Excellent energy storage density and efficiency in blend polymer-based composites by design of core-shell structured inorganic fibers and sandwich structured films

Composites Part B 177 (2019) 107429

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Excellent energy storage density and efficiency in blend polymer-based composites by design of core-shell structured inorganic fibers and sandwich structured films Yang Cui a, b, Tiandong Zhang a, b, Yu Feng a, b, Changhai Zhang a, b, **, Qingguo Chi a, b, c, *, Yongquan Zhang a, b, Qingguo Chen a, b, Xuan Wang a, b, Qingquan Lei a, b a

Key Laboratory of Engineering Dielectrics and Its Application, Ministry of Education, Harbin University of Science and Technology, Harbin, 150080, PR China School of Electrical and Electronic Engineering, Harbin University of Science and Technology, Harbin, 150080, PR China c State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, 610054, PR China b

A R T I C L E I N F O

A B S T R A C T

Keywords: Blended polymer Core-shell structured Sandwich structured films Energy storage performances

In this study, the ferroelectric polyvinylidene fluoride (PVDF) blended with linear dielectric polymethyl meth­ acrylate (PMMA) is chosen as polymer matrix (named as PMMA/PVDF). The effect of PMMA content and differently structured fillers on the microstructures and electrical properties of PMMA/PVDF-based composites have been investigated. The inorganic 0.5Ba(Zr0.2Ti0.8)O3- 0.5(Ba0.7Ca0.3)TiO3 fibers (abbreviated as BCZT), BCZT embedded with Ag particles (BCZTþAg), and core-shell structured BCZTþAg@Al2O3 fibers as fillers were designed and prepared by electrospinning technology. As the insulating Al2O3 shell layer could relieve the dielectric difference between the BCZT and polymer matrix and confine the mobility of carriers provided by Ag, the energy storage density and efficiency are 4.02 J/cm3 and 78.0% for the 3 vol% BCZTþAg@Al2O3/40% PMMA/PVDF composites at an electric field of 320 kV/mm. To further improve the energy storage properties of the single-layered composites, the sandwich-structured composite films have been designed to improve the breakdown strength. The results show that the energy storage density and efficiency of sandwich-structured composite film with 40 wt% PMMA/PVDF as outer layer and 3 vol% BCZTþAg@Al2O3/40% PMMA/PVDF as inter layer are 9.6 J/cm3 and 69.8%, respectively, at an electric field of 400 kV/mm. This work presents an effective way to improve the energy storage properties of inorganic/polymer composites.

1. Introduction In recent years, energy storage and conservation have been devel­ oped significantly with increasing intensive research. Among the current energy storage capacitors, compared with the batteries and super­ capacitors, the dielectric energy storage capacitors with the high power density and instantaneous charging ability have been attracted much attention. However, the relatively lower energy storage density limits the broad application of dielectric capacitors [1]. For example, biaxially oriented polypropylene (BOPP) is used as a commercial energy storage material, but its energy storage density is 1–4 J/cm3 [2–4]. The dielec­ tric material with high energy storage density is much expected to be

obtained to reduce the size and weight of the dielectric capacitors, which is significant for the pulsed power system and electric vehicles [5,6]. Therefore, it is urgent to find an effective way that can significantly improve the energy storage density of dielectric energy storage materials. Generally, energy storage materials can be divided into linear dielectric material and nonlinear dielectric material. Such as polymethyl methacrylate (PMMA) is one kind of representative linear dielectric, the energy storage efficiency (η) is close to ~92% under an electric field of 350 kV/mm [7]. However, there is a portion of stored energy that cannot be released from the nonlinear material during the discharge process, decreasing the energy storage efficiency of capacitors [8]. The

* Corresponding author. Key Laboratory of Engineering Dielectrics and Its Application, Ministry of Education, Harbin University of Science and Technology, Harbin, 150080, PR China. ** Corresponding author. Key Laboratory of Engineering Dielectrics and Its Application, Ministry of Education, Harbin University of Science and Technology, Harbin, 150080, PR China. E-mail addresses: [email protected] (C. Zhang), [email protected] (Q. Chi). https://doi.org/10.1016/j.compositesb.2019.107429 Received 8 May 2019; Received in revised form 24 August 2019; Accepted 10 September 2019 Available online 10 September 2019 1359-8368/© 2019 Published by Elsevier Ltd.

