PVDF-Based Dielectric Composite Films with Excellent Energy

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PVDF-Based Dielectric Composite Films with Excellent Energy Storage Performances by Design of Nanofibers Composition Gradient Structure Yue Zhang, Qingguo Chi, Lizhu Liu, Tiandong Zhang, Changhai Zhang, Qingguo Chen, Xuan Wang, and Qingquan Lei ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01306 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 9, 2018

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ACS Applied Energy Materials

PVDF-Based Dielectric Composite Films with Excellent Energy Storage Performances by Design of Nanofibers Composition Gradient Structure Yue Zhang,†,‡ Qingguo Chi,*‡,§,∥ Lizhu Liu,†,‡ Tiandong Zhang,*‡,§ Changhai Zhang,‡ Qingguo Chen,‡,§ Xuan Wang,‡,§ and Qingquan Lei‡ †School

of Materials Science and Engineering, Harbin University of Science and Technology,

52 Xuefu Road, Harbin, Heilongjiang 150080, P. R. China ‡Key

Laboratory of Engineering Dielectrics and Its Application, Ministry of Education, Harbin

University of Science and Technology, 52 Xuefu Road, Harbin, Heilongjiang 150080, P. R. China §School

of Electrical and Electronic Engineering, Harbin University of Science and Technology,

52 Xuefu Road, Harbin, Heilongjiang 150080, P. R. China ∥State

Key Laboratory of Electronic Thin Films and Integrated Devices, University of

Electronic Science and Technology of China,4 2nd Section on Jianshe North Road, Chengdu, Sichuan 610054, P. R. China

1

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ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: The dielectric polymer-based films with excellent energy storage properties have been considered as potential candidates for flexible capacitors. In this study, the hierarchical gradient structures of the 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 nanofibers (BZCT NFs) are designed in the polyvinylidene fluoride(PVDF) composite films. The composite films with different compositions BZCT NFs gradient structure were prepared by layer-by-layer electrospinning, hot-pressing and quenching processes. The microstructures and electrical properties have been studied, and the results indicate that the BZCT NFs gradient structures can simultaneously improve the polarization and the breakdown strength of the PVDF-based composite films, leading to the enhancement of energy storage density (9.8 J/cm3) compared to that of pristine PVDF films (4.2 J/cm3). This work provides an efficient route to improve the energy storage properties of polymer-based composite films by structure modulate. KEYWORDS: Electrospinning, gradient structure, breakdown strength, energy storage, composite film, polyvinylidene fluoride 1.INTRODUCTION Energy storage technology is essential element in advanced electrical power systems.1-5 Compared to the electrochemical methods of batteries and supercapacitors, the dielectric capacitors possess the higher power density and attracted much more attentions.1,6,7 Especially for the flexible dielectric capacitors which exhibit excellent insulation and energy storage properties, can be considered as promising components in electronics and electrical power systems for commercial, aerospace, and military fields. 2


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In principle, the energy storage density (U) of the dielectrics is governed by

U   EdD ,

(1)

where E is applied electric field and D is electric displacement. Further, the D can be determined by the electric polarization (P) and the dielectric constant (εr) as

D  P   r 0 E ,

(2)

where ε0 is vacuum permittivity of 8.85×10-12 F/m. As a special case of linear dielectrics, U can also be expressed by U

1 1 DE   r 0 E 2 . 2 2

(3)

Hence, it is significant to enhance the polarization and the dielectric breakdown strength (Eb) for optimizing the energy storage density.8-10 In particular, the influence of Eb on energy storage performance is more significant than P or εr according to Eqs. (3). Notably, the inorganic nanofiller/polymer matrix dielectric composites have attracted wide attention and interest in recent years. Nevertheless, with the ever-increasing requirements for high energy storage dielectrics, the lower Eb, εr or P limits the improvement of the energy storage density and the development of high energy storage dielectric capacitors.11,12 Besides, there is still a critical challenge that the increased εr of polymer dielectrics, such as doping with inorganic fillers with high dielectric constant, always obtains at the expense of Eb for high-performance energy storage dielectrics.13 The degradation of Eb may be caused by the following factors: (i) the imperfections caused by large loading of inorganic fillers, such as agglomeration introduced by the great surface energy of nanoparticles or voids induced by the poor compatibility of inorganic fillers 3



