PVDF flexible nanocomposites for high energy storage density in lower electric field

Journal Pre-proof Reduced conductivity in sandwich-structured BFT@DA/PVDF flexible nanocomposites for high energy storage density in lower electric field Zhuo Wang, Yinbo Li, Xiaoying Wang, Jiahao Fan, Zhihui Yi, Menglei Kong, Ning Xu PII:

S2352-8478(19)30227-8

DOI:

https://doi.org/10.1016/j.jmat.2019.12.009

Reference:

JMAT 257

To appear in:

Journal of Materiomics

Received Date: 29 October 2019 Revised Date:

12 December 2019

Accepted Date: 23 December 2019

Please cite this article as: Wang Z, Li Y, Wang X, Fan J, Yi Z, Kong M, Xu N, Reduced conductivity in sandwich-structured BFT@DA/PVDF flexible nanocomposites for high energy storage density in lower electric field, Journal of Materiomics (2020), doi: https://doi.org/10.1016/j.jmat.2019.12.009. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. All rights reserved.

Graphical Abstract

Reduced conductivity in sandwich-structured BFT@DA/PVDF flexible nanocomposites for high energy storage density in lower electric field a,b,

Zhuo Wang

a,b

a,b

*, Yinbo Li ,, Xiaoying Wang

a,b

, Jiahao Fan , Zhihui Yia,b, Menglei Konga,b,

and Ning Xua,b a

School of Materials Science and Engineering, Shaanxi University of Science & Technology, 710021 Xi’an, China.

b

School of Materials Science and Engineering, Shaanxi Key Laboratory of Green Preparation

and Functionalization for Inorganic Materials, Shaanxi University of Science and Technology, Xi’an, 710021, People’s Republic of China *

Corresponding author: Zhuo Wang Tel: +86-15114845870; E-mail address: [email protected]

ABSTRACT The Ba(Fe0.5Ta0.5)O3@DA/PVDF flexible composite films with sandwich-structured are prepared by solution-casting method. According to SEM image, the thickness of the composite film is about 15 µm and the each layer is about 5 µm. The sandwich-structured nanocomposite films not only have higher permittivity but also lower AC conductivity. The high permittivity is due to the large permittivity of BFT and the enhanced interface polarization between ceramic particles and polymer matrix. The low AC conductivity is due to the absence of conductive pathways in the PVDF layer. At low electric field strength of 150 MV/m, the energy density of sandwich-structure composite films filled with 1 vol.% is 1.93 J/cm3. When the breakdown strength is 250 MV/m, the maximum energy storage density is increased to 4.87 J/cm3. The sandwich-structure Ba(Fe0.5Ta0.5)O3@DA/PVDF flexible composite films with outstanding energy storage properties in lower electric field can be used for wearable devices in the future. Keywords: sandwich structure; conductive pathways; energy storage; lower electric field

1. Introduction With the developing of modern society, new energy technology is paid more attention, and dielectric energy storage are widely used in many fields. It means that higher requirements are proposed for dielectric materials[1–3]. In recent years, the flexible polymer capacitors are noticed which have high energy density[4,5]. However, the high energy density may only be realized in high electric field. For example, when the energy density of polyvinylidene fluoride-hexafluoropropylene (P(VDF-HFP) is 4.5 J/cm3, the electric field strength required is as high as 317 MV/m[6]. And for the composite films of ZnO and PVDF, the energy storage density reaches 3.6 J/cm3 at 460 MV/m when the filling content of ZnO is 0.8 Vol.%[7]. There are three shortcomings about high electric field. In the first place, it poses a great challenge to the reliability of capacitors. While capacitors are operated in high electric field, the possibility of capacitor breakdown will be raised greatly. In the next place, the application of electrical condensers is restricted by high electric field, which makes it impossible to be used in many electronic or electrical devices operating in low electric field[8,9]. Lastly, the life safety of operators working near high voltage equipment is also difficult to be guaranteed. Therefore, it is imperative to invent a new dielectric materials with high energy density in lower electric field. The mathematical expression of the discharged energy density (U) is [10]:

