Excellent Energy Storage of Sandwich-Structured PVDF-Based

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Excellent energy storage of sandwich-structured PVDF-based composite at low electric field by introduced the hybrid CoFe2O4@BZT-BCT nanofibers Qingguo Chi, Tao Ma, Yue Zhang, Qingguo Chen, Changhai Zhang, Yang Cui, Tiandong Zhang, Jiaqi Lin, Xuan Wang, and Qingquan Lei ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02659 • Publication Date (Web): 23 Nov 2017 Downloaded from http://pubs.acs.org on November 23, 2017

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ACS Sustainable Chemistry & Engineering

Excellent energy storage of sandwich-structured PVDF-based composite at low electric field by introduced the hybrid CoFe2O4@BZT-BCT nanofibers Qingguo Chi,,†,‡,§ Tao Ma,† Yue Zhang,† Qingguo Chen,,† Changhai Zhang,† Yang Cui,† Tiandong Zhang,† Jiaqi Lin,† Xuan Wang,† and Qingquan Lei† †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 ‡ 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 §Key

Laboratory of Functional Materials Physics and Chemistry, Ministry of

Education, Jilin Normal University, 1301 Haifeng Street, Siping, Jilin 136000, P. R. China *Corresponding

author: Tel./Fax: +86-451-86391681; E-mail: [email protected];

[email protected].

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ABSTRACT The high-performance energy-storage dielectric capacitors are increasingly necessary for the development of miniaturization, integration and multifunctionality of electronic device. Here, we describe a new strategy of sandwich-structured polymer-based

dielectric

composite

with

inorganic

fillers

of

semiconductor@perovskite hybrid fibers, and this novel dielectric composite possesses an excellent energy-storage performance at low electric field. It is of crucial importance

to

achieve

the

hybrid

nanofibers

of

0.5Ba(Zr0.2Ti0.8)O3-

0.5(Ba0.7Ca0.3)TiO3 nanofibers (BZT-BCT NFs) deposited by CoFe2O4 nanoparticles (CFO) (hereafter, CFO@BZT-BCT NFs in short). Herein, the BZT-BCT NFs ceramic has the typical perovskite structure and the large dielectric constant, which is used as the ceramic-support for CFO. Meanwhile, the semiconductor of CFO works as the electron donor to offer a great interfacial polarization for the improvement of overall dielectric constant of composite. Remarkably, the trilayer structure is composed of outer poly(vinylidene fluoride) (PVDF) layer to improve the breakdown strength and middle CFO@BZT-BCT NFs-PVDF nanocomposite to enhance the dielectric properties. The BZT-BCT NFs were prepared by electrospinning, and then the CFO@BZT-BCT NFs were gained by hydrothermal method. Furthermore, the BZT-BCT NFs and the CFO@BZT-BCT NFs were modified by polydopamine (PDA). Finally, the sandwich-structured composites were gotten by a typical process of solution casting and hot pressing. The influences of fillers’ volume fraction and type on the performances of composites have been systematically investigated. The 2


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PVDF/CFO@BZT-BCT NFs-PVDF/PVDF composite (P/CFO@BZT-BCT NFs-P/P) exhibits an outstanding energy density performance under a low electric field. The trilayer structure composite with an optimized content of nanofibers possesses an excellent dielectric performance (dielectric constant ~20.1 at 100 Hz) and great energy storage performance (electric displacement ~10.7 μC/cm2, discharged energy density ~11.3 J/cm3 and efficiency ~55.5% at a low electric field of 350 kV/mm). This work paves the avenue for potential applications in integrated electronic devices. KEYWORDS: poly(vinylidene fluoride) (PVDF), dielectric material, electric polarization, dielectric properties, energy density 

INTRODUCTION

The questions of sustainability, energy and environment are the undoubted challenges in any period of development of human civilization. Nowadays, to build a harmonious home, researchers need to solve the key issues of how to effectively store energy, reduce energy loss and mitigate the environmental burden. While, the energy storage is an important research part of energy development, which generally includes the batteries, electrochemical supercapacitors and dielectric capacitors. The batteries usually possess a great energy density, but they have a small power density and are also harmful to the environment.1 At the same time, the electrochemical supercapacitors hold a medium energy storage density and power density, but also have a complex structure, a low operating voltage, a large leakage current and short cycle time.2-3 As compared with the batteries and electrochemical supercapacitors, the 3



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dielectric capacitors own the advantage of fast charge/discharge rate, long cycle time, environmental friendliness, which are also widely used in pulsed power devices. However, energy storage density of dielectric capacitors is relatively low. 4-6 Therefore, how to further improve the energy storage density of dielectric capacitors is the main problem for the development of dielectric capacitors. Polymer-based dielectric composite has an attracted worldwide attention for the potential applications in future flexible and integrated electronic devices.7-9 Especially, the dielectric composite with high energy storage, large dielectric constant and low dielectric loss under low electric field is increasingly required in modern high-performance power electronic system.10-12 Many researchers have carried out a series of experimental and theoretical studies on this field to explore or optimize an ideal high energy storage dielectric material.13-18 A typical formula can be given to express the relationship of energy storage performance with the electric field strength and intensity of polarization for the nonlinear dielectric materials as Ue  

