15-40-15 composite exhibits an impressive permittivity of approximately 432 at 10 kHz, which is approximately 2700% higher than that of the PVDF matrix (approximately 16). Despite a slight increase in the dielectric loss tangent as the thickness ratio decreases, it remains at a relatively low level of 0.001–0.09 @10 kHz. Moreover, the dielectric spectra of the tri-layer composite 15-12-15, calculated for different thickness ratios spanning from 1:100:1 to 1:11000:1, are depicted in Figure S2. The results is obtained based on the series capacitance model [17]. Amazingly, the tri-layer composite displays a permittivity boost effect along with an enhancement as large as 8900 @10kHz as the thickness ratio is decreased to 1:1000:1. This enhancement can be attributed to two key factors. Firstly, the electron accumulation and interfacial polarization at the micro-interfaces between the fillers and the matrix within each individual layer contribute to this effect. Secondly, macro-interfaces in adjacent layers are strengthened in tri-layer composites with small thickness ratios, `rs to enhance the synergistic effect between positive-ε´ and negative-ε´ layers. Subsequently, when the thickness ratio decreases to 1:5000:1 or even 1:11000:1, the calculated dielectric spectra reveal a continuous occurrence of dielectric resonances, accompanied by negative permittivity. These findings indicate that the contribution of the negative permittivity layer surpasses that of the positive permittivity layer.
3.4 Breakdown strength of the tri-layer composites
As previously stated, the permittivity of composites significantly influences their energy storage characteristics. Moreover, the attainment of a high breakdown strength (Eb) is crucial, particularly under conditions of elevated electric fields. Here, the Eb of the tri-layer composite is characterised by a two-parameter Weibull distribution, which can be expressed as follows:
In this equation, P(E) is the cumulative failure probability of electrics; α is the characteristic breakdown strength with a 63.2% cumulative failure probability of samples (Weibull breakdown strength, Eb); β is the shape parameter, illustrating a Gaussian distribution of the tested data; and E is the measured breakdown electric field. The fitting results and characteristic breakdown strength values are presented in Fig. 5. As shown in Fig. 5(a, b), an obvious increase in BDS is observed with decreasing the middle-layer thickness of the tri-layer composites from 1:1:1 to 1: 0.5:1. This can be attributed to the negative permittivity (ε) layer becoming a vulnerable region along the path of dielectric breakdown following the introduction of conductive MWCNTs fillers into tri-layer composite. Consequently, the breakdown resistance worsens as the negative permittivity layer becomes thicker and the loading content of MWCNTs increases. Interestingly, when the thickness ratio reaches 1:0.5:1, the BDS ceases to improve and even exhibits a slight decline as the MWCNTs loading content increases in composites 15-2-15, 15-12-15, and 15-40-15. However, when compared to the tri-layer composite 15-2-15 consisting solely of positive layers, a significantly enhanced BDS value of 22.3 kV/mm is achieved in the composite 15-12-15 featuring a negative permittivity layer in the middle. This value exceeds those of all tri-layer composites considered in this work, indicating feasibility of introducing a negative-ε´ layer. Moreover, for the same MWCNTs content (12 wt%) and thickness ratio (1:0.5:1) in the middle layer, the dielectric breakdown strength (BDS) improves as the loading of BN in the outer layers increases from 5 wt% to 15%. This improvement can be attributed to the exceptional BDS of the BN sheets. However, a slight decline in BDS is observed upon further increasing the BN content to 25 wt%. In contrast, due to the inherent incompatibility between ceramic fillers and the polymer matrix, a substantial accumulation of BN fillers is observed predominantly in the outer layers of the tri-layer composite. This accumulation adversely affects the dielectric properties of the composite by introducing defects, consequently leading to an increased risk of breakdown. Beyond a critical BN content threshold (15 wt% in this case), the positive influence of BN sheets on the breakdown strength (BDS) becomes overshadowed by the detrimental impact of the escalating defect density, thereby further compromising the BDS [30].
In the sandwich-structured composite, the applied electric field will be redistributed due to the variation in dielectric constants between the filler and matrix in a single or adjacent layers. The electric field intensity within the negative-ε´ layer is relieved, enabling the positive-ε´ layers to withstand higher electric fields. Consequently, the development of some branches of electrical trees around the interfaces is impeded. This pronounced interfacial barrier effect stems from the discontinuity and redistribution of electric field intensities between adjacent layers, thereby enhancing the breakdown strength (BDS) of the tri-layer composite. Furthermore, the introduction of an insulating BN/PVDF composite as the top and bottom layers effectively suppresses the initiation of electrical trees at the onset of electrical breakdown. As a result, the sandwich-structured composites achieve a favorable and moderate breakdown strength in this study.
3.5 Energy density of the tri-layer composites
Figure 6 illustrates the obtained discharged energy density (Ud), stored energy density (Us), and efficiency (η) of both the PVDF matrix and tri-layer composites. These values were extracted from the P-E loops presented in Figures S3 and S4 in the Supporting Information. For a comprehensive and intuitive comparison of the PVDF matrix and tri-layer composites with different thickness ratios, the discrepancies in Us and Ud are depicted in Figure 7. Notably, a remarkable enhancement in Ud is observed for all tri-layer composites when the thickness of the middle negative-ε´ layer is reduced from 1:1:1 to 1:0.5:1, as shown in Figure 7(a, b).