U = ∫ EdP

(1)

which is based on the electric field (E) and polarization (P). In order to make composites have high energy density in lower electric field, it is important to choose dielectric materials with high polarization strength. The relationship between the polarization and permittivity can be expressed as[3]: P = (ε '− 1)ε 0 E

(2)

where εo is permittivity of vacuum, ε' is relative permittivity of materials. Therefore, the key of achieving high energy density in lower electric field is large permittivity. At present, high permittivity flexible polymer is poly(vinylidene fiuoride) (~10). However, the permittivity is

still very small. Thus, ceramic/polymer flexible nanocomposites represent a promising approach, which combines the advantages of ceramics and polymers. It has high permittivity and is possible to be achieved high energy density in lower electric field. In general, TiO2[11], BaTiO3[12], Na0.5Bi0.5TiO3[13], Ba(Fe0.5Nb0.5)O3[14] and Pb(Zr,Ti)O3[15] are used as common fillers in polymer nanocomposites. But the permittivity of those ceramic powders are about 103 magnitude. In order to obtain high energy density, Wang et al. researched sandwich structure barium titanate/poly(vinylidene fluoride), adding nanowires into the films[16], and Hu et al. prepared gradient structure films[17], were researched. Zhang et al. studied the preparation of new nanowire array composites[18]. Luo et al. researched the core-shell composite and the influence of the interface on the dielectric properties of the composite[19,20]. However, the large energy density obtained in these works are all in high electric field, which is difficult to achieve in low electric field. At the same time, Ba(Fe0.5Ta0.5)O3 (BFT) ceramic have a tremendous permittivity (ε′＞104), excellent variation of permittivity with frequency and temperature[21]. BFT powder is used a great choice filler because the large permittivity of BFT ceramics in ceramic-polymer composites in order to achieve high energy density in lower electric field. In our previous work, the flexibility composite films were prepared by solution-casting method, comprising of crosslinking agent (DA), nanopowders fillers (BFT) and polymer matrix (PVDF), which exhibited great energy storage properties in lower electric field[22]. However, it is prone to form conductive paths between nanopowders and polymers by adding the nano-powders which have large value of permittivity into polymer matrix. Hence, sandwich-structured BFT@DA/PVDF flexible composite films are fabricated to decrease the conductivity. We hope to further improve the energy storage properties in lower electric field. 2. Experimental 2.1.

Materials.

Barium

nitrate

(Ba(NO3)2),

iron(III)

nitrate

nonahydrate

(Fe(NO3)3•9H2O), oxalic acid dehydrate ((COOH)2•2H2O), ammonium hydroxide (NH3•H2O), N,N-dimethylformamide (DMF) and tantalum chloride (TaCl5) were acquired from

Sinopharm Chemical Reagent Co., Ltd. Dopamine (DA) and Poly(vinylidene fluoride) (PVDF, molecular weight=534,000) were procured from Alfa Aesar Co., Ltd. 2.2. Preparation of the BFT@DA nanopowders. BFT nano-powders were made by oxalate-coprecipitation method. Firstly, TaCl5 was melted in anhydrous ethanol, other three solutions were formed by dissolving Fe(NO3)3•9H2O, (COOH)2•2H2O and Ba(NO3)2 for deionized water and stirred, respectively. Secondly, these solutions were mixed and stirred to form precursors. And moderate ammonia was added to make the pH of precursor liquid was 10. Thirdly, the solution was then centrifuged and dried. The precursors of the required powders were obtained. Finally, The precursor was calcined at 1000 ℃ for 2 h to prepared BFT nanopowders. In order to prepare BFT powder modified by DA, BFT nano-powderswas put into 0.01M of dopamine hydrochloric and stirred to obtain BFT nano-powders coated by DA for 12 hours at 60 ℃。 2.3. Preparation of sandwich-structured BFT@DA/PVDF nanocomposite films. In order to prepare nanocomposite films, PVDF was dissolved in DMF. After 4 h of stirring, BFT@DA nano-powder was added into PVDF solution with a certain amount of filling. Then, the precursor solution was successfully prepared by ultrasound for 30 min and stirring for 30 min, after four cycles and stirring for 12 h. Using precursor as raw material, single layer films were successfully prepared by tape casting. After a little drying, the second film is prepared on the first layer. The sandwich-structured BFT@DA/PVDF nanocomposite films were prepared by layer-by-layer tape casting, as illustrated in Fig. 1. Subsequently, the film was desiccated to remove DMF at 100 ℃ for 12 h. After the films were heat treated by vacuum dried for 7 min at 200 ℃, the film with glass substrate was put into ice water for quenching immediately. Lastly, the composite films were removed from the glass templet and desiccated for 3 h at 80 ℃. The thickness of the flexible composite films was about 15 µm and every layer is 5µm. It should be specially noted that the filler content of outer layer is 1.5 vol.%, 4.5 vol.%, 7.5 vol.%,10.5 vol.% and the average filler content in nanocomposites in is 1 vol.%, 3 vol.%, 5 vol.%, 7 vol.%.