0

Dmax

EdD ,

where Ue, E and D represent the energy storage density, the electric field strength and the electric displacement. While the D can be described as D=P+εrε0E, in which P and εr mean the electric polarization and the relative permittivity, respectively. Therefore, it can be seen clearly from these expressions that the electric polarization, dielectric constant and electric field strength significantly influence on the energy storage performance of dielectric materials. At present, the research on the energy-storage dielectric materials with the PVDF-ferroelectric-polymer has attracted much more considerable interests. 4


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Compared with the commercial biaxially oriented polypropylene (εr~2.2, Ue~2 J/cm3),19 the energy density of the PVDF with γ-phase (trans-trans-trans-gauche (TTTG) chain conformation) exhibits as high as 6 J/cm3 at 350 kV/mm, and the dielectric constant of PVDF is up to 8 at 100 Hz.15-17, 20 Obviously, the energy density and the electric field strength of γ-phase PVDF is greater than that of inorganic dielectric ceramics under a low voltage,21-22 but the dielectric constant of PVDF is much lower than that of inorganic ceramics. More remarkably, the γ phase can be transformed from β phase by quenching process,20,23 which offers an inspiration for the preparation of PVDF based dielectric composite. Recently, the dielectric nanocomposite, achieved by introduction of the inorganic fillers into one kind of polymer matrix, possesses the wondrous performances over any single component of the filler or the matrix.24-26 Meanwhile, the dielectric properties and the energy storage performance of the composite can be tailored by adding fillers with different structure and types.27 However, the discharged energy density of polymer-based nanocomposite is not improved apparently because of the deteriorated breakdown strength by the introduction of inorganic fillers.28-30 Moreover, the large doping content of fillers damages the comprehensive performance of the dielectric materials, because some voids, pores and other macro-structural imperfections are induced into the matrix. Besides, the agglomeration of nanoparticles can happen and cause the rapid accumulation of internal carriers, which are harmful to reinforce energy storage performance of materials.8,31 In addition, another barrier in the way of enhancing overall performance is to relieve the difference in dielectric 5



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property between the matrix and the fillers. There is still much work to be done around above mentioned issues. Fortunately, three promising approaches can be carried out to improve the energy storage performance of dielectric materials. The first point, what matters is that the multilayer structure can be chosen to reduce breakdown probability and reinforce breakdown strength of composite due to the redistribution

of internal electric

field.32

Another effective

measure,

the

one-dimensional inorganic fillers can be chosen to combine with the polymer matrix to reduce the doping content of fillers and to twist the growth of electrical treeing at low electric field.29 More important step, the surface modification for high dielectric ceramics can be employed to improve the compatibility between the ceramic and the polymer, which is beneficial to avoid aggregation of nanofillers and to relieve the dielectric difference.31 These clever ideas provide us with more highlights. Inspired by the above mentioned literatures, we have carried out the following research work in this paper. A novel strategy of sandwich-structured PVDF-based dielectric nanocomposite with one-dimensional BZT-BCT NFs decorated by CoFe2O4 nanoparticles was proposed in the present study. The hybrid nanofibers combining the high-polarization perovskite BZT-BCT fibers and the semiconductor of CoFe2O4 nanoparticles were prepared by the electrospinning technique and the hydrothermal synthesis. The introduction of CoFe2O4 nanoparticles onto the surface of BZT-BCT fibers was intended to increase the number of carriers to further improve the polarizability and interfacial polarization of composite under the low applied voltage. Simultaneously, the trilayer dielectric composite was achieved by the typical solution 6


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casting and the hot-pressing process. The purpose of design of sandwich structure for the composite is to build the redistribution of internal electric field among each layer, which can reinforce the breakdown strength of composite. The PDA, which worked as the modifier for the inorganic-filler, could improve the compatibility and avoid the aggregation of the hybrid nanofibers in the composite. Meanwhile, the influences of content and type of the nanofillers on the performances of composites were systematically investigated. This present work is expected to paves the avenue for preparation of polymer-based dielectric material with the excellent energy storage performance at a low electric field for energy-saving sustainability in modern electrical and electronic industry. 

EXPERIMENTAL Materials. The Poly(vinylidene fluoride) (PVDF) was provided by Shanghai 3F

New Material Co., Ltd., China. The polyvinylpyrrolidone (PVP) was purchased from Alfa Aesar. Dopamine hydrochloride and tris-(hydroxymethyl)-aminomethane (Tris) were offered by Aladdin. The Ba(OH)2·8H2O, Ca(OH)2, CoCl2·6H2O and FeCl3·6H2O were obtained from Tianjin TIANLI Chemical Reagents Ltd., China. All other chemicals were supplied by Sinopharm Chemical Reagent Co., Ltd., China. And all the chemicals were analytical grade and were used as received without further purification. Preparation of CFO@BZT-BCT nanofibers. The BZT-BCT nanofibers were prepared by the sol-gel and electrospinning method, which could be seen in the 7



supplementary

information.