with organic matrix.14 (ii) the electric field distortion and local electric field concentration at the interface due to the large dielectric difference between inorganic fillers and organic matrix.15 Based on the above-mentioned, it is still a challenge for polymer composite to improve the dielectric constant and the electric breakdown strength simultaneously. Currently, previous studies have suggested that the improved εr or P for dielectrics can be achieved by the introduction of inorganic perovskite ceramic with huge dipole polarization.16-19 Zhang et al. obtained the polyvinylidene fluoride (PVDF) composite loading with poly(dopamine) (PDA) encapsulated BaTiO3 (BT) nanoparticles with discharged energy density of 2.9 J/cm3 at breakdown strength of 250 kV/mm.20 Luo et al. proposed a poly(vinylidenefluoride-hexafluoropropylene) (P(VDF-HFP)) composite combined with BaTiO3 nanowire with discharged energy density of 7.5 J/cm3 at 300 kV/mm.21 Huang et al. synthesized a copolymer PBNPF-b-PBHPF-(b-PTNP)2 with 8.99 J/cm3 at 300 kV/mm.22 Although the energy storage density of composite materials can be improved by introducing the inorganic fillers, the lower Eb is another key factor that limit the further improvement of energy storage density. In order to enhance the Eb of polymer-based dielectrics, the following ways are usually adopted: (i) introduction of one-dimensional nanofibers, which can effectively twist the paths in during breakdown process.23-27 For instance, Zhai et al. reported that one-dimensional (1D) nanofibers with large dipole moments was introduced into polymer matrixes to achieve high energy storage capacity.28 (ii) optimization of the compatibility and dispersion of inorganic fillers with polymer matrix.20,29 For instance, Dang et al. fabricated the core-shell 4


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structural nanoparticles to optimum dispersion state of inorganic fillers in polymer.30 (iii) design of multilayered polymer films based on interfacial engineering, which can block the expansion of electrical treeing because of the redistribution of electricfield.31-35 For instance, Hu et al. fabricated a multilayer poly(vinylidene fluoride)-based film with outer layer with TiO2 nanoparticles and central layer with Bi2O3-doped Ba0.3Sr0.7TiO3 nanofibers by layer-by-layer casting process, and the extractable energy density of which was ~8 J/cm3 at 300 kV/mm.36 In this study, we propose a promising method for fabricating the PVDF-based dielectric

composites

doped

with

the

hierarchical

gradient

structured

0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 nanofibers (BZCT NFs) multilayered structure. Because the lead-free 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BZCT) ceramic is widely used as a filling phase in the reinforced energy storage performance of composite. Importantly, it is well known that the BZCT ceramic always possesses excellent dielectric constant (εr) of ~3200 with pseudo-perovskite structure.37,38 Moreover, the composite films were prepared by layer-by-layer electrospinning, hot-pressing and quenching processes. By regulating the sequence of the fillers gradient, both of improved P and Eb values have been obtained in the composite films, leading to the significantly enhanced energy storage density compared to that of pristine PVDF films and homo-dispersed BCZT/PVDF films. The results indicate that constructing the structure of the fillers gradient distribution in the composite films is an efficient route to improve the energy storage properties of polymer-based capacitors. 5



2. EXPERIMENTAL SECTION PVDF polymer was provided by Shanghai 3F New Material Co., Ltd. Polyvinylpyrrolidone (PVP, Mw=1,300,000) obtained from Aladdin. Zirconium acetyl acetone (C20H28ZrO8) were purchased from Alfa Aesar Chemical Co., Ltd. Calcium hydroxide (Ca(OH)2), barium hydroxide octahydrate (Ba(OH)2·8H2O), tetrabutyltitanate (Ti(OCH2CH2CH2CH3)4) and acetyl acetone (C5H8O2) were offered by Shandong Sinocera Functional Material Co., Ltd. Dimethylformamide (DMF), ammonia (28%), acetic acid and ethyl alcohol were supplied by Sinopharm Chemical Reagent Co., Ltd. And all the chemicals were without further purification and analytical grade. Deionized water was used in all experiments. The BZCT nanofibers were prepared via the sol-gel and electrospinning process, which can be referred our previous work.8,10,17 Precursor solutions were prepared using sol-gel method. The Ba(OH)2·8H2O and Ca(OH)2 were dissolved in acetic acid, stirring at 60 °C for a stable precursor solution (designated Sol A). Then C20H28ZrO8 and Ti(OC4H9)4 were dissolved into C5H8O2, stirring at room temperature for a stable precursor solution (designated Sol B). After Sol A was cooled down to the room temperature, the Sol A was added into the Sol B, continuously stirring for a stable mixture solution (designated Sol C). Finally, the PVP were added into Sol C. After stirring for 12 h, the BZCT precursor solution was sucked into a syringe and electrospun, and the applied electric field under the needle and the receiver was about 2 kV/cm, meanwhile, the propulsion speed of the solution was 0.05 mm/min. And then calcinated in air at 750 °C for 3 h to get the 6


