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Largely enhanced energy storage performance of sandwich-structured polymer nanocomposites with synergistic inorganic nanowires
Author’s Accepted Manuscript Largely Enhanced Energy Storage Performance of Sandwich-Structured Polymer Nanocomposites with Synergistic Inorganic Nanowires Zeyu Li, Feihua Liu, He Li, Lulu Ren, Lijie Dong, Chuanxi Xiong, Qing Wang www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(19)30129-4 https://doi.org/10.1016/j.ceramint.2019.01.124 CERI20579
To appear in: Ceramics International Received date: 17 October 2018 Revised date: 4 January 2019 Accepted date: 16 January 2019 Cite this article as: Zeyu Li, Feihua Liu, He Li, Lulu Ren, Lijie Dong, Chuanxi Xiong and Qing Wang, Largely Enhanced Energy Storage Performance of Sandwich-Structured Polymer Nanocomposites with Synergistic Inorganic N a n o w i r e s , Ceramics International, https://doi.org/10.1016/j.ceramint.2019.01.124 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Largely Enhanced Energy Storage Performance of Sandwich-Structured Polymer Nanocomposites with Synergistic Inorganic Nanowires
Zeyu Li,a Feihua Liu,a,b He Li,b Lulu Ren,b Lijie Dong,a, Chuanxi Xiong,a, and Qing Wang,b,*
a
Center for Smart Materials and Device Integration, School of Materials Science and
Engineering, Wuhan University of Technology, Wuhan 430070, China b
Department of Materials Science and Engineering, The Pennsylvania State
University, University Park, PA 16802, USA *Corresponding author: Qing Wang (Email: [email protected]), Lijie Dong (Email: [email protected]), Chuanxi Xiong (Email: [email protected])
Abstract It is of crucial importance to develop novel dielectric materials with high discharge energy densities to meet the urgent requirements in the rapid advances in modern electronics and electrical power systems. In this work, we describe a new class of sandwich-structured ceramic-polymer nanocomposites base on poly(vinylidene 1
fluoride-co-hexafluoropropylene) exhibiting outstanding energy storage performance. The central layer of the composite consists of MgO nanowires to offer high breakdown strength, whereas the outer layers contain BaTiO3 nanofibers to provide improved dielectric constant of the composites. The systematic investigation gave the composites with 2 wt% MgO in the central layer and 20 wt% BaTiO3 in the outer layers to yield the best energy density of 15.55 J cm-3 with a charge-discharge efficiency of 68% at the Weibull breakdown strength of 416 MV m-1. It is found that the trilayered nanocomposite exhibits superior energy storage performance as compared to the corresponding single-layered composite films. The result demonstrates the uniqueness of the layered structure to incorporate multiple inorganic components into polymer with enhanced collective properties for energy applications.
Key words: MgO nanowires, BaTiO3 nanofibers, ceramic-polymer nanocomposites, energy storage.
1. Introduction Dielectric materials with high energy densities are used to store and convert electrical energy in film capacitors, which are key components in modern electronics and electric power systems [1-7]. For instance, film capacitors with fast charge-discharge abilities and ultra-high power densities are utilized in hybrid and electric vehicles to 2
convert direct currents from batteries to alternating currents to drive the engine of vehicles [8]. The key drawback of film capacitors is their energy densities that are more than one order of magnitude lower than those of electrochemical devices such as batteries and supercapacitors. For example, biaxially oriented polypropylene (BOPP), the state-of-the-art commercial dielectric polymer, possesses a limited energy density of about 2 J cm-3. Therefore, it is essential to enhance the energy densities of dielectric materials, which not only improves capacitor performance but also enables the minimization of electronics and power systems [3]. The energy density (U) of dielectric materials is determined by the applied electric field (E) and electric displacement (D), which is given by 𝑈 = ∫ 𝐸d 𝐷
(1)
𝑈=1/2DE = 1/2𝐾𝜀0 𝐸 2
(2)
For linear dielectrics
where K is the dielectric constant and ε0 is the vacuum permittivity. Ferroelectric polymers are preferred candidates for polymer film capacitors due to their highest K among the known polymers. Yet, the K values of ferroelectric polymers are still much lower than those of ferroelectric perovskites, such as BaTiO3, BaxSr1-xTiO3 and PZT [9-11], which could have permittivities of hundreds or even thousands. It has been demonstrated to enhance the energy storage performance of dielectric polymers by introducing electroceramics into polymer matrix to form the composites with 3
improved permittivity [10, 12-14]. However, one paradox is that the enhancement of permittivity is often achieved at the expense of breakdown strength (Eb). This is owing to the inhomogeneous field distribution because of large mismatch of the permittivity between ceramic fillers and polymer matrix. Since U depends on the square of Eb, improving the Eb of dielectric composites is a more effective route to realize greatly improved energy densities. For this purpose, a number of innovative approaches have been developed, including utilization of two-dimensional nanofillers, surface modification of nanofillers, cross-link structures and multilayered structures [15-22]. Among
the
aforementioned
methods,
the
sandwich-structured
polymer
nanocomposites have aroused intensive attentions because the layered structures provide unique features of simultaneous enhancement of K and Eb that is normally unattainable in the conventional single-layer films [23-25]. Hu et al. reported the trilayer-structured polymer nanocomposites with a small content of BaTiO3 nanofibers (BT nfs) in the central layer to deliver higher Eb and impede electric breakdown, while the outer layers of the composite contain a high content of BT nfs to offer enhanced D of nanocomposites. As a result, a dramatic enhancement of energy density of about 9.7 J cm-3 was achieved. In this work, we present a class of sandwich-structured polymer nanocomposite with different ceramic fillers that are spatially organized in the layered structures. Specifically, MgO nanowires (nws) were introduced into the central layer of the 4
composites to enhance the Eb, whereas the outer layers contain BT nfs to improve the D. Note that, to our knowledge, this is the first example to employ MgO nws as the nanofillers to enhance the Eb of the layered composites. MgO with a wide band gap of ~7.8 eV would efficiently reduce electrical conduction, which is the main loss mechanism of dielectric material at high fields, and thus improve the Eb. Moreover, the K of MgO, i.e. ~9-10, is close to that of the ferroelectric polymer matrix, which would yield homogeneous field distribution and further contribute to the enhanced overall Eb of the composites. In addition, in comparison to other highly insulating nanofillers such as boron nitride nanosheets (BNNSs) and montmorillonite nanoplates, MgO nws can be facilely prepared. The trilayered nanocomposites show a remarkable energy density of 15.55 J cm-3 at an applied field of 416 MV m-1 along with a charge-discharge efficiency of 68%, which represents an enhancement of ~700% over that of BOPP. When compared to the conventional single-layer composite films, the sandwich-structured films exhibit superior energy storage performance, including breakdown strength, discharge energy density and charge-discharge efficiency.