The

CoFe2O4@BZT-BCT

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NFs

(hereafter,

CFO@BZT-BCT NFs in short) were produced by the sol-gel, electrospinning and hydrothermal synthesis method. To make the hybrid nanofibers, BZT-BCT nanofibers were firstly achieved by above mentioned method. Then the BZT-BCT nanofibers were dissolved in 1 g/ml NaOH solution for ultrasonic dispersion to get the suspension A solution. Afterwards CoCl2·6H2O and FeCl3·6H2O powders were dissolved in deionized water and stirred for 30min to get the mixed solution with mole ratio of Fe3+ : Co2+ = 2:1, as B solution. Finally, the solution B was slowly dropped into the suspension A, and the mixed solution was continuously stirred for 20 min to obtain a homogeneous reaction solution.

Figure 1 Schematic preparation process of CoFe2O4@BZT-BCT NFs.

The homogeneous mixture solution was slowly transferred into the 50 ml the stainless steel autoclave with the Teflon-liner and was kept at a temperature of 180 °C for about 12 h. Then the reactor was cooled to room temperature, finally, the CFO@BZT-BCT NFs hybrids are obtained after centrifuging and washing with distilled water to the neutral pH, and then drying at 80 °C for 8 h. The schematic preparation process of CoFe2O4@BZT-BCT NFs can be seen from Figure 1.

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Surface modification for BZT-BCT NFs or CFO@BZT-BCT NFs. To make the fibers possess an excellent dispersion and compatibility, the PDA was coated on the surface of nanofibers.31 Tris-HCl buffer agent was first dissolved in deionized water at a concentration of 2 mg/ml (pH=8.5). 0.1 g PDA was dissolved in last solution to obtain the PDA solution. Then the BZT-BCT NFs or CFO@BZT-BCT NFs were immersed into above PDA solution in the mass ratio of dopamine and modified nanofibers for 1:10 and stirred vigorously at room temperature for 12 h. Followed by, the BZT-BCT NFs or CFO@BZT-BCT NFs modified by PDA was gained by centrifuged, washed and dried in a vacuum oven at 80 °C for 12 h. Preparation of sandwich-structured dielectric composites. The Preparation of monolayer BZT-BCT NFs/PVDF or CFO@BZT-BCT NFs/PVDF composite can be browsed in the supplementary information. Typically, the sandwich-structured dielectric composite was fabricated by hot-pressing. Firstly, outer monolayer was PVDF, and middle monolayer was the BZT-BCT NFs/PVDF or the CFO@BZT-BCT NFs/PVDF composite. After that, these three layers were stacked together, followed by hot-pressing at 170 °C for 10 min. Then the hot sandwich-structured films were cooled to room temperature quickly. Finally, the sandwich-structured PVDF/x vol.% BZT-BCT NFs-PVDF/PVDF and PVDF/x vol.% CFO@BZT-BCT NFs-PVDF/PVDF (hereafter, P/x vol.% BZT-BCT NFs-P/P and P/x vol.% CFO@BZT-BCT NFs-P/P in short) (x=0, 1, 3, 5 and 7) composite. The typical thickness of outer layer is around 7 μm, and the thickness of middle layer is ~ 10 μm. Characterization. X-ray diffraction (XRD) analysis was performed with a PANalytical Empyrean, and all data were analyzed through HighScore Plus software. Field emission scanning electron microscope (FE-SEM) tests were carried out through 9



Hitachi SU8020 Uhr. Transmission electron microscope (TEM) images were obtained under a FEI TECNAI2-12. Aluminum electrodes with diameter 3 mm and 25 mm were evaporated on two sides of the composites for the electrical measurements. Dielectric properties were collected using a broadband impedance analyzer of GmbH Novocontrol Alpha-A in the frequency range of 100 Hz-1 MHz. Dielectric breakdown strength tests were conducted by using a precision DC power supply test equipment YDZ-560. Electric displacement-electric field (D-E) loops were acquired over a Radiant Premier II Ferroelectric Test System at frequency of 10 Hz. Finite element simulation was performed on the COMSOL Multiphysics. 

RESULTS AND DISCUSSION

A series of sandwich-structured PVDF/BZT-BCT-PVDF/PVDF (P/BZT-BCT NFs-P/P in short, hereafter) composites were prepared by solution casting and hot-pressing. In order to investigate crystalline structure of the P/BZT-BCT NFs-P/P nanocomposites with different volume percent, the X-ray diffraction (XRD) patterns of the films are presented in Figure S1 in the supporting information. Which indicate that the major phase in the matrix of the sandwich-structured composite films is a mixture of non-polar γ, α and polar β phase. Obviously, the crystalline structure of PVDF is not influenced by introducing the BZT-BCT nanofibers. Meanwhile, all other diffraction peaks appearing in Figure S1 could be indexed to the BZT-BCT nanofibers, the pure perovskite structure of ABO 3,33 and the larger volume percent of the BZT-BCT nanofibers is introduced into the matrix, the more obvious the peak of the BZT-BCT nanofibers can be found. Meanwhile, the SEM of cross-sectional morphology for the sandwich-structured P/x vol.% BZT-BCT NFs-P/P (x=1, 3, 5, 7) 10