2.4. Characterization. X-ray powder diffraction instrument (Rigaku D/max-2200PC) was used to characterize the crystallinity of PVDF powder, PVDF films, composite films and BFT powders. The fracture appearance of the flexible composite films was photographed by scanning electron microscope (Hitachi S-4800). Before testing, the composite films were hardened and broken in liquid nitrogen, and identical gold layer were sprayed at the cross section. The dielectric permittivity, dielectric loss tangent and impedance of the composite films were measured using Agilent E4980A Precision Impedance Analyzer at a range from 100 to 2×106 Hz at room temperature. The gold electrodes were symmetrically spurted on composite films, and ferroelectric and electrical breakdown tests were completed through the ferroelectric analyzer (Radiant Premier II) at 10 Hz at room temperature. 3. Results and discussion XRD patterns of PVDF powders, PVDF films, composite films with different filling and BFT nanopowers are displayed in Fig. 1. The crystalline phase of PVDF powders is mainly α phase. The main crystalline phases of PVDF films are β phase. Because the change from α phase to β phase of PVDF after be heat treatment at high temperature and be quenched at ice water. The reason are casting method, preparation temperature, holding time, vacuum, film stretching and annealing temperature and other factors[23]. BFT nanopowders exhibit a typical perovskite structure. For the outer layer sandwich-structured BFT@DA/PVDF composite films, PVDF was maintained in two different crystalline phases. The crystal plane {(100), (020) , (110), (021)} are represented the diffraction peaks at 17.9°, 18.5°, 18.5°, 26.7°, respectively. They are corresponded to α-phase of PVDF. The crystal plane {(110), (200)} are represented the peaks at 20.9°. It is corresponded to β-phase of PVDF. The diffraction peak of PVDF is strongly influenced by the filling of BFT nanopowders, which is attributed to the fact that the diffraction peak of PVDF is masked because of the strong diffraction peak of BFT powders[24]. SEM

photo

and

the

corresponding

EDS

mappings

of

sandwich-structured

BFT@DA/PVDF flexible composite films loaded 7 vol.% fillers are shown in Fig. 2. The thickness of the sandwich-structured films is 15 µm, and the thickness of each layer is about 5

µm. The different interfaces are tightly connected in the film, without pore and defect[25]. In order to further confirm the sandwich-structure, the corresponding EDS mappings of C, Ba, Fe, Ta element are performed. The C element are homogenously distributed in the whole films. But the Ba, Fe, Ta element are distributed in the layers of both sides of the film. The results show that the middle layer is pure PVDF and the other two layers are BFT@DA/PVDF composites. The foundation of excellent dielectric properties is laid because successful preparation of flexible composite films of BFT@DA/PVDF composite films. Fig. 4(a) and Fig. 4(b) show the dependence of permittivity and dielectric loss tangent on the frequency for sandwich-structured BFT@DA/PVDF composite films, respectively. The results show that the permittivity and dielectric loss of the composite films are raised with the increase of BFT filler concentration, which is due to the large permittivity of BFT and the increase of charge accumulated at the interface. The permittivity of the composite films loaded 7 vol.% fillers is twice as much as the pure PVDF at 100 Hz, which helps to achieve high energy density in lower electric field. The permittivity of all composite films are abated depending on increasing of frequency, which is especially evident in the high-filled components. It is because the transformation of frequency is faster than the interface polarization. Only electron displacement polarization and ion displacement polarization are utilized in composite films, so the permittivity of composite films is reduced at high frequencies. And the dielectric loss tangent first decreases and then increased with the increasing of frequency. The phenomenon is due to the large dielectric loss at low frequencies. With the raising of frequencies, dielectric loss began to decrease, which was considered to the decrease of polarization at the Maxwell-Wagner-Sillars interface. Further as enhance of frequencies, dielectric loss increased sharply, which is due to the withdrawal of dipole orientation polarization[3,26]. The AC conductivity is tested by impedance analyzer in the following test conditions: voltage is 1 V, waveform is sinusoidal, and frequency is 100~2×106 Hz. Fig. 5 (a) shows the AC conductivity of the sandwich-structured and the single-layer composite films which were filled BFT powders with diverse quantities at 100 Hz. Obviously, as expressed in Fig. 5 (a),