2. Experimental 2.1 Preparation of BT nfs The BT nfs were synthesized by electrospinning method [26, 27]. In brief, barium acetate and tetrabutyl titanium at a molar ratio of 1:1 were dissolved in acetic acid and 5
stirred to yield homogeneous barium titanate precursor gel. The gel was then mixed with poly(vinyl pyrrolidone) (PVP, Mn = 1,300,000) ethanol solution to form electrospinning solution. The electrospinning was conducted at an electric field of 1.0 kV m-1 with a feeding rate of 1.0 mL h-1. The as-spun nanofibers were then calcined at 650 oC for 1.5 h in air at a heating rate of 5 oC min-1 to remove PVP completely and obtain BT nfs. Finally, The BT nfs were broke into short nanofibers by ultrasonication for 40 min. 2.2 Preparation of MgO nws MgO nws were synthesized via hydrothermal technique [28], followed by a calcination process. In a typical experiment, 6.44 g magnesium acetate and 1.2 g urea were dissolved in 100 mL distilled water and stirred to generate homogeneous solution. The solution was then transferred into a 100 mL Teflon-lined autoclave. The autoclave was maintained at 140 oC for 30 min. Afterward, the as-prepared power were washed by distill water three times and dried at 60 oC in an oven for 24 h. Finally, the obtained white power was calcined at 450 oC for 30 min in air at a heating rate of 5 oC min-1 to yield MgO nws. 2.3 Preparation of the sandwich-structured polymer nanocomposites films The sandwich-structured polymer nanocomposites films were prepared via solution-casting method, followed by a hot-press process. First, BT nfs, MgO nws and poly(vinylidene fluoride-co-hexafluoropropylene) P(VDF-HFP) were vacuum-dried 6
at 80
o
C for 12 h. Afterwards, 0.5 g P(VDF-HFP) was dissolved in 6 g
dimethylformamide (DMF) and stirred overnight at 70 oC to generate a homogeneous solution. BaTiO3 or MgO nanofillers were dispersed in the above solution by stirring and ultrasonication. The as-prepared suspension was then cast onto a clean glass and dried at 60 oC in an oven for 18 h. After that, the nanocomposite films were peeled off from the glass, stacked together and hot pressed at 170 oC at 10 MPa for 10 min, followed by an immediately quenched process in iced water. The thickness of each single layer was controlled to be 6-8 μm, and the final thickness of thus-prepared films
is
15-20
μm.
The
single-layered
P(VDF-HFP),
BT/P(VDF-HFP),
MgO/P(VDF-HFP) and BT/MgO/P(VDF-HFP) films were prepared by the same method. 2.4 Characterization Crystal structure of the nanofillers were characterized by power X-Ray diffraction (XRD) on a Bruker D8 Advanced diffractomer using Cu Kα (λ = 1.5418 Å). The morphology of nanofillers and dielectric films were investigated by scanning electron microscopy (SEM Hitachi S4800). The dielectric properties were measured using E4980A precision LCR Meter. The electric polarization-electric hysteresis field (P-E) loops and breakdown strength were measured at 100 Hz with a precision Multiferroelectric Materials Analyzer equipped with Precision 10 kV HVI-SC and Trek MODEL 609B (Radiant Inc.).
7
3. Results and Discussion We design the sandwich structure consisting of a higher insulating central layer and outer layers with enhanced electrical polarization. Ferroelectric P(VDF-HFP) was chosen as the polymer matrix. The introduction of bulky HFP units into PVDF facilitate dipole switching by breaking apart the large crystals and decoupling the ferroelectric domains, which leads to slimmer electric hysteresis loops and a high charge-discharge efficiency [8]. The central layer is composed of MgO nws dispersed in P(VDF-HFP). Recently, it has been shown that the addition of a small content of MgO nanoparticles can effectively increase the breakdown strength of polymer matrix [29]. MgO, a wide band gap insulator [30], can act as traps for the injected charges and reduce the electrical conduction in the polymer composites. Moreover, the comparable K values of MgO filler and P(VDF-HFP) matrix would avoid the local field distortion and lead to homogeneous distribution of the internal fields, thus benefiting the overall breakdown strength of the nanocomposites. BT nfs were dispersed in P(VDF-HFP) as the outer layers. It has been demonstrated previously that the nanocomposites with BT nfs possess higher D than BT nanoparticles because of the high aspect ratio of one-dimensional nanofillers [31, 32]. More interestingly, the most recent experimental and theoretical studies have demonstrated that the nanocomposites containing one-dimensional nanofillers exhibit much higher breakdown strength in comparison to the corresponding nanoparticle-filled nanocomposites, which has been attributed to the fact that the one-dimensional 8
nanofillers can afford more significant restrain to the growth of electric trees [33-35]. The morphologies of as-prepared BT nfs and MgO nws are displayed in Figs. 1a and b, respectively. BT nfs and MgO nws exhibit a similar structure with a diameter of 200-250 nm and an aspect ratio of 15-20. As shown in the XRD patterns (Fig. 1c), the Bragg reflection peaks of BT nfs and MgO nws have been successfully assigned to the tetragonal phase of BaTiO3 and the cubic phase of MgO, respectively. This result indicates that the nanofillers are highly crystalline and no byproducts are generated in the synthesis process. Fig. 1d displays the cross-section SEM micrograph of sandwiched nanocomposites film. It can be seen that the nanofillers are well dispersed in the polymer matrix and no obvious agglomeration or structure defect appears. Interestingly, these two nanoinclusion are nearly oriented in the direction parallel to the film surface and perpendicular to the electric field direction; this could contribute to restrain the development of electric trees and give rise to high breakdown strength. Moreover, the cross-section SEM micrograph clearly demonstrate the sandwich structure in the as-prepared multilayered nanocomposites film. To determine the optimal MgO nws content in the central layer of the sandwiched nanocomposites, we first carried out systematic studies of the breakdown strength and P-E loops of the single-layered MgO/P(VDF-HFP) films. The breakdown strength of the films was analyzed by a two-parameter Weibull statistic distribution according to:
9
𝐸
𝑃(𝐸) = 1 − 𝑒𝑥𝑝(−( )𝛽 ) 𝐸0
(3)