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composite (Figure S2 in the supporting information) can been observed and analyzed in the supplementary information. Figures 2(a1) and (a2) show the dielectric properties of the sandwich-structured P/BZT-BCT NFs-P/P nanocomposites with different volume contents of BZT-BCT NFs, respectively. It is worth noting that the dielectric constant is sharply increased from ~9.7 of the P/P/P film to ~16.9 of the P/7 vol.% BZT-BCT NFs-P/P nanocomposite. These results indicate that the large aspect ratio of BZT-BCT NFs is conductive to enhance the dielectric property of the composite, which possess the greater polarization and higher dielectric constant.

Figure 2 The relationship between dielectric properties and frequency and the energy storage performance of the sandwich-structured P/x vol.% BZT-BCT NFs-P/P (x=0, 1, 3, 5, 7) composite with different filler contents at room temperature. (a 1) dielectric constant, (a2) dielectric loss, (b1) 11



discharged energy density, (b2) energy efficiency.

At the same time, as can be seen from Figure 2(a2), there is no great difference of the dielectric loss for the composites with different BZT-BCT NFs doping content when the measured frequency is lower than 3×104 Hz, the loss tangent value is less than 0.05. Especially, the dielectric loss tangent is less than 0.10 at 1 MHz. The reason for this phenomenon is that the BZT-BCT NFs block the movement of internal electronic and space charge conduction in composites, meanwhile, the high aspect ratio nanofibers hinder the rotation of organic chain in matrix and inhibit the charge carrier movement and the formation of conductive path to reduce the conduction loss of composite. Moreover, with the increase of the volume fraction of BZT-BCT NFs, the dielectric loss does not change too much. It is worth to mention that the distinctive feature is the enhancement of dielectric constant with a lower dielectric loss. In order to analyze the energy storage performance of the composite more intuitively, the relationship between the energy storage performance and the electric field of composite is obtained by calculation and the discharged energy density (b1) and energy efficiency (b2) vs. the electric field of the sandwich-structured P/x vol.% BZT-BCT NFs-P/P (x=0, 1, 3, 5, 7) at 10 Hz under a low applied voltage are given in Figure 2(b). Moreover, the relationship between the electric displacement and the electric field of the sandwich-structured P/x vol.% BZT-BCT (x=0, 1, 3, 5, 7) at a low electric field of 350 kV/mm is presented and discussed in Figure S3 in the supporting information, which shows the electric displacement of composite increases from the ~4.1 μC/cm2 for the P/P/P composite to ~5.8 μC/cm2 for P/7 vol.% BZT-BCT 12


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NFs-P/P composite. Furthermore, from the Figure 2(b1), the energy density of P/x vol.% BZT-BCT NFs-P/P composite is improved with the increase of electric field strength, however, the energy efficiency decreases slowly as the external electric field increases. The energy density of composite also increases gradually with the increased content of fillers at the same field. With the content of filling phase of BZT-BCT NFs increasing from 0 to 7 vol.%, the discharged energy density of composite also increases. While the x is 0, 1, 3, 5 and 7, the discharged energy density is 4.1 J/cm 3, 5.2 J/cm3, 6.0 J/cm3, 6.8 J/cm3 and 7.4 J/cm3 at low electric field of 350 kV/mm, respectively. The introduction of the BZT-BCT NFs has a higher polarization intensity, so that the discharged energy density of the composite increases. Figure 2(b2) shows that the energy efficiency of the composite maintains at a high level (≥ 48.2%) in the low electric field range of 25-350 kV/mm. From the figure, it can be also found that the energy efficiency decreases obviously with the increase of electric field strength. Meanwhile, the energy efficiency also decreases obviously as the content of BZT-BCT NFs increases. The reduction of the energy efficiency for the composite can be ascribed to the following two aspects. Firstly, the imperfections appear in the matrix under a larger filling phase content (such as 7 vol.%), which causes the energy efficiency of composites decreases faster under the high electric field. Secondly, the BZT-BCT NFs has excellent ferroelectric properties, which leads to the increase of the hysteresis loss. According to the previously reported literatures,34-36 the growth of a little conductive nanoparticles on the surface of inorganic fillers can enhance the dielectric 13



and polarization performance of the composite under a low volume fraction, which can make the composite maintain a high breakdown strength and a good processing characteristic. On this basis, the CFO@BZT-BCT NFs were designed and synthesized here. The XRD and the transmission electron microscopy (TEM) have been employed to analyze the crystal structure, morphology and components of hybrid inorganic nanofillers. Figure 3(a) shows the XRD diffraction patterns of BZT-BCT NFs and CFO@BZT-BCT NFs. The characteristic diffraction peaks of the BZT-BCT in the hybrid of CFO@BZT-BCT NFs can be accurately indexed as pure perovskite structure and there is no other impurity phase, indicating that the crystal structure of BZT-BCT NFs is unchanged during the hydrothermal reaction of CFO grown on the surface of BZT-BCT NFs. It is worth to note that the characteristic diffraction peak (311) appeared lightly at 2θ=35.4 ° is a typical spinel crystal structure of CFO from the XRD pattern,37-41 indicating that a small amount of CFO was combined with BZT-BCT NFs successfully. However, the characteristic diffraction peak of CFO was weak because of the low relative content of CFO nanoparticles and the X ray fluorescence by using the Cu target to characterize the CFO.