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genuinely useful energy storage density is the discharge energy density. For example, the energy storage density of ferroelectric polyvinylidene fluoride (PVDF) is about 4.8 J/cm3 under an electric field of 350 kV/mm, and the η is about 75.0% [9]. It can be seen that linear materials have higher efficiency, while nonlinear materials have higher energy storage densities. Among multitudinous dielectric materials, polymer-based compos­ ites are favored by many researchers because they combine the merits of ceramics (e.g., high dielectric constant) and polymers (e.g., high breakdown strength) [4,9–14]. For polymer-based composites filled with high dielectric ceramics, as the filler loading is low, the dielectric constant of the composites slightly increases compared with that of the un-filled polymer matrix. Although the high filler loading can induce high dielectric constant for the polymer composites, it is accompanied with a rapid reduction in breakdown strength [11–13]. It should be noted that the improved dielectric constant of the composites filled with a high content of fillers is always at the expense of breakdown strength, which leads to unobvious improvement in energy storage properties of composites. To reduce the content of the filler, the composite filler containing conductive or semi-conductive phase is always introduced into the polymer matrix [15,16]. Besides the positive effect of high dielectric constant on the energy storage density of composites, it is also obvious that the improved breakdown strength can lead to the signifi­ cantly further increased energy storage density of the composite. Much efforts have been done to improve the breakdown strength and the dielectric constant simultaneously, and some typical methods have been proposed. (i) The one-dimensional inorganic nanofibers with large aspect ratio are employed as fillers instead of particles, which is bene­ ficial to reduce the filler content [17–21]. (ii) The fillers with a core-shell structure and multiple interfacial structures have been proposed for improving the interfacial polarization and relieving the distortion of the electric field [22–25]. (iii) The sandwiched structural composite films or multilayer composite films have been constructed for enhancing the dielectric constant and breakdown strength simultaneously [26–31]. It is not difficult to find from previous studies that multiphase composites are considered as one of the effective ways to achieve high energy density of composites [22–32]. Among them, the core-shell structure has been widely concerned for improving the breakdown field strength and energy storage density of composites. Because fillers with a core-shell structure have better dispersion and compatibility in the polymer ma­ trix [30]. Although the energy storage density of the composite has been improved by various methods, the energy storage efficiency has not been paid enough attention. There are many kinds of energy storage ferro­ electric materials with high energy storage density and low efficiency [16–19,29]. The low efficiency not only causes waste of electrical en­ ergy, but also induces severe accumulation of joule heat which gives rise to the damage of capacitors [33]. So it is also significant that the energy efficiency of capacitors should be equally considered as that of energy storage density. The high remnant polarization and serious hysteresis of the ferroelectric materials lead to the massive energy loss during the charge-discharge process. However, the linear dielectrics always exhibit high energy storage efficiency due to the remnant polarization is close to zero. Based on the above mentioned, in this study, both the energy storage density and the energy storage efficiency are taken into account for the polymer-based composites. Although high maximum polarization can be obtained in PVDF, the relatively low breakdown strength and high remnant polarization limit its energy storage density and efficiency [16–19,29]. To improve the intrinsic polarization behaviors of the PVDF for energy storage, herein, the ferroelectric PVDF blended with linear dielectric PMMA is chosen as the polymer matrix (abbreviated as PMMA/PVDF), where the PMMA possesses the relative higher break­ down strength and ultra-lower remnant polarization [7,34–36]. Simul­ taneously, the inorganic composite fillers with multiple interfacial structures have been designed. The composite fiber (named as

BCZTþAg@Al2O3) includes Al2O3 as the outer layer, 0.5Ba(Zr0.2Ti0.8) O3-0.5(Ba0.7Ca0.3)TiO3 (BCZT) and silver particles (Ag) as the inner layer. And the BCZTþAg@Al2O3 composite fiber is fabricated by coaxial spinning technique. The BCZT NFs and Ag are used to improve the dielectric constant of the composites, where the Al2O3 fibers outer layer cannot only relieve the dielectric difference between the BCZT NFs and the polymer matrix and weaken the distortion of the electric field, but also limit the mobility of electronics offered by Ag and the interfacial charges. The results show that the 3 vol% BCZTþAg@Al2O3/40 wt% PMMA/PVDF composite has an energy storage density of 4.02 J/cm3 and an efficiency of 78.0% under the electric field of 320 kV/mm. In order to further improve the energy storage properties of composites, the sandwich structured composite films have been designed. The 3 vol % BCZTþAg@Al2O3/40 wt% PMMA/PVDF is chosen as a inter layer to provide high dielectric constant, and the 40 wt% PMMA/PVDF is chosen as outer layers to improve the breakdown strength of the sandwich-structured composite films. The breakdowns strength of sandwich-structured composite films increases up to 398.5 kV/mm, accompanying with a discharged energy storage density of 9.6 J/cm3 and an efficiency of 69.8%. 2. Experimental section 2.1. Materials PVDF was supplied by Shanghai 3F New Materials Technology Co., Ltd. PMMA, Al(NO3)3⋅9H2O, Ba(OH)2⋅8H2O, Ca(OH)2, C16H36O4Ti, AgNO3, NH3⋅H2O, N2H4⋅H2O, ethanol (C2H6O), acetic acid (CH3COOH), acetylacetone (C5H8O2), N,N-dimethylformamide (DMF) were pur­ chased from Sinopharm Chemical Reagent Co., Ltd.. HCl was purchased from Tianjin Zhiyuan chemical reagent co., Ltd.. C20H28O8Zr, poly­ vinylpyrrolidone (PVP, K90), tris(hydroxymethyl)aminomethane (tris) were purchased from Aladdin. 2.2. Preparation of fillers The Ag particles were prepared as follows. NH3⋅H2O (6 ml) was mixed with deionized water (10 ml). AgNO3 (3 g) was added to the above solution under stirring, and the solution A was obtained. N2H4⋅H2O (1.8 ml) was mixed with ethanol (10 ml) to get solution B. And then, PVP (1.2 g) was dissolved in ethanol (70 ml) and the solution C was obtained. Solution A and B were alternately dripped into solution C under the condition of 1000 r/min. After all the solution A and B were dripped, the mixture was stirred at 500 r/min for 30 min. The solution was dried at 80 � C for 12 h and then calcined at 300 � C for 4 h. Finally, Ag particles were obtained after grinding. Three kinds of fillers, BCZT fiber, BCZTþAg composite fiber and BCZTþAg@Al2O3 composite fiber, were prepared via electrostatic spinning technique. The BCZTþAg@Al2O3 fiber was synthesized via the coaxial electrospinning technique as shown in Fig. 1. The inner pre­ cursor solution for the BCZTþAg nanofibers was prepared as follows. First, the ethanol (7.6 ml), acetic acid (18.1 ml) and acetylacetone (3.08 ml) were mixed to form a homogeneous solution A (sol A). Second, the Ba(OH)2⋅8H2O (4.344 g), Ca(OH)2 (0.18 g) and C20H28O8Zr (0.788 g) were dissolved in sol A and stirred at 500 r/min for 2 h to form a stable solution B (sol B). Third, the C16H36O4Ti (4.95 ml) was dripped into the sol B to form a stable solution C (sol C). And then, the Ag particles (0.5 g) were dispersed in sol C by ultrasonic dispersion for 2 h to form a sus­ pension D (sus D). The PVP (0.4 g) was dissolved in sus D to control the viscosity of the suspension to form a stable inner precursor solution. The outer precursor solution for the Al2O3 nanofibers was prepared by dis­ solving Al(NO3)3 (2 g) in deionized water (2 ml) and then dripping into ethanol (20 ml) form a stable solution. The viscosity of the solution was controlled by 0.7 g PVP. The inner precursor solution and outer pre­ cursor solution were moved to two syringes, respectively. The syringes were connected to a double layer coaxial needle to realize the coaxial 2