the biggest conductivity in all composite films isn't achieved to 10-8 S/m due to the PVDF matrix, which indicated that all composite films are insulated.[27]. At the same time, the AC conductivity is raised with enhancing of BFT proportion. The spacing of BFT particles is diminished exponentially with the raising of BFT loading.[28]. By comparing the AC conductivity of sandwich-structured and single layer BFT@DA/PVDF composite films, the effect of sandwich structure on the conductivity of materials is revealed. As shown, for BFT@DA/PVDF composite films, the conductivity of single-layer is higher than that of sandwich-structured at the same filling amount, because the pure PVDF layer in the middle inhibits the formation of conductive pathways. To explain this mechanism, Fig. 5 (b) shows a schematic diagram of conductive pathways are formed in composite films. The conductive pathways are formed in singer-layer composite films. When an electric field is applied to the film, the charge is transferred easily between the particles and the conductive pathways is generated. (Fig. 5(b) left). On the contrary, it is difficult to form conductive paths in sandwich BFT@DA/PVDF composite films. When an electric-field is applied to the film, the motion of the accumulated charge is prevented to form a conductive pathways due to the pure PVDF of the middle layer (Fig. 5 (b) right). Thus, sandwich-structured exhibit lower AC conductivity than single-layer at the same filling amount. The breakdown strength data of sandwich BFT@DA/PVDF composite films at room temperature were analyzed by Weibull distribution. The results were expressed in Fig. 4 (a) and 4 (b), respectively. The mathematical definition of Weibull formula is:

P ( E ) = 1 − exp(−( E / Eb ) m )

(3)

P is the accumulative breakdown possibility of the sample. The actual breakdown electric field of the sample is expressed as E. Eb is the breakdown electric field of the sample when the cumulative breakdown probability is 63.2%. It is called characteristic breakdown strength. The relationship between Weibull modulus distribution and linear regression fitting is regarded as m. And the reliability of the breakdown field strength measured many times by represented by m. It can be seen Fig. 6(a) that the m value of all samples higher than one.

Thus, the breakdown strength of sandwich-structured BFT@DA/PVDF flexibility composite films has a high reliability. As show in Fig. 6(b), With increasing of BFT powder content, the breakdown strength of sandwich-structured BFT@DA/PVDF flexibility composite films decreased. It was attributed to the fact that when more fillers were loaded in composite films, the dispersion of particles decreases, the bonding strength was low and the density was high, which greatly increased the probability of structural defects such as porosity, microvoids and cracks in composite films[29]. The average breakdown electric field of pure PVDF reached 278 MV/m. The composite films loaded with 1 vol.% filler had a larger average breakdown electric field of 249 MV/m., which is in favor of its energy storage properties[1]. Energy storage density is calculated using hysteresis loop integral. As a result, the results show that the best energy density of BFT filler with 1 vol.% is 4.87 J/cm3 of 250 MV/m in all composite films (Fig. 6(c) and (d)). It is essential that the energy density in a lower electric field of 150 MV/m was up to 1.93 J/cm3. The discharge energy efficiency is also shown in Fig. 6 (d). At 280 MV/m, the energy storage efficiency of PDVF membrane is 33%. And in the 1 vol.% BFT / PVDF composite film with the maximum energy density, the energy storage efficiency is 39% at 250 MV/m, and is 68% at 150 MV/m. It will serve important significance in expanding the application fields of composite films. The energy density was bigger than single-layered BFT@DA/PVDF flexibility composite films in our previous work[22]. The energy storage densities of other polymer-based composite membranes at 150 MV/m are compared as shown in Table 1. Compared with other composite films, the sandwichstructured composite films were prepared in this work have higher energy density with smaller filling volume in the same lower electric field. This is conducive to reducing costs in large-scale production. 4. Conclusions Sandwich-structured BFT@DA/PVDF flexible composite films are prepared by solutioncasting method. According to SEM image, the film thickness of all components is about 15 µm, and the thickness of each layer is about 5 µm. High permittivity and low AC conductivity are possessed by composite films. The high permittivity is caused by the large permittivity of