where E is the measured breakdown strength, P(E) is the cumulative probability as a function of E. E0 is the breakdown strength when the cumulative failure probability is 63.2% and used to represent the Weibull breakdown strength (Weibull Eb). β is a shape parameter to evaluate the scatter of the measured breakdown strength [23]. In this study, at least 8 samples were tested for Weibull fitting. The Weibull distribution of the single-layer MgO/P(VDF-HFP) nanocomposites was plotted in Fig. S1a, and the Eb calculated from Weibull distribution was shown in Fig. 2a. It can be seen that the Eb of the nanocomposites exhibits a dramatic increase with the loading of MgO nws and reaches to 508 MV m-1 at 3 wt% MgO nws. Comparatively, pure P(VDF-HFP) films have a moderate Eb of 411 MV m-1. The electric resistivity of the nanocomposite films displays an obvious increase with the content of MgO nws as shown in Fig. S1b, which is consistent to the trend shown in Eb. This demonstrates that the enhancement of Eb results from the synergistic effect of the highly insulating property and a large aspect ratio of MgO nws. Nevertheless, the P-E loops (as shown in Fig. S1c) indicate that the nanocomposites with the increased MgO nws content show limited maximum polarization and almost the same remanent polarization, hence offering a reduced charge-discharge efficiency from 60% to 55% as summarized in Fig. 2b. The decrease of the charge-discharge efficiency indicates high dielectric loss, which generates waste heating and thus impairs capacitor lifetime [36]. Based on these results, the MgO/P(VDF-HFP) containing 2 wt% MgO with the Eb of 10
485 MV m-1 was chosen as the central layer in the sandwiched structure. A series of the BT/MgO/P(VDF-HFP) sandwiched nanocomposites with varied BT nfs contents from 5 wt% to 25 wt% were prepared. The frequency dependence of dielectric properties of the trilayered films was shown in Fig. 3. Obviously, K consistently increases with increasing BT nfs content over the tested frequency range. For instance, K of the sample 25B/2M/25B (standing for the trilayerd nanocomposite with 25 wt% BT nfs in outer layers and 2 wt% MgO nws in the central layer) reaches up to 11.21 at 1 kHz, and K of the sample 5M/2B/5M is 7.56 at 1 kHz. K of the nanocomposites decreases with the increase of frequency, which is due to the molecular dipoles in P(VDF-HFP) that are unable to follow the frequency at high frequencies. The dielectric loss of all the samples still remains a low value, i.e. around 0.05 at 1 kHz, which is beneficial to small leakage current and low energy loss [23]. An obvious rising in dielectric loss can be observed from 104 to 106 Hz in all the trilayered nanocomposite films, which results from the dielectric relaxation in P(VDF-HFP) matrix. With the increase of BT nfs content, the trilayered nanocomposites show a slight increase in the dielectric loss due to the ferroelectric nature of BT nfs. The Weibull plots of the sandwiched films and the single-layer BT/P(VDF-HFP) films are given in Fig. S2. The corresponding Eb of the as-prepared nanocomposite films are shown in Fig. 4. The most striking feature is that, with the increase of BT nfs content, the sandwiched films show a largely enhanced Eb compared to the 11
BT/P(VDF-HFP) single films. For example, the 20B/2M/20B sample possesses the Eb of 416 MV m-1, which is 29% higher than the BT/P(VDF-HFP) single-layer film (i.e. 322 MV m-1). With the increase of BT nfs content, the Eb of the sandwiched nanocomposites decreases gradually. The Eb of the sandwiched films with BT content varying from 5 wt% to 20 wt% only decreases slightly from 476 MV m-1 to 416 MV m-1. When the BT nfs content reaches to 25 wt%, Eb sharply drops to 350 MV m-1, which probably results from the agglomeration of nanofillers and/or appearance of structure defects in the nanocomposites. These results demonstrate that the multilayer films show great advantage in the breakdown strength, which is attributed to the highly enhanced Eb in central layer loading with MgO nws. The energy storage properties of the sandwiched nanocomposites were determined from P-E loops at Eb as given in Fig. S3a. Fig. 5a shows the maximum D and the energy density of the as-prepared sandwich-structured nanocomposites at Eb, respectively. The discharge energy densities of the sandwiched films at different applied fields are shown in Fig. 5b. Obviously, due to the enhancement of D as a consequence of the addition of BT nfs, the trilayered nanocomposites present increased maximum D with the increase of the BT content, while the remnant polarization still remains at a limited level. As a result, the sandwiched nanocomposites afford much increased discharge energy densities. Particularly, the 20B/2M/20B film shows the uppermost maximum electrical displacement of 10.15 μC cm-2 as well as a remnant polarization of 1.63 μC cm-2 at the Eb of 416 MV m-1. 12
Consequently, a largely enhanced discharge energy density of about 15.55 J cm -3 was realized, and the charge-discharge efficiency still reaches up to 69% (as shown in Fig. S3b). In comparison, while P(VDF-HFP) possesses a similar Weibull Eb of 411 MV m-1, its discharge energy density is only 9.77 J cm-3 due to the lower maximum D. In fact, as shown in Fig. 5a, all the sandwiched nanocomposites show praiseworthy discharge energy density because of the enhanced Eb and D. Interestingly, when the applied electric field exceeds 300 MV m-1, the charge-discharge efficiency of almost all the sandwiched samples show a slight increase. This phenomenon can be explained as follows. In the sandwiched films, the discrepancy of K in different layers can lead to electric field redistribution [16, 24]. Briefly, the outer layers with higher K undertake much less electric field compared with the central layer. In the outer layers, electronic and ionic displacement of BT nfs can only be activated under higher electric fields. When the sandwich films withstand lower electric fields, the central layer is the dominant contribution to the dielectric responses. With the increase of the applied electric field, the outer layers with a high content of BT nfs gradually withstand higher voltages, which subsequently activate the responses of electronic and ionic displacement of BT nfs to yield a large increase of the maximum polarization [23]. Consequently, the efficiency steadily grows at a certain applied electric field. The major peaks at 18.4o and 20.1o in the XRD pattern of as prepared sandwiched nanocomposites (as shown in Fig. S5b) confirm the α and γ phases of P(VDF-HFP) matrix after quenching process during the preparation of the composite 13
films [13, 23]. According to the areas of different fitting curves, the contents of α and γ phases are 12.3% and 27.6%, respectively. The major γ phase facilitates the switching of ferroelectric phase to paraelectric phase during discharging, thus leading to lower remnant polarization and a higher charge-discharge efficiency [23]. To our knowledge, the energy storage performance of the sandwiched films at about 410 MV m-1 are among the best values reported so far. For instance, Wang et al. presented the sandwiched nanocomposites with optimized spatial organizations of BT/P(VDF-HFP), which achieved the best values of the energy density and efficiency by far in the sandwich-structured
polymer
nanocomposites.