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3. Results and discussion In previous work, it was found that the composition of 0.5 (Ba0.7Ca0.3)TiO3-0.5Ba(Zr0.2Ti0.8)O3 (BCZT) is near the morphotropic phase boundary (MPB) at room temperature, which results in a nearly vanishing polarization anisotropy and is beneficial to the polarization rotation, thus the BCZT with MPB composition possesses the excellent saturation polarization and dielectric constant [37]. It has been also reported that 0.5(Ba0.7Ca0.3)TiO3-0.5Ba(Zr0.2Ti0.8)O3 as a free lead material which is environmentally friendly, plays an important role in making MLCCs due to its high permittivity and low loss [38]. Therefore, in this work, BCZT and its composite fibers are selected as the filling phase. The morphology characterization of the prepared fibers is shown in Fig. 2. BCZT, BCZTþAg and BCZTþAg@Al2O3 have a large aspect ratio. The diameters of BCZT and BCZTþAg are about 200 nm, and the lengths are about 3–5 μm (Fig. 2a1,b1,a2,b2). The total diameter of BCZTþAg@Al2O3 coaxial fiber is about 260 nm in which the inner diameter of BCZTþAg inner layer is about 200 nm and the thickness of Al2O3 outer layer is about 30 nm, and the length of the coaxial fiber is 3–5 μm (Fig. 2c1,c2). In addition, compared with that of BCZT fiber, the surface of BCZTþAg fiber is rougher (Fig. 2a2,b2), which may be caused by Ag particles not only embed inside of BCZT fiber but also distribute on the surface of BCZT fiber. However, the surface of BCZTþAg@Al2O3 coaxial fiber is very smooth due to the introduction of Al2O3 outer layer (Fig. 2c2). In order to further confirm the microstructure more clearly, the EDS spectras were employed to demonstrate the element distribu­ tion of Ba, Ca, Zr, Ti, Ag and Al in fibers (Fig. 2a3,b3,c3). In addition, the observed Cu signals come from the copper grides in the EDS test. The emergence of the elemental signals of Ag in the BCZTþAg and BCZTþAg@Al2O3 indicates that the Ag has been introduced (Fig. 2a3, b3). And the Al element appeared in BCZTþAg@Al2O3 means that the Al2O3 outer layer has been introduced successfully onto the surfaces of BCZTþAg inner layer. The phase compositions of different types of nanofibers were studied by XRD as shown in Fig. 3a. The characteristic peaks of BCZT are observed at 2θ of 22.12� , 31.52� , 38.81� , 45.17� , 50.80� , 56.10� , 65.76� , 71.42� , 74.75� , 79.21� , 83.45� , which are consistent with JCPDS card (No. 31-0174). For BCZTþAg, in addition to the characteristic peaks of BCZT, the characteristic peaks of Ag can also be observed at 2θ of 38.1� , 44.25� , 64.45� , 77.25� , which are consistent with the JCPDS card (No.04-0783). Combined with the results in Fig. 2, it can be further explained that Ag particles have been successfully introduced into BCZT fibers. And there are other characteristic peaks other than BCZTþAg@Al2O3, which correspond to the amorphous Al2O3. The

Fig. 1. The flowchart for the preparation process of composite films.

electrospinning process, as shown in Fig. 1, the ratio of injection velocity for the inner and outer precursor solution is set to 1:2, and a 20 kV voltage of voltage was applied. The obtained composite fibers were calcinated at 700 � C for 3 h to form BCZTþAg@Al2O3 ceramic fibers. Besides, the BCZT fibers and BCZTþAg composite fibers were prepared via a facile electrospinning technique. And the details of the preparation process were described in the supporting information. The surfaces of the three kinds of fibers were modified by dopamine to improve the filler-matrix compatibility. The tirs (0.4 g) was dissolved in deionized water (200 ml) to form a buffer solution. The HCl was dripped in the buffer solution to adjust the PH of the solution to 8.5. The dopamine hydrochloride (0.1 g) was dissolved in buffer solution. All the fillers were dispersed in the above solutions under stirring at 800 r/min for 12 h, followed by drying at 80 � C for 12 h. 2.3. Preparation of composite films The PMMA/PVDF composite films were prepared as follows. The PVDF and PMMA were proportionally dissolved into DMF by stirring at 500 r/min for 12 h, forming a stable suspension. The inorganic fillers were added into the suspension. The suspension was then cast onto a glass plate. The as-cast films were dried at 80 � C for 12 h to volatilize the solvent and the composite films were obtained. The sandwich-structured composite films were prepared by hot pressing for 15 min under the condition of 150 � C and 10 Mpa. The preparation flowchart is shown in Fig. 1. 2.4. Characterization The morphology of fillers and cross-sectional images of composite films were conducted by scanning electron microscopy (SEM, SU 8020), transmission electron microscopy (TEM, JEM-2010F). X-ray diffraction (XRD, Empyrean, PANalytical, Holland) was used to study the crystal structure of fillers and composites using a Cu Kα source. The elements of fillers were characterized using X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD, UK). Fourier transform infrared spectroscopy (FT-IR) of the nanofibers was conducted on a NEXUS 670 FT-IR spectrometer from 2000 to 650 cm 1 in the transmittance mode. The dielectric perfor­ mance of composite films was characterized by a broadband dielectric spectrometer (Novocontrol GmbH, Germany) from frequency 100–107 Hz. The D-E loops and current density were characterized by a Precision LC ferroelectric test system (Radiant Technologies, USA).