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Figure 3 The XRD (a) of BZT-BCT NFs and CFO@BZT-BCT NFs and the TEM (b) of CFO@BZT-BCT NFs. (b1) High-resolution TEM image of CFO, (b2) the partial enlarged TEM of CFO@BZT-BCT NFs, (b3) CFO@BZT-BCT NFs, (b4) the EDS of CFO@BZT-BCT NFs.

In order to obtain the microstructure information, such as the size of CFO and BZT-BCT NFs, the distribution state and the growth morphology of CFO on the surface of BZT-BCT NFs, the TEM and energy dispersive spectrometer (EDS) of CFO@BZT-BCT NFs have been investigated and given in Figure 3(b). It can be seen from the Figure 3(b1) of high resolution figure that the diameter of CFO nanoparticle dispersed on the surface of BZT-BCT NFs is about 20 nm, and the CFO nanoparticles have the characteristic diffraction plane of (311), which is consistent with the XRD results. From Figures 3(b2) and (b3) can be seen, the diameter of BZT-BCT NFs is about 300 nm, and the length is about 5 μm, which indicates that the CFO@BZT-BCT NFs has a larger ratio of length and diameter. Meanwhile, the surface of BZT-BCT NFs is very smooth, and the CFO nanoparticles on the surface of BZT-BCT NFs are along with only a small local area cluster, because the CFO nanoparticles possess a larger surface area and surface energy resulting in slight agglomerate. As shown in 15



Figure 3(b3), the CFO nanoparticles discretely grows on the BZT-BCT NFs surface, and no obvious aggregation can be found in a large area, which also indicates that the CFO does not form a conductive channel among the BZT-BCT NFs. As illustrated in Figure 3(b4), the peaks of O, Ba, Ti, Fe, Co, Ca and Zr seven elements appear in the EDS of CFO@BZT-BCT NFs, which is further confirmed that CFO@BZT-BCT NFs has been successfully prepared. Figure 4(a) presents the XRD patterns of sandwich-structured P/x vol.% CFO@BZT-BCT/P (x=1, 3, 5, 7) composite. According to Figure 4(a2), the diffraction peaks of γ (020), γ (110) and γ (022) of PVDF matrix after quenching exist at 18.5 °, 20.1 ° and 26.8 °, respectively. The existence of γ phase can reduce the residual displacement (Dr) and increase the maximum displacement (Dm), which is helpful to enhance the energy density (Ue) of the composite. It can also be seen that the (100) diffraction peak of the nonpolar α phase appears at 17.9 °, and the (200) diffraction peak of the ferroelectric β phase appears at 20.7 ° in the quenched PVDF matrix. The results indicate that the phase in the sandwich-structured composite is mainly the γ phase and a small amount of non-polar α phase and polar β phase. Furthermore, each diffraction peak of the BZT-BCT NFs can be clearly indexed in the Figure 4(a), and the diffraction peak intensity gradually increases with the content of fillers increasing. The characteristic peak (311) of spinel crystal CFO with weak intensity appears at 2θ=35.4 ° in composite,37-41 indicating that a small amount of CFO successfully combined with the BZT-BCT NFs. The formed CFO@BZT-BCT NFs have been 16


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introduced into the PVDF matrix for constructing the sandwich-structured P/x vol.% CFO@BZT-BCT NFs-P/P (x=1, 3, 5, 7) composite. Obviously, the introduction of CFO@BZT-BCT NFs does not change the crystal structure of the PVDF matrix. Figure 4(b) gives the SEM of cross-sectional morphology for the P/x vol.% CFO@BZT-BCT NFs-P/P (x=1, 3, 5, 7) sandwich-structured composite. As can be seen from Figure 4(b), the thickness of outer PVDF film is about 7 μm and the thickness of middle CFO@BZT-BCT NFs-PVDF layer is about 10 μm for the sandwich-structured P/x vol.% NFs-P/P (x=1, 3, 5, 7) of dielectric composite. It is important that the sandwich-structured composite has no obvious interface between the outer PVDF and the middle CFO@BZT-BCT NFs-PVDF layer, indicating that the trilayer be combined well with each other. Meanwhile, the dopamine modified CFO@BZT-BCT NFs has good compatibility with the PVDF matrix, no obvious gaps and imperfections can be found, which may be beneficial for improving the performance of the composite.

