High Energy Storage Density for Poly(vinylidene fluoride) Composites

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High energy storage density for poly(vinylidene fluoride) composites by introduced core-shell CaCu3Ti4O12@Al2O3 nanofibers Qingguo Chi, Xubin Wang, Changhai Zhang, Qingguo Chen, Minghua Chen, Tiandong Zhang, Liang Gao, Yue Zhang, Yang Cui, Xuan Wang, and Qingquan Lei ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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ACS Sustainable Chemistry & Engineering

High energy storage density for poly(vinylidene fluoride) composites by introduced core-shell CaCu3Ti4O12@Al2O3 nanofibers Qingguo Chi,†,‡,§ Xubin Wang,†,‡ Changhai Zhang,†,* Qingguo Chen,†,‡ Minghua Chen,†,‡ Tiandong Zhang,†,‡,* Liang Gao,†,‡ Yue Zhang,† Yang Cui,†,‡ 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 ‡

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 *Corresponding Author:

E-mail: [email protected], [email protected] ABSTRACT: In this paper, the one-dimensional (1D) Al2O3 nanofibers (Al2O3 NFs), CaCu3Ti4O12 nanofibers (CCTO NFs) and core-shell CaCu3Ti4O12@Al2O3 nanofibers (CCTO@Al2O3 NFs) were prepared via electrospinning technique. The surface modification with dopamine (PDA) was employed for the above three kinds of nanofibers before being filled the PVDF matrix, which can improve their dispersion and compatibility with the matrix. The microstructure, dielectric properties, leakage

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current density, breakdown strength and energy storage performance of composites with three kinds of filler, CCTO NFs/PVDF, Al2O3 NFs/PVDF and CCTO@Al2O3 NFs/PVDF, were systematically investigated. By comparing the three composites, it can be found that energy storage density of CCTO@Al2O3 NFs/PVDF were enhanced compared to that of pure PVDF, which can be attributed to improvement of polarization and electric breakdown strength. The energy density of 8.46 J/cm3 at 340 kV/mm was obtained for 4 vol.% CCTO@Al2O3 NFs/PVDF nanocomposites, which is 230% larger than that of PVDF (3.68 J/cm3 at 330 kV/mm). This study provides a method for preparing high energy storage PVDF-based composite film which can be used for the next generation of dielectric capacitors. KEYWORDS: core-shell CaCu3Ti4O12@Al2O3 nanofibers, poly(vinylidene fluoride) nanocomposite, dielectric properties, energy density. 

INTRODUTION

In recent years, the dielectric capacitors with excellent energy storage performance have attracted much attention, which have been considered as the candidates for high power energy storage applications because of fast charge and discharge rates.1, 2, 4, 5 It is well known that the batteries have a high energy storage density, however, accompanying with a low power density and serious environmental pollution problem.3 Super capacitors have a medium energy density and power density, but they have the shortcomings of complicated structure, large leakage current and short cycle.3 Dielectric capacitors not only have a high power density, but also possesses high power density and long cycle, however, its energy storage density is low,4, 5 2


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which is needed to be further enhanced. In recent years, the polymer materials, which has high mechanical properties, light weight, low cost, easy processing, high electric breakdown strength and excellent dielectric constant, have become a widely used dielectric material with the development of power electronic technology.4-6 The commonly used polymer materials, such as Polyethylene (PE), biaxially oriented polypropylenes (BOPP), rubber, epoxy, Polyimide (PI), especially for Poly(vinylidene fluoride) (PVDF) which possesses high dielectric constant, have attracted much attention.7-15 For a dielectric capacitor, the energy density (U) of the dielectric material is described as: 𝑈 = ∫ 𝐸d𝐷

(1)

where E represents the breakdown strength and D is the electric displacement,15, 16

herein, D can be further expressed as: D=ε0εrE

(2)

where εr means the dielectric constant of dielectric materials. In order to improve the energy density of the PVDF, one way is to improve its breakdown strength by adding some nano-inorganic fillers with wide band-gap, such as two-dimensional BN nanosheets (BN NTs). Another way is to enhance its polarization by filling high dielectric nano-inorganic fillers, such as zero-dimensional BaTiO3 nanoparticles (BT NPs), one-dimensional BaxSr1-xTiO3 nanowires (BST NWs), CaCu3Ti4O12 nanofibers (CCTO NFs). During the introduction of nano-inorganic fillers into the polymer matrix, some problems were appeared. For instance, the agglomeration phenomenon can be found due to the nano-inorganic fillers with large specific surface area and