BFT and the interface overpotential between ceramics and polymers. The reason for low AC conductivity is that the formation of conductive pathways is inhibited by the presence of pure PVDF in the middle layer. At low electric field of 150 MV/m, the energy density of sandwich-structured composite films loaded with 1 vol.% filler of BFT is 1.93 J/cm3. When the breakdown electric field of the sample is 250 MV/m, the energy storage density can be achieved to 4.87 J/cm3. The sandwich-structured BFT@DA/PVDF flexible composite films with outstanding energy storage properties are expected to be used in wearable devices in the future. Conflict of interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The present work was supported by National Natural Science Foundation of China (5157 2160), the Natural Science Foundation of Shaanxi Province (2017KJXX44), China Postdoctoral Science Foundation (2016T90881) and Graduate Innovation Fund of Shaanxi University of Science and Technology. References [1]

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Figure caption Fig. 1. Fabrication of sandwich-structured BFT@DA/PVDF composite films. Fig. 2. XRD patterns of PVDF powders, PVDF films, composite films with different filling and BFT nanopowers Fig. 3. SEM pictures and the corresponding EDS mappings of sandwich-structured BFT@DA/PVDF flexibility composite films loaded 7 vol.% fillers Fig. 4. Dependence of permittivity and dielectric loss tangent on the frequency of sandwichstructured BFT@DA/PVDF composite films Fig. 5. (a) AC conductivity of the sandwich-structured and the single-layer composite films with filled diverse loading of BFT; (b) mechanism diagram of conductive pathways in composite film Fig. 6. (a) failure possibility of electric breakdown inferred from Weibull distribution, (b) breakdown field strength, (c) electric polarization-electric field (P-E) loops and (d) discharge energy

density

and

discharge

efficiency

sandwich-structured

BFT@DA/PVDF

nanocomposite films Table 1 Comparison of energy density for dielectric composite films with different fillers at 150 MV/m

Fig.1

Fig.2

Fig.3

Fig.4

Fig.5

Fig.6

Table 1 Structured

single layer

sandwich

Composite films

Energy Density(J/cm3)

References

2.5 % NF-APS BSTO3/PVDF

1.50

[30]

2.5% BT NF-APS/PVDF

1.63

[31]

1% BFT/PVDF

1.81

[22]

3% NaNbO3/PVDF

1.52

[25]

5%CoFe2O4@BZT-BCT/PVDF

1.83

[32]

1% NH2-MWNT/PI

1.61

[33]

1% BFT/PVDF

1.93

This work

1. 2. 3.

The Ba(Fe0.5Ta0.5)O3@DA/PVDF flexible composite films with sandwich-structured are prepared by solution-casting method. The formation of conductive paths in Ba(Fe0.5Ta0.5)O3@DA/PVDF composite films is inhibited due to sandwich structure. The sandwich structure enables the composite films to obtain a high energy storage density at low electric field.

Zhuo Wang obtained her PhD degree in Material Science and Engineering from Zhejiang University in 2010, and she joined the School of Materials Science and Engineering, Shaanxi University of Science and Technology. She has worked as an Associate Professor at the Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials since 2010. Her work has concentrated on interface design, polymer based dielectrics for energy storage and ferroelectric ceramics.

Conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.