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Figure 4 The XRD (a) and SEM (b) of the sandwich-structured P/x vol.% CFO@BZT-BCT NFs-P/P (x=1, 3, 5, 7) composite. (a1) 2θ=10-100 °, (a2) 2θ=15-38 °, (b1) x=1, (b2) x=3, (b3) x=5, (b4) x=7.

Figures 5(a1) and (a2) are the dependence of dielectric constant and dielectric loss on the frequency for the sandwich-structured P/CFO@BZT-BCT NFs-P/P composite with different volume of CFO@BZT-BCT NFs in middle layer. As shown in the Figure 5(a1), the dielectric constant of composite increases from ~12.1 for P/1 vol.% CFO@BZT-BCT NFs-P/P to ~20.1 for P/5 vol.% CFO@BZT-BCT NFs-P/P 18


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composite at 100 Hz, which is attributed to the BZT-BCT NFs possesses a higher dielectric constant and a good polarization. Besides, the enhancement of dielectric constant of hybrids CFO@BZT-BCT NFs is induced by the semiconductor CFO nanoparticles.

Figure 5 The relationship between dielectric properties and frequency and the energy storage performance of the sandwich-structured P/x vol.% CFO@BZT-BCT NFs-P/P (x=1, 3, 5, 7) composite with different filler contents at room temperature. (a 1) dielectric constant, (a2) dielectric loss, (b1) energy storage efficiency, (b2) energy efficiency.

Furthermore, the introduction of a small amount of CFO can increase the number of electrons and charges, which are conducive to increasing the quantity of dipoles and improving the polarizability for the composite. More importantly, the introduced CFO and BZT-BCT NFs for the hybrid CFO@BZT-BCT NFs enhance the interfacial 19



polarization of the composite. However, the dielectric constant of composite decreases to ~18.3 when the filling content increases to x=7. The uniformly dispersed CFO@BZT-BCT NFs can be used as a miniature capacitor. The number of micro capacitance reasonably increases when the content of BZT-BCT NFs increases. Therefore, the composites exhibit a greater dielectric constant. 42-43 However, the phenomenon of agglomeration is obvious at a larger volume fraction, so that the capacitances are difficult to be connected effectively, which lead the dielectric constant to be reduced. From the Figure 5(a2), we can see that the dielectric loss tangent of composite is less than 0.04 when the measured frequency is lower than 4×104 Hz, when the frequency is higher than 4×104 Hz, dielectric loss tangent of the P/x vol.% CFO@BZT-BCT NFs-P/P composite increases with the frequency increasing, because the dipoles cannot fellow the shift of electric field at a higher frequency, so that the dipole relaxation and the relaxation loss happen.44 Meanwhile, there is no obvious difference of dielectric loss for the composite with different CFO@BZT-BCT NFs doping content. To sum up, compared with the P/x vol.% BZT-BCT NFs-P/P sandwich-structured (x=0, 1, 3, 5, 7) composite, the dielectric properties of sandwich-structured P/x vol.% CFO@BZT-BCT NFs-P/P (x=1, 3, 5, 7) composite have an obvious increase, when the BZT-BCT NFs is optimized to the CFO@BZT-BCT NFs. Figure 5(b) shows the relationship between the energy performance of the sandwich structure P/x vol.% CFO@BZT-BCT NFs-P/P (x=1, 3, 5, 7) of the 20


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composite and the electric field at 10 Hz under a low applied voltage. Meanwhile, the relationship between the electric displacement and the electric field of the sandwich-structured P/x vol.% CFO@BZT-BCT NFs-P/P (x=1, 3, 5, 7) composite is presented and discussed in Figure S4 in the supporting information, which shows that the maximum electric displacement of P/x vol.% CFO@BZT-BCT NFs-P/P (x=1, 3, 5, 7) composite is 6.5 μC/cm2 (at 350 kV/mm), 8.2 μC/cm2 (at 350 kV/mm), 10.7 μC/cm2 (at 350 kV/mm) and 8.3 μC/cm2 (at 295 kV/mm), respectively. From the Figure

5(b),

the

discharge

energy

density

of

a

series

of

P/x

vol.%

CFO@BZT-BCT-P/P (x=1, 3, 5, 7) increases, but the energy efficiency decreases with the external electric field increasing. It can be seen from Figure 5(b1) that the magnitude of applied electric field and discharge energy density of the composite are obviously different for the composite with different CFO@BZT-BCT NFs doping content. For instance, the discharge energy density of composite increases firstly and then decreases with CFO@BZT-BCT NFs increasing. The maximum discharge energy density of P/x vol.% CFO@BZT-BCT NFs-P/P (x=1, 3, 5, 7) composite is 6.6 J/cm3 (350 kV/ mm), 8.0 J/cm3 (350 kV/mm), 11.3 J/cm3 (350 kV/mm) and 7.4 J/cm3 (295 kV/mm), respectively. As illustrated in Figure 5(b1), the displacement polarization of composite increases with CFO@BZT-BCT NFs increasing at the same electric field, which is good for enhancing the discharge energy density. But the more CFO@BZT-BCT NFs are added into the composite, the larger numbers of imperfections are produced in matrix, and the more conductive paths are formed, which are not conducive to enhance the electric displacement, and then the 21