The
energy
density
of
the
nanocomposites is about 15 J cm-3 at 400 MV m-1, similar to the results obtained in our work [37]. To further study the energy storage performance of the sandwiched nanocomposites. The corresponding single-layer nanocomposite films including BT/P(VDF-HFP) and BT/MgO/P(VDF-HFP) were prepared. Fig. 4S presents the K and dielectric loss of the single-layered nanocomposite films. It is clearly seen that all the dielectric nanocomposites show enhanced K and the 20B/2M/20B nanocomposite possesses a moderate K value compared to those of BT/P(VDF-HFP) and BT/MgO/P(VDFF-HFP) single-layered films. This is due to the fact that the actual content of BT nfs in the sandwich nanocomposite is less than that of the BT/P(VDF-HFP) single-layered film. Meanwhile, the dielectric loss of all the samples is similar. Eb and energy density of different dielectric films were given in Fig. 6a. 14
The 20B/2M/20B sandwiched film has the highest dielectric strength and energy density among all the samples. It should be noted that, with the same content of BT nfs, the BT/MgO/P(VDF-HFP) single-layered film also possesses a slightly increase in Eb (i.e. 351 MV m-1) compared to the BT/P(VDF-HFP) film (i.e. 322 MV m-1). Yet this reinforcement is much weaker than that of the sandwiched films. This further proves that the spatial arrangement of multiple nanoinclusion in the layered structures can give rise to remarkable improvements in the dielectric breakdown strength of the nanocomposites. Discharge energy densities of the as-prepared dielectric films are shown in Fig. 6b. Apparently, owing to the most outstanding Eb and maximum D, the sandwiched 20B/2M/20B film exhibit the best energy density compared to those of the corresponding single-layered films.
4. Conclusion In this work, we have demonstrated a new route to design the sandwich-structured polymer nanocomposites based on P(VDF-HFP), which consists of a central layer with wide-band-gap MgO nanowires and the outer layers with high-K BaTiO3 nanofibers. To our knowledge, MgO nws, for the first time, is utilized as the nanofillers in the sandwiched nanocomposites. It is found that the introduction of a small amount of MgO nws in the central layer can effectively increase the breakdown strength of the sandwiched nanocomposites, while the loading of BT nfs in the outer layers can obviously enhance the electric displacement of the nanocomposites. With 15
the optimized nanofiller content, the resulting sandwiched nanocomposites show a dramatically enhanced energy density of 15.55 J cm-3 at Weibull breakdown strength of 416 MV m-1, which is 60% higher than that of polymer matrix (i.e. 9.77 J cm-3 at 410 MV m-1) and ~700% over that of BOPP. The improved dielectric and energy-storage performance are attributed to synergistic effects of the spatial arrangement of multiple nanoinclusion in the trilayered structures. This study provides a facile and versatile route to develop the polymer nanocomposites with outstanding energy-storage properties.
Acknowledgements This work is supported by the national Natural Science Foundation of China (No.51773163, 51673154). We also appreciate Prof. Suling Zhao, Wanting Zhu, Yi Guo and other lab technicians of the Center for Materials Research and Analysis of Wuhan University of Technology for materials characterization.
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and restrained conductive loss in sandwich-structured ceramic/polymer nanocomposites, J. Mater. Chem. A, 5 (2017) 4710-4718. [26] J. Yuh, J.C. Nino, W.M. Sigmund, Synthesis of barium titanate (BaTiO3) nanofibers via electrospinning, Mater. Lett., 59 (2005) 3645-3647. [27] H. Li, H. Wu, D. Lin, W. Pan, High Tc in electrospun BaTiO3 nanofibers, J. Am. Ceram. Soc., 92 (2009) 2162-2164. [28] N.M.A. Hadia, H.A.-H. Mohamed, Characteristics and optical properties of MgO nanowires synthesized by solvothermal method, Mate. Sci. in Semicond. Process., 29 (2015) 238-244. [29] S.-S. Chen, J. Hu, L. Gao, Y. Zhou, S.-m. Peng, J.-l. He, Z.-m. Dang, Enhanced breakdown strength and energy density in PVDF nanocomposites with functionalized MgO nanoparticles, RSC Adv., 6 (2016) 33599-33605. [30] M.L. Bortz, R.H. French, D.J. Jones, R.V. Kasowski, F.S. Ohuchi, Temperature dependence of the electronic structure of oxides: MgO, MgAl 2O4 and Al2O3, Physica Scripta, 41 (1990) 537. [31] H. Tang, Z. Zhou, H.A. Sodano, Relationship between BaTiO3 nanowire aspect ratio and the dielectric permittivity of nanocomposites, ACS Appl. Mater. Interfaces, 6 (2014) 5450-5455. [32] X. Zhang, Y. Shen, Q. Zhang, L. Gu, Y. Hu, J. Du, Y. Lin, C.W. Nan, Ultrahigh energy density of polymer nanocomposites containing BaTiO3@TiO2 nanofibers by atomic-scale interface engineering, Adv. Mater., 27 (2015) 819-824.
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[33] Z. Li, F. Liu, G. Yang, H. Li, L. Dong, C. Xiong, Q. Wang, Enhanced energy storage performance of ferroelectric polymer nanocomposites at relatively low electric fields induced by surface modified BaTiO3 nanofibers, Compos. Sci. Technol., (2018) 214-221. [34] S. Liu, J. Zhai, Improving the dielectric constant and energy density of poly(vinylidene fluoride) composites induced by surface-modified SrTiO3 nanofibers by polyvinylpyrrolidone, J. Mater. Chem. A, 3 (2015) 1511-1517. [35] Z.H. Shen, J.J. Wang, Y. Lin, C.W. Nan, L.Q. Chen, Y. Shen, High-throughput phase-field design of high-energy-density polymer nanocomposites, Adv. Mater, 30 (2018) 1704380. [36] Q. Chen, Y. Shen, S. Zhang, Q.M. Zhang, Polymer-based dielectrics with high energy storage density, Annu. Rev. Mater. Res., 45 (2015) 433-458. [37] Y. Wang, L. Wang, Q. Yuan, J. Chen, Y. Niu, X. Xu, Y. Cheng, B. Yao, Q. Wang, H. Wang, Ultrahigh energy density and greatly enhanced discharged efficiency of sandwich-structured polymer nanocomposites with optimized spatial organization, Nano Energy, 44 (2017) 364-370.