This observation mirrors the regulation of breakdown strength and stems from the exceptional breakdown strength (BDS) exhibited by the samples. Notably, among the composites denoted as 15-2-15, 15-12-15, and 15-40-15 in Figure 7(a), the energy density (Ud) of the tri-layer composite, with a thickness ratio of 1:0.5:1, experiences a remarkable increase to 23.4 J/cm3 upon the incorporation of 12 wt% multi-walled carbon nanotubes (MWCNTs) in the intermediate negative permittivity (negative-ε´) layer. However, this trend begins to reverse as the MWCNTs content reaches 40 wt%, resulting in a decrease to 19.3 J/cm3, which still exceeds that of pure PVDF. The efficiency (η) of the 15-12-15 composite is comparable to that of the PVDF matrix. Hence, the synergistic effect of positive and negative permittivity layers, together with the sandwich structure, assumes a pivotal role in attaining a high energy density upon discharge. As demonstrated in Figure 7(b), the discharged energy density (Ud) exhibits an initial increase followed by a subsequent decrease as the BN content is elevated, considering a thickness ratio of 1:0.5:1. This observation is primarily attributed to the significantly enhanced breakdown strength conferred by the incorporation of BN sheets and the utilization of a sandwich structure in composite 15-12-15. Furthermore, as depicted in Figure S5 (Supporting Information), the maximum saturated polarization (Pmax) of composite 15-12-15 (1:0.5:1) displays comparable outcomes to that of the other samples, implying that the improved discharged energy density primarily originates from the notable enhancement in the breakdown strength of composite 15-12-15.
The radar plots in Figure 8 illustrate the characteristics of tri-layer composites with thickness ratios of 1:1:1 and 1:0.5:1. These plots facilitate a comprehensive assessment of the composites' energy storage properties by comparing ε´, tan δ, Eb, Ud, and η. When compared to composite 15-2-15, which exhibits low Ud and small ε´, the sandwich-structured composites incorporating a negative ε´ layer demonstrate increased ε´ and Ud as the MWCNTs content is raised to 12 wt% or even 40 wt% (transitioning from positive ε´ to negative ε´). This improvement can be attributed to the synergistic effects of positive and negative ε´ layers. Notably, the significantly expanded region marked by the shadow coordinates in the radar plots indicates the outstanding energy storage capabilities of these composites. Obviously, the composite 15-12-15 (1:0.5:1) exhibits significantly larger surface areas compared to other composites, showing exceptional comprehensive dielectric performance and energy storage properties. In contrast, composites 5-12-5 and 25-12-25 demonstrate reduced breakdown strength or discharged energy density as the BN content varies from 15 wt% to 5 wt% or 25 wt%, respectively. Thus, composite 15-12-15 displays the most optimal combination of positive permittivity and negative permittivity layers, resulting in outstanding energy storage capabilities and presenting a unique design approach for achieving high-performance dielectrics.
In this study, a novel approach is investigated for the design of a tri-layer composite, incorporating a negative-ε´ layer sandwiched by positive-ε´ layers. The results obtained demonstrate the remarkable synergistic effects between the positive and negative-ε´ layers, as well as the interfacial polarizations occurring at the micro-interfaces within the layers and the macro-interface between the layers. These effects contribute to the attainment of high permittivity and high energy density. Additionally, the outer positive-ε´ layer with high breakdown strength (BDS) and the presence of numerous interfaces effectively block the expansion of electric trees within the composites, thereby significantly enhancing the breakdown strength. Consequently, a tri-layer composite with a specific thickness ratio achieves a high permittivity of approximately 432, accompanied by a moderate dielectric loss of 0.06. Notably, composite 15-12-15 exhibits a favourable energy density of 22.3 J/cm3, along with a high breakdown strength of 23.4 J kJ/mm. It is believed that the design of a sandwich structure with a middle negative-ε´ layer will play a vital role in the development of energy storage composites.
Conflict of interests: The authors declare no competing interests.
Funding: The authors have no relevant financial or non-financial interests to disclose.
Conflict of Interest statement：I write on behalf of myself and all co-authors to confirm that the results reported in the manuscript are original and neither the entire work, nor any of its parts have been previously published. The authors confirm that the article has not been submitted to peer review, nor has been accepted for publishing in another journal. The author(s) confirms that the research in their work is original, and that all the data given in the article are real and authentic. If necessary, the article can be recalled, and errors corrected.
Author's contribution
Xiaoting Song: Conceptualization, Methodology, Investigation, Writing-original draft.
Zidong Zhang, Guohua Fan: Data curation.
Zaixin Wei: Visualization.
Yao Liu: Supervision, Funding acquisition.
Runhua Fan: Funding acquisition, Writing-review & editing.
All authors reviewed the manuscript.
Acknowledge
This study was funded by the National Natural Science Foundation of China [grant No. 52171141], the Shandong Natural Science Foundation for Excellent Young Scholars [grant No. ZR2020YQ32], National Youth Fund [grant No. 52207239].
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No competing interests reported.