breakdown strength of composite can be deteriorated. Therefore, the breakdown strength of P/7 vol.% CFO@BZT-BCT NFs-P/P dielectric composite decreases significantly. Figure 5(b2) shows the energy efficiency of all composites. Clearly, the energy efficiency of P/1 vol.% CFO@BZT-BCT NFs-P/P composite is 48.8%, which is lower than that of P/x vol.% CFO@BZT-BCT NFs-P/P (x=3, 5, 7) . Meanwhile, the energy efficiency of composite decreases with the applied electric field strength increasing. Although the introduction of CFO@BZT-BCT NFs improves the electric displacement intensity of polarization, simultaneously, the imperfections, hysteresis loss and conduction loss can be induced by the high doping content of inorganic fillers, resulting in the reduction of electric field strength, which is not conducive to the discharge energy density and energy efficiency. Figure 6(a) is the two-parameter Weibull distribution of sandwich-structured P/x vol.% NFs-P/P (x=1, 3, 5, 7) composite. The ln(-ln(1-P(E))) and lnE can be gained by the breakdown strength test, and the intercept and slope of the curve are represented by the Eb and β. The Weibull distribution statistical parameters and calculation of breakdown strength of the sandwich-structured P/x vol.% CFO@BZT-BCT NFs-P/P (x=1, 3, 5, 7) composite are given in the Table S1 in the supporting information. Hence, the breakdown strength of sandwich-structured P/x vol.% CFO@BZT-BCT NFs-P/P (x=1, 3, 5, 7) is 418 kV/mm, 380 kV/mm, 372 kV/mm and 304 kV/mm, respectively.

22


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Figure 6 Weibull distribution (a) and Finite element simulation (b) of the sandwich-structured P/x vol.% CFO@BZT-BCT NFs-P/P (x=1, 3, 5, 7) dielectric composite. (b1) x=1, (b2) x=3, (b3) x=5, (b4) x=7.

Based on the analysis of the breakdown strength of the composite, the distribution of the internal field strength for the sandwich structure is investigated deeply. The P/x vol.% CFO@BZT-BCT NFs-P/P sandwich-structured (x=1, 3, 5, 7) composite with different hybrid nanofibers doping content in the middle layer exhibits different breakdown strength, which stems from the redistribution of local electric field in each layer of sandwich-structured composite. The redistribution of local electric field happens much more obviously in the composite with much greater dielectric difference between the outer and middle layers. Figure 6(b) is the finite element simulation of the local electric field redistribution for the P/x vol.% CFO@BZT-BCT NFs-P/P sandwich-structures (x=1, 3, 5, 7) composite. As described in figure, the local electric field of the middle layer is smaller due to larger dielectric constant (electric field lines are sparser), oppositely, the local electric field of the outer PVDF layer is greater due to the smaller dielectric constant (electric field lines are denser). Combined with the analysis of Table 1, we can get the detailed electric field 23



Page 24 of 44

distribution. The sandwich structure can be regarded as three kinds of capacitors connected in series, and the local electric field redistribution of each layer can be obtained according to the series capacitance partial pressure formula as follow:45 E1 

E2 

V

(1)

V

(2)

 2 d1  p d 2 b

 d 2  2 b d1 p

E1 is the local electric field of the outer PVDF film, and E2 is the local electric field of the middle x vol.% CFO@BZT-BCT NFs-PVDF (x=1, 3, 5, 7) layer, and V is the applied voltage on the composite, and d1 is the thickness of outer PVDF layer, d2 is the thickness of middle x vol.% CFO@BZT-BCT NFs-PVDF (x=1, 3, 5, 7) layer, and εp = 10.2. Table 1 Local electric field of every layer calculated by the capacitive voltage divider Composites P/1 vol.% CFO@ BZT-BCT NFs-P/P P/3 vol.% CFO@ BZT-BCT NFs-P/P P/5 vol.% CFO@ BZT-BCT NFs-P/P P/7 vol.% CFO@ BZT-BCT NFs-P/P

d1 (μm)

d2 (μm)

εb

E1 (kV/mm)

E2 (kV/mm)

Em (kV/mm)

E1/E2

6

10

12

452

376

418

1.2

6

11

25

533

213

380

2.5

7

9

36

519

144

372

3.6

7

10

20

384

192

304

2.0

According to the calculation formula of series capacitor partial pressure, the numerical value of local electric field is shown in Table 1. It is found that the local electric field of each layer is different due to the redistribution of electric field caused by the various dielectric constant of each layer. The outer layer possesses a higher 24


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polarization because of the greater redistributed local electric field under a smaller applied electric field. More importantly, the middle layer owns a weakened breakdown probability due to the lower redistributed local electric field under a smaller applied electric field. Accordingly, the middle layer with low redistributed electric field can hinder the growth of electrical treeing to improve breakdown strength of composite.45 The introduction of a small amount of large aspect ratio and high dielectric constant CFO@BZT-BCT NFs can improve the breakdown strength of sandwich composite, which is beneficial to further enhance the energy storage density.