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Fig. 1. The SEM micrograph of BT nfs (a) and MgO nws (b). (c) XRD patterns of BT nfs and MgO nws. (d) Cross-section SEM micrograph of the sandwiched nanocomposites.
22
Fig. 2. (a) Weibull breakdown strength of MgO/P(VDF-HFP) nanocomposites. (b) Electric displacement and charge-discharge efficiency of MgO/P(VDF-HFP) nanocomposites at 300 MV m-1.
23
Fig. 3. Frequency dependent dielectric constant (a) and dielectric loss (b) of the sandwiched nanocomposites with various amounts of BT nfs.
24
Fig. 4. Weibull breakdown strength of the sandwich nanocomposites and BT/P(VDF-HFP) single-layered film with various amounts of BT nfs.
25
Fig. 5. (a) The maximum electric displacement and energy density of the sandwiched nanocomposites. (b) Discharge energy density of the sandwiched nanocomposites at different electric fields.
26
27
Fig. 6. (a) Weibull breakdown strength and energy density, (b) Discharge energy density at different electric fields of P(VDF-HFP), single-layered BT/P(VDF-HFP) nanocomposite with 20 wt% BT nfs, single-layered BT/MgO/P(VDF-HFP) nanocomposite with 20 wt% BT nfs and 2 wt% MgO nws and the trilayered nanocomposites.
28
PII: DOI: Reference:
S0272-8842(19)30129-4 https://doi.org/10.1016/j.ceramint.2019.01.124 CERI20579
To appear in: Ceramics International Received date: 17 October 2018 Revised date: 4 January 2019 Accepted date: 16 January 2019 Cite this article as: Zeyu Li, Feihua Liu, He Li, Lulu Ren, Lijie Dong, Chuanxi Xiong and Qing Wang, Largely Enhanced Energy Storage Performance of Sandwich-Structured Polymer Nanocomposites with Synergistic Inorganic N a n o w i r e s , Ceramics International, https://doi.org/10.1016/j.ceramint.2019.01.124 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Largely Enhanced Energy Storage Performance of Sandwich-Structured Polymer Nanocomposites with Synergistic Inorganic Nanowires
Zeyu Li,a Feihua Liu,a,b He Li,b Lulu Ren,b Lijie Dong,a, Chuanxi Xiong,a, and Qing Wang,b,*
a
Center for Smart Materials and Device Integration, School of Materials Science and
Engineering, Wuhan University of Technology, Wuhan 430070, China b
Department of Materials Science and Engineering, The Pennsylvania State
University, University Park, PA 16802, USA *Corresponding author: Qing Wang (Email: [email protected]), Lijie Dong (Email: [email protected]), Chuanxi Xiong (Email: [email protected])
Abstract It is of crucial importance to develop novel dielectric materials with high discharge energy densities to meet the urgent requirements in the rapid advances in modern electronics and electrical power systems. In this work, we describe a new class of sandwich-structured ceramic-polymer nanocomposites base on poly(vinylidene 1
fluoride-co-hexafluoropropylene) exhibiting outstanding energy storage performance. The central layer of the composite consists of MgO nanowires to offer high breakdown strength, whereas the outer layers contain BaTiO3 nanofibers to provide improved dielectric constant of the composites. The systematic investigation gave the composites with 2 wt% MgO in the central layer and 20 wt% BaTiO3 in the outer layers to yield the best energy density of 15.55 J cm-3 with a charge-discharge efficiency of 68% at the Weibull breakdown strength of 416 MV m-1. It is found that the trilayered nanocomposite exhibits superior energy storage performance as compared to the corresponding single-layered composite films. The result demonstrates the uniqueness of the layered structure to incorporate multiple inorganic components into polymer with enhanced collective properties for energy applications.
Key words: MgO nanowires, BaTiO3 nanofibers, ceramic-polymer nanocomposites, energy storage.
1. Introduction Dielectric materials with high energy densities are used to store and convert electrical energy in film capacitors, which are key components in modern electronics and electric power systems [1-7]. For instance, film capacitors with fast charge-discharge abilities and ultra-high power densities are utilized in hybrid and electric vehicles to 2
convert direct currents from batteries to alternating currents to drive the engine of vehicles [8]. The key drawback of film capacitors is their energy densities that are more than one order of magnitude lower than those of electrochemical devices such as batteries and supercapacitors. For example, biaxially oriented polypropylene (BOPP), the state-of-the-art commercial dielectric polymer, possesses a limited energy density of about 2 J cm-3. Therefore, it is essential to enhance the energy densities of dielectric materials, which not only improves capacitor performance but also enables the minimization of electronics and power systems [3]. The energy density (U) of dielectric materials is determined by the applied electric field (E) and electric displacement (D), which is given by 𝑈 = ∫ 𝐸d 𝐷
(1)
𝑈=1/2DE = 1/2𝐾𝜀0 𝐸 2
(2)
For linear dielectrics
where K is the dielectric constant and ε0 is the vacuum permittivity. Ferroelectric polymers are preferred candidates for polymer film capacitors due to their highest K among the known polymers. Yet, the K values of ferroelectric polymers are still much lower than those of ferroelectric perovskites, such as BaTiO3, BaxSr1-xTiO3 and PZT [9-11], which could have permittivities of hundreds or even thousands. It has been demonstrated to enhance the energy storage performance of dielectric polymers by introducing electroceramics into polymer matrix to form the composites with 3
improved permittivity [10, 12-14]. However, one paradox is that the enhancement of permittivity is often achieved at the expense of breakdown strength (Eb). This is owing to the inhomogeneous field distribution because of large mismatch of the permittivity between ceramic fillers and polymer matrix. Since U depends on the square of Eb, improving the Eb of dielectric composites is a more effective route to realize greatly improved energy densities. For this purpose, a number of innovative approaches have been developed, including utilization of two-dimensional nanofillers, surface modification of nanofillers, cross-link structures and multilayered structures [15-22]. Among
the
aforementioned
methods,
the
sandwich-structured
polymer
nanocomposites have aroused intensive attentions because the layered structures provide unique features of simultaneous enhancement of K and Eb that is normally unattainable in the conventional single-layer films [23-25]. Hu et al. reported the trilayer-structured polymer nanocomposites with a small content of BaTiO3 nanofibers (BT nfs) in the central layer to deliver higher Eb and impede electric breakdown, while the outer layers of the composite contain a high content of BT nfs to offer enhanced D of nanocomposites. As a result, a dramatic enhancement of energy density of about 9.7 J cm-3 was achieved. In this work, we present a class of sandwich-structured polymer nanocomposite with different ceramic fillers that are spatially organized in the layered structures. Specifically, MgO nanowires (nws) were introduced into the central layer of the 4
composites to enhance the Eb, whereas the outer layers contain BT nfs to improve the D. Note that, to our knowledge, this is the first example to employ MgO nws as the nanofillers to enhance the Eb of the layered composites. MgO with a wide band gap of ~7.8 eV would efficiently reduce electrical conduction, which is the main loss mechanism of dielectric material at high fields, and thus improve the Eb. Moreover, the K of MgO, i.e. ~9-10, is close to that of the ferroelectric polymer matrix, which would yield homogeneous field distribution and further contribute to the enhanced overall Eb of the composites. In addition, in comparison to other highly insulating nanofillers such as boron nitride nanosheets (BNNSs) and montmorillonite nanoplates, MgO nws can be facilely prepared. The trilayered nanocomposites show a remarkable energy density of 15.55 J cm-3 at an applied field of 416 MV m-1 along with a charge-discharge efficiency of 68%, which represents an enhancement of ~700% over that of BOPP. When compared to the conventional single-layer composite films, the sandwich-structured films exhibit superior energy storage performance, including breakdown strength, discharge energy density and charge-discharge efficiency.