Fig. 2. The microstructure images of BCZT, BCZTþAg and BCZTþAg@Al2O3, (a) SEM, (b) TEM, (c) EDS. 3

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Fig. 3. (a) XRD patterns of BCZT, BCZTþAg, and BCZTþAg@Al2O3 fibers, respectively, XPS patterns of (b) BCZTþAg@Al2O3, (c) Al 2p, (d) O 1s, respectively, (e) XRD patterns and (f) FTIR diagrams of PVDF blended with different content of PMMA.

structural characteristics of BCZTþAg@Al2O3 coaxial fiber could be further confirmed by XPS spectra (Fig. 3b,c,d). The results of the XPS test are processed by Origin software, and the method used for XPS fitting selects Gaussian/Lorentzian function and Shirley-type background. As illustrated in Fig. 3b, the coaxial fiber is made of elements such as Ca, Ba, Zr, Ti, O, and Ag, which is consistent with the EDS results (Fig. 2c3), and the detailed maps of Al 2p and O 1s are shown in Fig. 3c and d, respectively. The binding energy peak located at 74.02 eV represents the Al 2p, which features the formation of Al (3þ) state [39,40]. But the binding energy position of Al element is slightly lower than that reported in references, that may be due to the competition of oxygen in BCZT and Al2O3 weakens the Al–O bond and makes the electron cloud disperse [39–41]. As shown in Fig. 3d, the corresponding peak of O 1s can be divided into three peaks, corresponding to Ca–O bond, Al–O band and Ba–O band, respectively [42]. In conclusion, combining with the results as mentioned above, the BCZT, BCZTþAg and BCZTþAg@Al2O3 fibers were successfully prepared. In addition to the structural design of inorganic fillers, the polymer matrix was also optimized in this work. The linear dielectric PMMA was selected as a copolymer to improve the electrical performance of PVDF, which can effectively reduce the loss and remnant polarization of PVDF. Besides, the PMMA exhibits excellent compatibility with PVDF. The compatibility between different polymers can be measured by the dif­ ference between the solubility parameters (δ), that is, the closer the δ is, the better the compatibility is. The solubility parameters δ equation is as follows:

δ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi δ2d þ δ2p þ δ2h

(1)

where δd is dispersion force, δp is dipole force and δh is hydrogen bonding force. According to the reported references [43,44], the solu­ bility parameters of PVDF and PMMA are given in Table 1: It can be seen from Table 1 that the solubility parameters of PVDF are very close to that of PMMA, so it can be deduced that PVDF and PMMA have good compatibility. Fig. S4 shows the SEM of PVDF and PMMA/ PVDF blend films. No obvious cavities and cracks can be observed for both PVDF and PMMA/PVDF blend films, which indicates that PVDF and PMMA/PVDF have good homogeneity. This phenomenon is consistent with the calculating results in Table 1. The XRD and FTIR of the polymer matrix are shown in Fig. 3e and f. The characteristic peaks of PVDF can be observed and indexed as (020) and (110)/(200) crystal plane, which indicates that α, β and γ phase of PVDF coexist. The diffraction peaks appeared in the range of 12.4� –17.4� correspond to the amorphous PMMA. With the increase of PMMA content, the diffraction peak of PMMA in blend films is more obvious. In order to further confirm the phase composition and molec­ ular chain structures, the FTIR is also employed (Fig. 3f). It can be seen Table 1 Solubility parameters of polymers.

4

Materials

Molecular formula

δd

δp

δh

δ

PVDF PMMA

-(CH2-CF2)n-[CH2C(CH3)(COOCH3)]n-

17.2 18.6

12.5 10.5

9.2 7.5

23.2 22.7

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3.56 μC/cm2, 3.19 μC/cm2 and 2.66 μC/cm2, respectively, which is consistent with the results of measured dielectric constant. However, compared with pure PVDF, the D-E curves of PMMA/PVDF blend films become much slimmer with the increased PMMA content, which is attributed to the intrinsic linear dielectrics of PMMA and the decreased dielectric loss for the blend films. As shown in Fig. 4c, the current density of PMMA/PVDF blend films is lower than that of pure PVDF, which is attributed to the lower conductivity and dielectric loss. Besides, the breakdown strength of the three kinds of polymer matrix has been investigated and shown in Fig. S5 and Fig. S6. It can be found that the breakdown strength of the polymer matrix increases with the increase of PMMA content. The breakdown strength of pure PVDF, 15% PMMA/ PVDF and 40% PMMA/PVDF are 351.5 kV/mm, 369.9 kV/mm and 399.6 kV/mm, respectively. The improvement of breakdown strength is caused by the decreased conductivity and dielectric loss due to the introduction of PMMA. Although the breakdown strength of PVDF films can be enhanced by blending with PMMA, which is at the expense of decreased dielectric constant and polarization, leading to the unim­ proved energy storage density for PVDF, 15% PMMA/PVDF and 40% PMMA/PVDF, which is 4.2 J/cm3, 4.1 J/cm3 and 4.3 J/cm3, respec­ tively. However, the energy storage efficiency has been significantly improved, which is 57.2%, 58.1% and 67.4%, respectively. It indicates that the introduction of linear dielectric PMMA is beneficial to relieve the hysteresis loss and improve energy storage efficiency. It was reported that the inorganic fillers with high dielectric constant and conductive were usually used to improve the dielectric constant of the polymer. The dielectric constant of the polymer can be improved by filling the BCZT fibers with high dielectric constant, which can be further enhanced by adding conductive particles into the composites [17,18,42,50]. To further improve the energy storage density of the PMMA/PVDF blend films, the inorganic fillers have been chosen for improving the dielectric constant. In this study, the inorganic fillers of BCZT fibers, BCZTþAg composite fibers, and BCZTþAg@Al2O3 com­ posite fibers have been designed. The fillers with different structure were filled into the polymer matrix for obtaining the single-layered composites. Herein, in order to better reveal the effect of filler struc­ ture on the electrical properties, the 3 vol% content is selected, which is consistent with our previous works [17]. The XRD and SEM of the composites with different fillers of 40% PMMA/PVDF are shown in Fig. S7. The characteristic peaks of the polymer matrix and fillers can be