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pseudocubic perovskite crystal structure of BZCT NFs with high aspect ratio. The different compositions of BZCT NFs (0 vol. %, 2.5 vol. %, 7.5 vol. % and 10 vol. % in polymer matrix) were fully mixed with 3g PVDF in 30 mL DMF. Then the BZCT NFs and PVDF dispersion solution in DMF was sucked into a syringe and electrospun, and the applied electric field under the needle and the receiver was about 2 kV/cm, meanwhile, the propulsion speed of the solution was 0.5 mm/min. Then the fiber gradient composite was obtained by tuning the composition of BZCT NFs, for example, the outer layer is 0 vol% BCZT/PVDF, the next layer is 2.5 vol% BCZT/PVDF, 5 vol% BCZT/PVDF, 7.5 vol% BCZT/PVDF and 10 vol% BCZT/PVDF, successively, and then, we reversed the growth sequences from10 vol% BCZT/PVDF to 0 vol% BCZT/PVDF by the layer-by-layer electrospinning process. At last, the composite films endured the hot-pressing process at 180 °C and 15 MPa for 30 min, followed by the quenching process to room temperature. And the preparation for composite films by layer-by-layer electrospinning and hot-pressing is shown in Scheme 1. Finally, the obtained fiber gradient composite films was defined as 0-10-0 BCZT/PVDF. Similarly, the oppositely composition of BZCT NFs in PVDF was also fabricated by regulating the growth sequence, which was defined as the fiber gradient 10-0-10 BZCT/PVDF composite. For comparison, the pristine PVDF and fiber random 5 vol% BZCT/PVDF films were also prepared.

7



Scheme 1. The preparation for composite films by layer-by-layer electrospinning and hot-pressing.

The X-ray diffraction (XRD) analysis was performed by the PANalytical Empyrean for getting the crystal structure information of BZCT and PVDF-based films with Cu Kα at 40 kV and 40mA. Fourier-transform infrared spectroscopy (FTIR) was detected by the FTIR spectrometer (FT/IR 6100, SHIMADZU). The morphology information (for instance, the size and dispersion of nanofibers in PVDF matrixes) of series of PVDF-based composites were collected by field emission scanning electron microscope (FE-SEM, Hitachi SU8020 Uhr). For electrical performance measurements, the aluminum electrodes (3 mm and 25 mm in diameter) were evaporated on two sides of PVDF-based composites for the following electric measurements. Dielectric properties were collected at the frequency range of 10 Hz to 1 MHz via the broadband impedance analyzer (GmbH Novocontrol Alpha-A). And the PVDF-based composites were subjected to the electric breakdown strength tests via YDZ-560. Radiant Premier II Ferroelectric Test System was used to test the electric polarization-electric field (P-E) hysteresis loops and the current-voltage (I-V) behavior of PVDF-based films at frequency of 10 Hz. 3. RESULTS AND DISCUSSION 8