2. Experimental 2.1 Preparation of BT nfs The BT nfs were synthesized by electrospinning method [26, 27]. In brief, barium acetate and tetrabutyl titanium at a molar ratio of 1:1 were dissolved in acetic acid and 5
stirred to yield homogeneous barium titanate precursor gel. The gel was then mixed with poly(vinyl pyrrolidone) (PVP, Mn = 1,300,000) ethanol solution to form electrospinning solution. The electrospinning was conducted at an electric field of 1.0 kV m-1 with a feeding rate of 1.0 mL h-1. The as-spun nanofibers were then calcined at 650 oC for 1.5 h in air at a heating rate of 5 oC min-1 to remove PVP completely and obtain BT nfs. Finally, The BT nfs were broke into short nanofibers by ultrasonication for 40 min. 2.2 Preparation of MgO nws MgO nws were synthesized via hydrothermal technique [28], followed by a calcination process. In a typical experiment, 6.44 g magnesium acetate and 1.2 g urea were dissolved in 100 mL distilled water and stirred to generate homogeneous solution. The solution was then transferred into a 100 mL Teflon-lined autoclave. The autoclave was maintained at 140 oC for 30 min. Afterward, the as-prepared power were washed by distill water three times and dried at 60 oC in an oven for 24 h. Finally, the obtained white power was calcined at 450 oC for 30 min in air at a heating rate of 5 oC min-1 to yield MgO nws. 2.3 Preparation of the sandwich-structured polymer nanocomposites films The sandwich-structured polymer nanocomposites films were prepared via solution-casting method, followed by a hot-press process. First, BT nfs, MgO nws and poly(vinylidene fluoride-co-hexafluoropropylene) P(VDF-HFP) were vacuum-dried 6
at 80
o
C for 12 h. Afterwards, 0.5 g P(VDF-HFP) was dissolved in 6 g
dimethylformamide (DMF) and stirred overnight at 70 oC to generate a homogeneous solution. BaTiO3 or MgO nanofillers were dispersed in the above solution by stirring and ultrasonication. The as-prepared suspension was then cast onto a clean glass and dried at 60 oC in an oven for 18 h. After that, the nanocomposite films were peeled off from the glass, stacked together and hot pressed at 170 oC at 10 MPa for 10 min, followed by an immediately quenched process in iced water. The thickness of each single layer was controlled to be 6-8 μm, and the final thickness of thus-prepared films
is
15-20
μm.
The
single-layered
P(VDF-HFP),
BT/P(VDF-HFP),
MgO/P(VDF-HFP) and BT/MgO/P(VDF-HFP) films were prepared by the same method. 2.4 Characterization Crystal structure of the nanofillers were characterized by power X-Ray diffraction (XRD) on a Bruker D8 Advanced diffractomer using Cu Kα (λ = 1.5418 Å). The morphology of nanofillers and dielectric films were investigated by scanning electron microscopy (SEM Hitachi S4800). The dielectric properties were measured using E4980A precision LCR Meter. The electric polarization-electric hysteresis field (P-E) loops and breakdown strength were measured at 100 Hz with a precision Multiferroelectric Materials Analyzer equipped with Precision 10 kV HVI-SC and Trek MODEL 609B (Radiant Inc.).