from the diagram that there are α, β and γ in PVDF and PMMA/PVDF blend films [35]. α phase is a nonpolar phase, β and γ phase is the ferroelectric phase. The main characteristic peaks of the α phase appear at 1403, 1073, 764, 610 and 408 cm 1. The characteristic peaks of the β phase are 878 and 840 cm 1 [45–47]. And the peaks at 1234, 976, 838, 811, 510 and 430 cm 1 correspond to γ phase. The absorption peaks at 1727, 991 and 750 cm 1 can be observed in PMMA/PVDF blend films and are represented by ω in Fig. 3f, which correspond to the charac­ teristic peaks of PMMA [35,46]. Also, it is found that the γ phase is the main phase of PMMA/PVDF blend films. The PVDF with γ phase has higher breakdown strength, which is beneficial to restrain the early saturation of hysteresis loops and improve energy storage efficiency [35, 48,49]. Fig. 4 shows the electrical properties of pure PVDF and PMMA/PVDF blend films. The dielectric constant of pure PVDF is higher than that of blend films in the whole frequencies range, and the dielectric constant of pure PVDF is 9.34 at 1 Hz (Fig. 4a1). With the increase of PMMA blended content, the dielectric constant of the blend films decreases, for example, the dielectric constant at 1 Hz is 7.9 and 6.7 for 15% PMMA/PVDF and 40% PMMA/PVDF, respectively. This phenomenon can be explained by that the linear dielectric PMMA has a lower dielectric constant (ε ¼ 3.5) [35,36], resulting in the decreased average dielectric constant of PMMA/PVDF blend films. The dielectric constant of all films decreases with the increase of frequency, which is induced by the dipoles which are more accessible to follow the change of electric field at low fre­ quency. However, the dipoles orientation polarization cannot keep up with the changed frequency at high frequency, leading to the decreased dielectric constant. The dielectric loss of PMMA/PVDF blend films is lower than that of pure PVDF (Fig. 4a2). For example, the dielectric loss of 15% PMMA/PVDF and 40% PMMA/PVDF is 0.0634 and 0.0552 at 1 Hz. The decrease of dielectric loss, especially at low frequencies, is mainly attributed to the decreased chain mobility in PMMA/PVDF blend films. It can be found that the conductivity of PMMA/PVDF blend films is also lower than that of pure PVDF films. The conductivity of 15% PMMA/PVDF and 40% PMMA/PVDF is 2.685 � 10 13 S/cm and 2.129 � 10 13 S/cm, respectively (Fig. 4a3). Fig. 4b shows the D-E curves of PVDF and PMMA/PVDF blend films under the same electric field (350 kV/mm). The maximum polarization decreases with the increase of PMMA blended content. The maximum polarization of PVDF, 15% PMMA/PVDF and 40% PMMA/PVDF are

Fig. 4. Electrical properties of PVDF blended with different content of PMMA. (a1) dielectric constant, (a2) dielectric loss and (a3) conductivity as a function of frequency, (b) the electric displacement and (c) current density as a function of the electric field. 5

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found in the composites, which indicates that the polymer matrix and fillers are just physically mixing and no chemical reaction occurs. The BCZT, BCZTþAg and BCZTþAg@Al2O3 fillers can be seen in the poly­ mer matrix from the SEM results as given in Fig. S7 b,c,d. The larger aspect ratio of the fibers may be inclined to distribute in the in-plane direction during the coating process, so it can also be found that the fillers are tended to lie in the horizontal direction of the composite films [9]. Fig. 5a shows the dielectric constant measured at 1 Hz of singlelayered composite films with different matrix and fillers. For the same matrix, the dielectric constant of the composites as a descending sort is BCZTþAg/matrix, BCZT/matrix, BCZTþAg@Al2O3/matrix. In this work, for the composites filled with BCZTþAg, the highest dielectric constant is not only induced by the interfacial polarization between the BCZT and polymer matrix but also attributed to the electronics provided by Ag particles [9,51]. As shown in Table 2, the dielectric loss, con­ ductivity and current density of the BCZTþAg/matrix do not signifi­ cantly increase, which may be caused by the electronics confinement effect [21,52–54]. Remarkably, the BCZTþAg@Al2O3/matrix compos­ ites show the lower dielectric constant and dielectric loss compared with other composites, which can be explained as below. First, the interfacial polarization may be restrained in BCZTþAg@Al2O3/matrix composite films because of the introduction of Al2O3 shell with low dielectric constant [55]. Second, Al2O3 shell acting as an insulating layer effec­ tively confines the charge carrier movement between the fillers and polymer matrix [56]. For the same kind of fillers, the dielectric constant, dielectric loss, conductivity and current density of the composites decrease with the increase of PMMA content in the polymer matrix, which is consistent with the result of PMMA/PVDF matrix without fillers. It is exciting that the dielectric constant of the composite films has been improved by introducing the inorganic fillers, which may be beneficial to enhance the energy storage density. Besides, the break­ down strength also plays a vital role in improving energy storage den­ sity. The breakdown strength of single-layered composite films is analyzed using the Weibull distribution. The breakdown strength (Eb) of 40% PMMA/PVDF composite films with different fillers is summarized in Fig. 5b. The BCZTþAg@Al2O3/40% PMMA/PVDF composite film shows the largest value of breakdown strength (351.0 kV/mm) compared with other composite films. The improved breakdown strength may be caused by that: (i) The dielectric constant of Al2O3 (εr � 10) is close to that of the polymer matrix (εr � 7-10), so Al2O3 outer layer can relieve the dielectric difference between the BCZT fillers and the polymer matrix and mitigate the electric-field concentration [9,55]. (ii) The Al2O3 as an insulating outer layer can effectively reduce the current density and dielectric loss of the composites (see from Table 2). (iii) The PMMA possesses higher breakdown strength than that of PVDF,