Figure 7 Comparison of energy density of the sandwich-structured P/5 vol.% CFO@ BZT-BCT NFs-P/P composite and the related reported composites.

The Figure 7 and the Table S2 (in the supporting information) give the comparison of energy storage performance of the representative PVDF-based composite films.4, 20, 29, 46-51

It can be found that the P/5 vol.% CFO@BZT-BCT NFs-P/P composite in this

work shows a better overall performance than most of the reported composites. For instance, when the applied electric field is 350 kV/mm, the discharged energy density reaches to 11.3 J/cm3 at such a low electric field, meanwhile, the energy efficiency 25



maintains at a high level (~55.5%). This excellent performance is attributed to these following main reasons. Firstly, a little bit CoFe2O4 nanoparticles deposited on the surface of BZCT NFs increases the quantity of dipoles and further improves the interfacial polarization and dielectric constant of composite. More importantly, the larger aspect ratio BZT-BCT NFs with low doping can improve the polarization of the PVDF/BZT-BCT NFs-PVDF/PVDF composite at low electric field, which may hinder the migration of carriers and the formation of conductive path to reduce the conduction loss of composite. Furthermore, the large aspect ratio fibers can also inhibit the electrical tree growth to enhance the energy storage performance of composite under low applied voltage. Remarkably, the redistribution of local electric field in each layer of sandwich-structured composite has been occurred due to dielectric difference among three layers, which could hinder the growth of electrical treeing to reinforce breakdown strength of composite. Finally, the compatibility between the inorganic-filler and polymer-matrix can be improved by the modifier of PDA to avoid the aggregation of the hybrid nanofibers in the composite. So that the P/5 vol.% CFO@BZT-BCT NFs-P/P composite exhibits an excellent energy-storage performance than other reported materials. So this research has an important guiding value. Conclusions In summary, the sandwich-structured PVDF/x vol.% BZT-BCT NFs-PVDF/PVDF and PVDF/x vol.% CFO@BZT-BCT NFs-PVDF/PVDF dielectric composite with γ phase 26


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have been prepared by combing with the electrospinning, solution casting, quenching and hot pressing. The CFO@BZT-BCT NFs prepared by hydrothermal method enhances the interfacial polarization and increases the number of dipoles in the sandwich-structured P/x vol.% CFO@BZT-BCT NFs-P/P composite under low applied voltage, which can improve the dielectric properties of the composite. Meanwhile, the large aspect ratio of BZT-BCT NFs with strong polarization can improve the dielectric and energy storage properties of the composite at low electric field. More importantly, the middle layer of the sandwich-structured composite with a high dielectric constant induces the dielectric difference between the outer layer and the middle layer; therefore, there is the redistribution of internal electric field, which is conducive to improve the breakdown strength and the energy storage performance. Herein, the P/5 vol.% BZT-BCT NFs-P/P composite exhibits an excellent dielectric performance (the dielectric constant ~20.1 at 100 Hz) and great energy storage performance (the electric displacement ~10.7 μC/cm2, the discharged energy density ~11.3 J/cm3 and the efficiency ~55.5% at such a low electric field of 350 kV/mm). 

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS. Supplementary material for experimental methods, supplemental results, and additional figures discussed in the manuscript. 

AUTHOR INFORMATION

Corresponding Author 27



Page 28 of 44

*Tel./Fax: +86-451-86391681 E-mail: [email protected]; [email protected]. ORCID Qingguo Chi: 0000-0001-6923-337X Yue Zhang: 0000-0002-9325-3343 Notes The authors declare no competing financial interest. 

ACKNOWLEDGEMENTS

The authors thank the support of the National Natural Science Foundation of China (61640019), the Open Foundation of State Key Laboratory of Electronic Thin Films and Integrated Devices (KFJJ201601), the Youth Innovative Talents Training Plan of Ordinary Undergraduate Colleges in Heilongjiang (UNPYSCT-2016157), the Science Funds for the Young Innovative Talents of HUST (201102), the Supported by Key Laboratory of Functional Materials Physics and Chemistry, Ministry of Education (Jilin Normal University) (2015002). 

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(51) Guo, D.; Cai, K.; Wang, Y. Distinct mutual phase transition in new PVDF based lead-free composite film with enhanced dielectric and energy storage performance and low loss. J. Mater. Chem. C 2017, 5, 2531-2541. DOI:10.1039/C6TC04648G Table of Content Graphic

Synopsis:

Environment-friendly

PVDF/CFO@BZT-BCT

NFs-PVDF/PVDF

composite possesses high energy storage due to CoFe2O4@BZT-BCT hybrid nanofibers with energy-saving sustainability at low electric field.

37



Figure 1 54x21mm (300 x 300 DPI)


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Figure 2 108x84mm (300 x 300 DPI)





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Figure 4 145x256mm (300 x 300 DPI)



Figure 5 108x84mm (300 x 300 DPI)


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Excellent Energy Storage of Sandwich-Structured PVDF-Based

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