7
3. Results and Discussion We design the sandwich structure consisting of a higher insulating central layer and outer layers with enhanced electrical polarization. Ferroelectric P(VDF-HFP) was chosen as the polymer matrix. The introduction of bulky HFP units into PVDF facilitate dipole switching by breaking apart the large crystals and decoupling the ferroelectric domains, which leads to slimmer electric hysteresis loops and a high charge-discharge efficiency [8]. The central layer is composed of MgO nws dispersed in P(VDF-HFP). Recently, it has been shown that the addition of a small content of MgO nanoparticles can effectively increase the breakdown strength of polymer matrix [29]. MgO, a wide band gap insulator [30], can act as traps for the injected charges and reduce the electrical conduction in the polymer composites. Moreover, the comparable K values of MgO filler and P(VDF-HFP) matrix would avoid the local field distortion and lead to homogeneous distribution of the internal fields, thus benefiting the overall breakdown strength of the nanocomposites. BT nfs were dispersed in P(VDF-HFP) as the outer layers. It has been demonstrated previously that the nanocomposites with BT nfs possess higher D than BT nanoparticles because of the high aspect ratio of one-dimensional nanofillers [31, 32]. More interestingly, the most recent experimental and theoretical studies have demonstrated that the nanocomposites containing one-dimensional nanofillers exhibit much higher breakdown strength in comparison to the corresponding nanoparticle-filled nanocomposites, which has been attributed to the fact that the one-dimensional 8
nanofillers can afford more significant restrain to the growth of electric trees [33-35]. The morphologies of as-prepared BT nfs and MgO nws are displayed in Figs. 1a and b, respectively. BT nfs and MgO nws exhibit a similar structure with a diameter of 200-250 nm and an aspect ratio of 15-20. As shown in the XRD patterns (Fig. 1c), the Bragg reflection peaks of BT nfs and MgO nws have been successfully assigned to the tetragonal phase of BaTiO3 and the cubic phase of MgO, respectively. This result indicates that the nanofillers are highly crystalline and no byproducts are generated in the synthesis process. Fig. 1d displays the cross-section SEM micrograph of sandwiched nanocomposites film. It can be seen that the nanofillers are well dispersed in the polymer matrix and no obvious agglomeration or structure defect appears. Interestingly, these two nanoinclusion are nearly oriented in the direction parallel to the film surface and perpendicular to the electric field direction; this could contribute to restrain the development of electric trees and give rise to high breakdown strength. Moreover, the cross-section SEM micrograph clearly demonstrate the sandwich structure in the as-prepared multilayered nanocomposites film. To determine the optimal MgO nws content in the central layer of the sandwiched nanocomposites, we first carried out systematic studies of the breakdown strength and P-E loops of the single-layered MgO/P(VDF-HFP) films. The breakdown strength of the films was analyzed by a two-parameter Weibull statistic distribution according to:
9
𝐸
𝑃(𝐸) = 1 − 𝑒𝑥𝑝(−( )𝛽 ) 𝐸0
(3)
where E is the measured breakdown strength, P(E) is the cumulative probability as a function of E. E0 is the breakdown strength when the cumulative failure probability is 63.2% and used to represent the Weibull breakdown strength (Weibull Eb). β is a shape parameter to evaluate the scatter of the measured breakdown strength [23]. In this study, at least 8 samples were tested for Weibull fitting. The Weibull distribution of the single-layer MgO/P(VDF-HFP) nanocomposites was plotted in Fig. S1a, and the Eb calculated from Weibull distribution was shown in Fig. 2a. It can be seen that the Eb of the nanocomposites exhibits a dramatic increase with the loading of MgO nws and reaches to 508 MV m-1 at 3 wt% MgO nws. Comparatively, pure P(VDF-HFP) films have a moderate Eb of 411 MV m-1. The electric resistivity of the nanocomposite films displays an obvious increase with the content of MgO nws as shown in Fig. S1b, which is consistent to the trend shown in Eb. This demonstrates that the enhancement of Eb results from the synergistic effect of the highly insulating property and a large aspect ratio of MgO nws. Nevertheless, the P-E loops (as shown in Fig. S1c) indicate that the nanocomposites with the increased MgO nws content show limited maximum polarization and almost the same remanent polarization, hence offering a reduced charge-discharge efficiency from 60% to 55% as summarized in Fig. 2b. The decrease of the charge-discharge efficiency indicates high dielectric loss, which generates waste heating and thus impairs capacitor lifetime [36]. Based on these results, the MgO/P(VDF-HFP) containing 2 wt% MgO with the Eb of 10
485 MV m-1 was chosen as the central layer in the sandwiched structure. A series of the BT/MgO/P(VDF-HFP) sandwiched nanocomposites with varied BT nfs contents from 5 wt% to 25 wt% were prepared. The frequency dependence of dielectric properties of the trilayered films was shown in Fig. 3. Obviously, K consistently increases with increasing BT nfs content over the tested frequency range. For instance, K of the sample 25B/2M/25B (standing for the trilayerd nanocomposite with 25 wt% BT nfs in outer layers and 2 wt% MgO nws in the central layer) reaches up to 11.21 at 1 kHz, and K of the sample 5M/2B/5M is 7.56 at 1 kHz. K of the nanocomposites decreases with the increase of frequency, which is due to the molecular dipoles in P(VDF-HFP) that are unable to follow the frequency at high frequencies. The dielectric loss of all the samples still remains a low value, i.e. around 0.05 at 1 kHz, which is beneficial to small leakage current and low energy loss [23]. An obvious rising in dielectric loss can be observed from 104 to 106 Hz in all the trilayered nanocomposite films, which results from the dielectric relaxation in P(VDF-HFP) matrix. With the increase of BT nfs content, the trilayered nanocomposites show a slight increase in the dielectric loss due to the ferroelectric nature of BT nfs. The Weibull plots of the sandwiched films and the single-layer BT/P(VDF-HFP) films are given in Fig. S2. The corresponding Eb of the as-prepared nanocomposite films are shown in Fig. 4. The most striking feature is that, with the increase of BT nfs content, the sandwiched films show a largely enhanced Eb compared to the 11
BT/P(VDF-HFP) single films. For example, the 20B/2M/20B sample possesses the Eb of 416 MV m-1, which is 29% higher than the BT/P(VDF-HFP) single-layer film (i.e. 322 MV m-1). With the increase of BT nfs content, the Eb of the sandwiched nanocomposites decreases gradually. The Eb of the sandwiched films with BT content varying from 5 wt% to 20 wt% only decreases slightly from 476 MV m-1 to 416 MV m-1. When the BT nfs content reaches to 25 wt%, Eb sharply drops to 350 MV m-1, which probably results from the agglomeration of nanofillers and/or appearance of structure defects in the nanocomposites. These results demonstrate that the multilayer films show great advantage in the breakdown strength, which is attributed to the highly enhanced Eb in central layer loading with MgO nws. The energy storage properties of the sandwiched nanocomposites were determined from P-E loops at Eb as given in Fig. S3a. Fig. 5a shows the maximum D and the energy density of the as-prepared sandwich-structured nanocomposites at Eb, respectively. The discharge energy densities of the sandwiched films at different applied fields are shown in Fig. 5b. Obviously, due to the enhancement of D as a consequence of the addition of BT nfs, the trilayered nanocomposites present increased maximum D with the increase of the BT content, while the remnant polarization still remains at a limited level. As a result, the sandwiched nanocomposites afford much increased discharge energy densities. Particularly, the 20B/2M/20B film shows the uppermost maximum electrical displacement of 10.15 μC cm-2 as well as a remnant polarization of 1.63 μC cm-2 at the Eb of 416 MV m-1. 12