Table 2 Dielectric loss, electrical conductivity and current density of single-layered composite films. Materials

Dielectric Loss (at 1 Hz)

Conductivity (S/cm)

PVDF BCZT/PVDF BCZTþAg/PVDF BCZTþAg@Al2O3/PVDF 15% PMMA/PVDF BCZT/15% PMMA/ PVDF BCZTþAg/15% PMMA/ PVDF BCZTþAg@Al2O3/15% PMMA/PVDF 40% PMMA/PVDF BCZT/40% PMMA/ PVDF BCZTþAg/40% PMMA/ PVDF BCZTþAg@Al2O3/40% PMMA/PVDF

0.1531 0.1873 0.1991 0.1606 0.0628 0.0868

1.208 � 10 2.087 � 10 2.774 � 10 1.501 � 10 2.685 � 10 3.777 � 10

12

0.0970

4.696 � 10

0.0724

Current Density (A/cm2) 3.535 � 10 1.276 � 10 1.422 � 10 5.860 � 10 1.989 � 10 2.368 � 10

6

13

3.048 � 10

6

3.584 � 10

13

2.100 � 10

6

0.0559 0.0713

2.095 � 10 3.233 � 10

13

1.478 � 10 2.024 � 10

6

13

0.0843

4.064 � 10

13

2.264 � 10

6

0.0616

2.116 � 10

13

1.509 � 10

6

12 12 12 13 13

5 5 6 6 6

6

and the PMMA/PVDF is chosen as a polymer matrix which can improve the breakdown strength of the composite films. In order to further confirm the effect of PMMA on the breakdown strength of the composite films, the breakdown strength of the composite films with different PMMA blended content has been investigated and given as illustrated in Fig. 5b. It can be found that the breakdown strength of the composite films filled BCZTþAg@Al2O3 sharply reduces with the decrease of PMMA content. It is obvious that if the PMMA content is further increased, the higher breakdown strength may be obtained, however, which may be accompanied with the seriously reduced dielectric con­ stant. In consideration of both of breakdown strength and dielectric constant play an important role in the energy storage properties, the highest PMMA content is selected as 40 wt% in this study. The polarization and energy storage characteristics of 40% PMMA/ PVDF filled with different fillers and BCZTþAg@Al2O3/40% PMMA/ PVDF with different PMMA content composites are shown in Fig. 6. When an electric field of 250 kV/mm is applied, the BCZTþAg/40% PMMA/PVDF composite film shows the highest maximum polarization (Dmax) which is consistent with the results of dielectric properties, accompanying with the highest remnant polarization (Dr), as shown in Fig. 6a1. In contrast, BCZTþAg@Al2O3/40% PMMA/PVDF composite film shows the lowest Dmax and Dr that attributed to the Al2O3 outer layer. Although the high Dmax could result in high energy storage den­ sity, the high Dr could cause lower energy storage density owing to the decreasing integration area in the D-E loops. And low Dr also means that

Fig. 5. (a) The dielectric constant of the single-layered composite films; (b) Weibull distribution of breakdown strength for 40% PMMA/PVDF matrix with different fillers, and the illustration shows the breakdown strength for BCZTþAg@Al2O3/PMMA/PVDF with different PMMA content. 6

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Composites Part B 177 (2019) 107429

Fig. 6. Polarization and energy storage properties single-layered composite films, (a1) the value of displacement and (a2) energy density of 40% PMMA/PVDF matrix with different fillers, (b1) the value of displacement and (b2) energy density of BCZTþAg@Al2O3/PMMA/PVDF with different PMMA content.

once the electric field is removed, the stored energy can be released more entirely and the enhanced energy storage efficiency can be ach­ ieved [23,57]. As shown in Fig. 6a2, the energy storage density of BCZTþAg/40% PMMA/PVDF is just 3.92 J/cm3 under an electric field of 250 kV/mm. Because the BCZTþAg@Al2O3/40% PMMA/PVDF composite film possesses higher breakdown strength than that of other composite films, the higher energy storage density of 4.02 J/cm3 can be obtained at an electric field of 320 kV/mm. Additionally, efficiency (η) is another important parameter in practical applications. The equation of η is as follows:

η¼

Udischarge � 100% Ucharge

BCZTþAg@Al2O3/15% PMMA/PVDF (η ¼ 68.3%) is lower than that of BCZTþAg@Al2O3/40% PMMA/PVDF (η ¼ 78.0%). It is worth noting that although the BCZTþAg@Al2O3/40% PMMA/ PVDF exhibits excellent energy storage efficiency, its energy storage density is not satisfying. It is well known that the sandwich structure or multiple layer films were usually designed for further improving the energy storage properties of the composites. Because the sandwich structure can not only regulate the electric field distribution in com­ posites but also combine the advantage of each layer to achieve an enhancement in breakdown strength or polarization [30,58,59]. In this study, to further enhance the energy storage density of BCZTþAg@Al2O3/40% PMMA/PVDF composites, two kinds of sandwich-structured composite films have been designed. The one is to choose the pure PVDF with higher dielectric constant as outer layer to compensate the lower dielectric constant (or polarization) of BCZTþAg@Al2O3/40% PMMA/PVDF inter layer films, abbreviated as 0-33 40-0. The other one chooses the 40% PMMA/PVDF with higher breakdown strength (as given in Fig. S6) as outer layer to improve the breakdown strength of BCZTþAg@Al2O3/40% PMMA/PVDF inter layer films, abbreviated as 40-33 40-40. Fig. 7a and b shows the SEM images of the two kinds of sandwichstructured composite films. Compared with the two kinds of compos­ ite films, the interface between the PVDF outer layer films and BCZTþAg@Al2O3/40% PMMA/PVDF inter layer films (0-33 40-0) can be seen as shown in Fig. 7a, which may be caused by different melting points between PVDF and 40% PMMA/PVDF matrix. However, as shown in Fig. 7b, it is a little unclear between the 40% PMMA/PVDF outer layer films and BCZTþAg@Al2O3/40% PMMA/PVDF inter layer films (40-33 40-40), which is attributed to the same polymer matrix in outer layer films and inter layer films. The dielectric performances of two kinds of sandwich-structured composite films are shown in Fig. S9. At the same frequency, the dielectric constant of 0–33 40-0 is higher than the single-layered

(2)

where Udischarge is the discharge energy storage density, and Ucharge is the charge energy storage density. Although the η of the BCZTþAg/40% PMMA/PVDF is gradually decreasing with the increase of the applied electric field, the η of the BCZTþAg@Al2O3/40% PMMA/PVDF com­ posite film remains a relatively higher value (above 78.0%) at the maximum electric field, which might be ascribed to the low Dr value. In order to investigate the effect of PMMA content on the polariza­ tion and energy storage characteristics of the composites, the results for the composites filled with BCZTþAg@Al2O3 and blended with different content of PMMA are shown in Fig. 6b. Under an electric field of 250 kV/ mm, the Dmax and Dr of the composites decrease with the increase of PMMA content, which owes to the introduction of linear dielectric PMMA. It should be noted that although the bearable maximum electric field of the BCZTþAg@Al2O3/15% PMMA/PVDF is lower than that of BCZTþAg@Al2O3/40% PMMA/PVDF, the energy storage density of the former one (U ¼ 4.78 J/cm3) is slightly higher than the latter one. The results indicate that the higher polarization can be obtained by blending with the lower content of PMMA and beneficial to the improvement of energy storage density. However, the energy storage efficiency of 7

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comparison diagram of the energy storage properties for the represen­ tative polymer-based materials is given in Fig. 8 [2,5,9,11,17–19,30,32, 33,61–63,68,69]. It can be found that the 40-33 40-40 composite film exhibits both excellent energy storage density and efficiency. In order to better understand the improved breakdown strength of the sandwich-structured composite films, the effect of the structure of inorganic fillers and the composite films on the distribution behaviors of electric field, potential and space charge density have been investigated by the simulation method of COMSOL Multiphysics. Fig. 9a,b and c simulate the electric field distribution of a single-layered 40% PMMA/ PVDF composite films with BCZT, BCZTþAg and BCZTþAg@Al2O3 fillers, respectively. The direction of the applied electric field is from the top to the bottom of the composite films and the electric field intensity changes from low to high, marked as from to blue to red color. Compared with the simulation results, the distortion electric field of the inorganic filler tip and the polymer matrix in the BCZT/40% PMMA/ PVDF and BCZTþAg/40% PMMA/PVDF composite films is significantly red, while the distortion electric field of BCZTþAg@Al2O3/40% PMMA/ PVDF composite film is significantly reduced. The weakened distortion of the electric field is helpful to restrict the electric field concentration and prevent electric breakdown early. It was reported that large dielectric differences between the filler and the polymer matrix lead to local electric field distortion [23,25,64–68]. In this study, the dielectric differences between BCZT and polymer matrix can be relieved by introduction Al2O3 as the buffer layer, resulting in the higher breakdown strength in BCZTþAg@Al2O3/40% PMMA/PVDF single-layer composite film. The distribution of potential lines and space charges for sandwich-structured composite films is given in Fig. 9d and e, respec­ tively. Compared with two sandwich-structure composite films, it is found that the distribution of the potential line in 0–33 40-0 is more uniform in the whole composites, and the potential lines of 40-33 40-40 are denser in 40% PMMA/PVDF outer layers. For the 0-33 40-0, the dielectric constant of BCZTþAg@Al2O3/40% PMMA/PVDF inter layer film is close to PVDF outer layer, which is higher than that of 40% PMMA/PVDF outer layer in 40-33 40-40. It is known that the layer with a lower dielectric constant will allocate a higher electric field in sandwich-structured composite films [27]. The composite film with 40% PMMA/PVDF as outer layer possesses higher breakdown strength than that with PVDF as outer layer, which leads to the higher breakdown strength obtained in 40-33 40-40. In addition, the space charges tend to accumulated at the interface between two different dielectric layers in the sandwich-structured composite films [60]. From Fig. 9e1 and e2, it can be seen that the charge density of 0–33 40-0 is higher than 40-33

Fig. 7. The SEM images for sandwich structured films (a) 0-33 40-0 and (b) 4033 40-40, Weibull distribution (c), current density (d), polarization values (e) and energy storage properties (f) of sandwich-structured composite films.