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The XRD patterns of the BZCT NFs fillers and the BCZT/PVDF composite films are shown in Figure 1(a). It can be seen from the figure that the inorganic ceramic BZCT NFs with perovskite structure are observed clearly and without other phases, which are well indexed to the (100), (110), (111), (200), (210), (211), (220), (221), (310), (311) and (322) planes, which can be indexed by comparing with a standard BaTiO3 pattern (PDF no. 01-075-2116), indicating the solid solution perovskite structure of BZCT system is formed.37,39-41 Meanwhile, the high-aspect-ratio perovskite BZCT NFs in fiber random 5 vol% BZCT/PVDF, fiber gradient 0-10-0 and fiber gradient 10-0-10 BZCT/PVDF composites with different structures can be indexed from the XRD patterns and without any other inorganic phases, which reveal the existence of inorganic crystalline phase of BZCT in these nano-composites. Besides the diffraction peaks of semicrystalline PVDF polymer are also indexed. And three distinct main peaks locate at around 2θ=18.5°, 20.2° and 26.6°, which can be assigned to the (020) (110) and (022) planes of non-polar γ-phase [in trans-trans-trans-gauche chain conformation (TTTG)] (PDF no. 00-038-1638) for PVDF originated from the annealing and quenching processing.8,11,17,41,42 In addition, two minor peaks locate at around 2θ=17.8°, 18.4° and 20.1°, which can be assigned to the (100) (020) and (110) planes of non-polar α-phase [in trans-gauche chain conformation (TGTG)] (PDF no. 00-038-1638) for PVDF.8,11,17,41,43 Meanwhile, as shown in Figure 1(b), it is confirmed that the FTIR results of pristine PVDF films, fiber random 5 vol% BZCT/PVDF films, fiber gradient 0-10-0 and fiber gradient 10-0-10 BZCT/PVDF films are consistent with the XRD pattern results of the PDF no. 00-038-1638 for non-polar 9



α-phase PVDF and the PDF no. 01-075-2116 for γ-phase PVDF, indicating that the composite films mainly have the non-polar α-phase and γ-phase PVDF.44-46 According to the Figure 1(c) of SEM morphology of BZCT NFs have a diameter of around 200 nm and a length of around 15 μm with a large aspect ratio > 60.

Figure 1. (a) XRD patterns of BZCT NFs, pristine PVDF films, fiber random 5 vol% BZCT/PVDF films, fiber gradient 0-10-0 and fiber gradient 10-0-10 BZCT/PVDF films, and the inset is the partial enlargement at the diffraction angle range of 17-21°, (b) FTIR spectra of pristine PVDF films, fiber random 5 vol% BZCT/PVDF films, fiber gradient 0-10-0 and fiber gradient 10-0-10 BZCT/PVDF films, (c) SEM morphology of BZCT NFs (the bar is 2 μm).

As shown in Figure 2(a), it can be seen that the PVDF films have the smooth section with no obvious impurities or other defects. Figure 2(b) shows that BZCT NFs are evenly 10


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dispersed in the PVDF matrix and have a good compatibility with the matrix, and there is no obvious agglomeration phenomenon or voids for fiber random 5vol% BZCT/PVDF films. Figure 2(c) exhibits that the gradient layer gradually transits form the pristine PVDF layer to the 10 vol% BZCT/PVDF layer, and further transits to the pristine PVDF layer. Meanwhile, the transition layers are also evident in the reverse fiber gradient 10-0-10 BZCT/PVDF film shown in Figure 2(d). And we can observe that the BZCT NFs are uniformly dispersed in the PVDF matrix without obvious agglomeration or imperfection in all the three BZCT/PVDF composites.

Figure 2. Cross-sectional SEM images of pristine PVDF film (a), fiber random 5 vol% BZCT/PVDF film (b), fiber gradient 0-10-0 (c) and fiber gradient 10-0-10 (d) BZCT/PVDF films, and the bar is 5 μm. 11



The curves of the dielectric properties vs. frequency are presented in Figure 3. As seen from Figure 3(a), the introduction of inorganic BZCT NFs can significantly improve the dielectric polarization (P) and dielectric constant (εr) compared with pristine PVDF films, for instance, the εr of the fiber gradient 10-0-10 BZCT/PVDF, the fiber gradient 0-10-0 BZCT/PVDF, the fiber random 5 vol% BZCT/PVDF and the pristine PVDF composite are up to 25.6, 23.8, 21.2 and 9.3 at 10 Hz, respectively. The εr of the gradient composite films is higher than that of fiber random 5vol% BZCT/PVDF composite, which may be due to the special gradient structure of these composites, because the gradient multilayer structure ferroelectric may exhibit a colossal dielectric response due to the interlayer electrostatic interactions.47 The εr of four kinds of films decreases with the increase of frequency, which may be due to the relaxation phenomenon occurring in the dielectric composites at high frequencies, and the dipole polarization need to take a long time, leading to the increase of the dielectric loss of composites. Nevertheless, the dielectric loss of four kinds of films remains at a low level (

PVDF-Based Dielectric Composite Films with Excellent Energy

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