Consequently, a largely enhanced discharge energy density of about 15.55 J cm -3 was realized, and the charge-discharge efficiency still reaches up to 69% (as shown in Fig. S3b). In comparison, while P(VDF-HFP) possesses a similar Weibull Eb of 411 MV m-1, its discharge energy density is only 9.77 J cm-3 due to the lower maximum D. In fact, as shown in Fig. 5a, all the sandwiched nanocomposites show praiseworthy discharge energy density because of the enhanced Eb and D. Interestingly, when the applied electric field exceeds 300 MV m-1, the charge-discharge efficiency of almost all the sandwiched samples show a slight increase. This phenomenon can be explained as follows. In the sandwiched films, the discrepancy of K in different layers can lead to electric field redistribution [16, 24]. Briefly, the outer layers with higher K undertake much less electric field compared with the central layer. In the outer layers, electronic and ionic displacement of BT nfs can only be activated under higher electric fields. When the sandwich films withstand lower electric fields, the central layer is the dominant contribution to the dielectric responses. With the increase of the applied electric field, the outer layers with a high content of BT nfs gradually withstand higher voltages, which subsequently activate the responses of electronic and ionic displacement of BT nfs to yield a large increase of the maximum polarization [23]. Consequently, the efficiency steadily grows at a certain applied electric field. The major peaks at 18.4o and 20.1o in the XRD pattern of as prepared sandwiched nanocomposites (as shown in Fig. S5b) confirm the α and γ phases of P(VDF-HFP) matrix after quenching process during the preparation of the composite 13
films [13, 23]. According to the areas of different fitting curves, the contents of α and γ phases are 12.3% and 27.6%, respectively. The major γ phase facilitates the switching of ferroelectric phase to paraelectric phase during discharging, thus leading to lower remnant polarization and a higher charge-discharge efficiency [23]. To our knowledge, the energy storage performance of the sandwiched films at about 410 MV m-1 are among the best values reported so far. For instance, Wang et al. presented the sandwiched nanocomposites with optimized spatial organizations of BT/P(VDF-HFP), which achieved the best values of the energy density and efficiency by far in the sandwich-structured
polymer
nanocomposites.
The
energy
density
of
the
nanocomposites is about 15 J cm-3 at 400 MV m-1, similar to the results obtained in our work [37]. To further study the energy storage performance of the sandwiched nanocomposites. The corresponding single-layer nanocomposite films including BT/P(VDF-HFP) and BT/MgO/P(VDF-HFP) were prepared. Fig. 4S presents the K and dielectric loss of the single-layered nanocomposite films. It is clearly seen that all the dielectric nanocomposites show enhanced K and the 20B/2M/20B nanocomposite possesses a moderate K value compared to those of BT/P(VDF-HFP) and BT/MgO/P(VDFF-HFP) single-layered films. This is due to the fact that the actual content of BT nfs in the sandwich nanocomposite is less than that of the BT/P(VDF-HFP) single-layered film. Meanwhile, the dielectric loss of all the samples is similar. Eb and energy density of different dielectric films were given in Fig. 6a. 14
The 20B/2M/20B sandwiched film has the highest dielectric strength and energy density among all the samples. It should be noted that, with the same content of BT nfs, the BT/MgO/P(VDF-HFP) single-layered film also possesses a slightly increase in Eb (i.e. 351 MV m-1) compared to the BT/P(VDF-HFP) film (i.e. 322 MV m-1). Yet this reinforcement is much weaker than that of the sandwiched films. This further proves that the spatial arrangement of multiple nanoinclusion in the layered structures can give rise to remarkable improvements in the dielectric breakdown strength of the nanocomposites. Discharge energy densities of the as-prepared dielectric films are shown in Fig. 6b. Apparently, owing to the most outstanding Eb and maximum D, the sandwiched 20B/2M/20B film exhibit the best energy density compared to those of the corresponding single-layered films.
4. Conclusion In this work, we have demonstrated a new route to design the sandwich-structured polymer nanocomposites based on P(VDF-HFP), which consists of a central layer with wide-band-gap MgO nanowires and the outer layers with high-K BaTiO3 nanofibers. To our knowledge, MgO nws, for the first time, is utilized as the nanofillers in the sandwiched nanocomposites. It is found that the introduction of a small amount of MgO nws in the central layer can effectively increase the breakdown strength of the sandwiched nanocomposites, while the loading of BT nfs in the outer layers can obviously enhance the electric displacement of the nanocomposites. With 15
the optimized nanofiller content, the resulting sandwiched nanocomposites show a dramatically enhanced energy density of 15.55 J cm-3 at Weibull breakdown strength of 416 MV m-1, which is 60% higher than that of polymer matrix (i.e. 9.77 J cm-3 at 410 MV m-1) and ~700% over that of BOPP. The improved dielectric and energy-storage performance are attributed to synergistic effects of the spatial arrangement of multiple nanoinclusion in the trilayered structures. This study provides a facile and versatile route to develop the polymer nanocomposites with outstanding energy-storage properties.
Acknowledgements This work is supported by the national Natural Science Foundation of China (No.51773163, 51673154). We also appreciate Prof. Suling Zhao, Wanting Zhu, Yi Guo and other lab technicians of the Center for Materials Research and Analysis of Wuhan University of Technology for materials characterization.
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Fig. 1. The SEM micrograph of BT nfs (a) and MgO nws (b). (c) XRD patterns of BT nfs and MgO nws. (d) Cross-section SEM micrograph of the sandwiched nanocomposites.
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Fig. 2. (a) Weibull breakdown strength of MgO/P(VDF-HFP) nanocomposites. (b) Electric displacement and charge-discharge efficiency of MgO/P(VDF-HFP) nanocomposites at 300 MV m-1.
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Fig. 3. Frequency dependent dielectric constant (a) and dielectric loss (b) of the sandwiched nanocomposites with various amounts of BT nfs.
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Fig. 4. Weibull breakdown strength of the sandwich nanocomposites and BT/P(VDF-HFP) single-layered film with various amounts of BT nfs.
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Fig. 5. (a) The maximum electric displacement and energy density of the sandwiched nanocomposites. (b) Discharge energy density of the sandwiched nanocomposites at different electric fields.
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Fig. 6. (a) Weibull breakdown strength and energy density, (b) Discharge energy density at different electric fields of P(VDF-HFP), single-layered BT/P(VDF-HFP) nanocomposite with 20 wt% BT nfs, single-layered BT/MgO/P(VDF-HFP) nanocomposite with 20 wt% BT nfs and 2 wt% MgO nws and the trilayered nanocomposites.
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