BCZTþAg@Al2O3/40% PMMA/PVDF composites, which is attributed to the PVDF as the outer layer with higher dielectric constant (as shown in Fig. 4). In addition, the interfacial polarization is existed between the outer layer and inter layer, which is also helpful to improve the dielectric constant of the composites [60]. It can be seen from Fig. S9b that the dielectric loss and conductivity of 40-33 40-40 are lower than single-layered BCZTþAg@Al2O3/40% PMMA/PVDF and 0-33 40-0, which is induced by the introduction of 40% PMMA/PVDF as outer layer. From Fig. 7c, it can be found that the breakdown strength for both sandwich-structured composite films has been improved compared to that of single-layered BCZTþAg@Al2O3/40% PMMA/PVDF composite films (Eb ¼ 351.0 kV/mm), especially, the value of breakdown strength for 40-33 40-40 reaches 398.5 kV/mm. The 40% PMMA/PVDF outer layer with higher breakdown strength and lower dielectric constant can induce the redistribution of an electric field in the sandwich-structured composite films. Meanwhile, the interfaces of sandwich-structured composite films can restrain the development of electrical trees, resulting in the improved breakdown strength obtained. Fig. 7d shows the current density of the sandwich-structured composites. The 40-33 40-40 composite film has much lower current density than 0-33 400 composite film. This phenomenon is caused by that the 40% PMMA/ PVDF composite film possesses lower current density and more excellent insulativity than that of PVDF. At the same electric field, 0-33 400 composite film has higher Dmax and Dr than 40-33 40-40 composite film as seen in Fig. 7e, leading to higher energy storage density and lower efficiency for 0-33 40-0 composite film (U ¼ 10.4 J/cm3, η ¼ 48.2% at 380 kV/mm). However, 40-33 40-40 composite film has higher efficiency and slightly lower energy storage density at the maximum electric field (U ¼ 9.6 J/cm3, η ¼ 69.8% at 400 kV/mm). Both of the sandwich-structured composite films exhibit significantly improved energy storage density compared to that of single-layered BCZTþAg@Al2O3/40% PMMA/PVDF composite film. The results indi­ cate that the design of sandwich-structured composite films is beneficial to improve the breakdown strength and energy storage properties. The

Fig. 8. Comparison diagram of the energy storage properties for the repre­ sentative polymer-based materials. 8

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Composites Part B 177 (2019) 107429

Fig. 9. Electric field simulation diagram of single-layered composite films, (a1) BCZT/40% PMMA/PVDF, (b1) BCZTþAg/40% PMMA/PVDF, (c1) BCZTþAg@Al2O3/ 40% PMMA/PVDF, where (a2), (b2) and (c2) are locally amplified diagram, respectively, the potential line distribution for sandwich structured films (d1) 0-33 400 (d2) 40-33 40-40, and the space charge density simulation for sandwich structured films (e1) 0-33 40-0, (e2) 40-33 40-40.

40-40. This phenomenon will lead to the increased dielectric loss and leakage current density of 0–33 40-0 which are higher than that of 40-33 40-40.

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4. Conclusions In this study, it can be found that the PVDF blended with 40 vol% PMMA exhibits higher breakdown strength and energy efficiency. In order to further improve the dielectric constant of the polymer films, the inorganic fillers with different structure have been designed and pre­ pared by electrostatic spinning technique. The results show that the composite films filled with BCZTþAg@Al2O3 fibers possess higher breakdown strength, lower dielectric loss and leakage current. For further improving the energy storage properties, the sandwichstructured composite films have been designed. The results show that the sandwich-structured composite film with 40% PMMA/PVDF as outer layer and BCZTþAg@Al2O3/40% PMMA/PVDF as inter layer exhibits excellent energy storage properties, and the energy storage density and efficiency are 9.6 J/cm3 and 69.8%, respectively, at an electric field of 400 kV/mm. Both of the energy storage density and efficiency have been considered and significantly improved compared with that of pure PVDF films. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51807042), the National Natural Science Foundation of Heilongjiang Province (No. TD2019E002), China Postdoctoral Science Foundation (No. 2018M640303). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2019.107429. References [1] Whittingham MS. Materials Challenges facing electrical energy storage. MRS Bull 2008;33:411–9. [2] Liu FH, Li Q, Li ZY, Liu Y, Dong LJ, Xiong CX, Wang Q. Poly(methyl methacrylate)/ boron nitride nanocomposites with enhanced energy density as high temperature dielectrics. Compos Sci Technol 2017;142:139–44. [3] Maurizio R, Guido P. Status quo and future prospects for metallized polypropylene energy storage capacitors. IEEE Trans Plasma Sci 2002;30(5):1939–42. [4] Huang XY, Jiang PK. Core-shell structured high-k polymer nanocomposites for energy storage and dielectric applications. Adv Mater 2015;27(3):546–54. [5] Hu PH, Jia Z, Shen ZH, Wang P, Liu XR. High dielectric constant and energy density induced by the tunable TiO2, interfacial buffer layer in PVDF nanocomposite contained with core-shell structured TiO2@BaTiO3 nanoparticles. Appl Surf Sci 2018;31:824–31.

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[57] [58] [59]

[60] [61]

[62] [63] [64]

[65] [66] [67] [68]

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