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surface energy, which results in some voids and structural defects in the polymer materials, leading to the degradation of electric breakdown strength and mechanical properties.17-30 It was reported that surface modification for fillers is beneficial to uniform distribution in the polymer matrix.14-19 For example, Song et al. selected the poly(vinylidene fluoride-trifluoroethylene) as a matrix and BT NFs as fillers for preparing the composites, respectively. The BT NFs was modified by dopamine (PDA). The results showed that the dielectric constant and breakdown strength of the polymer nanocomposites were improved significantly. Because surface modification by dopamine improves the interfacial compatibility between BaTiO3 and polymer matrix.14 Hu et al. prepared Bi2O3-doped Ba0.3Sr0.7TiO3 nanofibers (BSBT NFs) by PDA modification, and fabricated BSBT NFs/P(VDF-TrFE) flexible nanocomposites by solution casting. It was also found that the dielectric constant and breakdown strength significantly enhanced several times in comparison with the pure polymer matrix, which could be attributed to the combined effects of the surface modification, large aspect ratio and paraelectric polarization behavior of the BSBT fibers.15 Meanwhile, another problem is that the obvious dielectric difference between nano-inorganic fillers and polymer matrix gives rise to local electric field distortion and reduces electric breakdown strength of the composites induced by the accumulation of amount of charge between the interface of fillers and matrix. Both of the nonuniform distribution of inorganic fillers and the distortion of local electric field are detrimental to electric breakdown strength and energy storage performance of composites. In another words, the increased polarization of composites by doping

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inorganic fillers with high dielectric constant was always obtained at the expense of electric breakdown strength, it is still a challenge for obtaining both of high polarization and high electric breakdown strength simultaneously. In order to solve the above main problems, the core-shell structure nano-inorganic fillers CaCu3Ti4O12@Al2O3 nanofibers have been designed and prepared, the Al2O3 NFs outer layer with a moderate dielectric constant can relieve the local electric filed distortion induced by large dielectric difference between the fillers and matrix, leading to the enhancement of electric breakdown strength of the composites. Meanwhile, the CCTO NFs with high dielectric constant and the interface polarization effect between CCTO NFs and Al2O3 NFs can obviously enhance the polarizability of composites. Besides, the surface modification with PDA for CCTO@Al2O3 NFs has been employed to improve the uniform distribution of fillers in the PVDF matrix. Simultaneously, for comparison with that of CCTO@Al2O3 NFs/PVDF composites, the Al2O3 NFs/PVDF composites and the CCTO NFs/PVDF composites with different contents of fillers have been systematically studied. The results indicate that the excellent energy storage performance has been achieved in CCTO@Al2O3 NFs/PVDF composites. 

EXPERIMENTAL AND METHODS Materials. Poly(vinylidene fluoride) (PVDF, FR 401) was purchased from

Shanghai 3F New Material Co., Ltd., China. Cupric acetate (Cu(CH3COO)2•H2O), calcium

nitrate

(Ca(NO3)2•4H2O),

tetrabutyltitanate

(Ti(OCH2CH2CH2CH3)4),

Aluminum nitrate(Al(NO3)3•9H2O), polyvinylpyrrolidone (PVP, Mw=1,300,000), 5



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N,N-dimethylformamide (DMF), ethanol and acetic acid were supplied by Sinopharm Chemical

Reagent

Co.,

China.

Dopamine

hydrochloride

and

tris-(hydroxymethyl)-aminomethane (Tris) were provided by Aladdin. All other chemicals were obtained as analytical grade products and used without further purification.

Preparation of inorganic nanofibers. The 1D core-shell CCTO@Al2O3 NFs were prepared via coaxial electrospinning technique as shown in Fig. S1 in Supporting Information and preparation process was given as below. Firstly, Al(NO3)3•9H2O was dissolved in water and stirred for 0.5 h to form a homogeneous solution A (Sol A). The molar ratio of Al(NO3)3•9H2O to water is 1:17.4. Secondly, PVP was dissolved in ethanol to form a mass fraction of 6% PVP alcohol solution B (Sol B). The mass ratio of Al(NO3)3•9H2O to PVP is 1:1. Then put Sol A into Sol B and stirred for 3 h to form a spinning precursor. Thirdly, (CH3COO)2Cu•H2O and Ca(NO3)2•4H2O were dissolved in ethanol and then acetic acid was added to promote the precipitation of copper ions. After stirring for 0.5 h, Ti(OCH2CH2CH2CH3)4 was added to form a homogeneous solution. The molar ratio of copper ions, calcium ions and titanium ions was 1:3:4. Then PVP was added to form a mass fraction of 5% solution and stirred for 3h to form a spinning precursor. The precursors were electrospun at 1.5 kV/cm and the injection rate was 0.1 ml/min for the CCTO core-layer and 0.15 ml/min for the Al2O3 shell-layer. The fibers was heated to 400 °C for 1 h with the raising temperature rate of 2 °C /min, and followed by heated at 950 °C for 1 h to obtain CCTO@Al2O3 NFs. Meanwhile, the preparation method of CCTO NFs and Al2O3 NFs is given in 6


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Supporting Information.

In order to improve the interface compatibility between the inorganic fillers and the PVDF matrix, the surface modification was employed for all inorganic fillers. The mass ratio of PDA to inorganic fillers was 1:10. Firstly, inorganic fillers were added into 2 g/L (C4H11NO3) standard buffer, then adjusted pH to 8.5 with dilute hydrochloric acid before the PDA was added. The modified inorganic fillers were obtained by stirring for 12 h before the centrifuge and evaporation.

Preparation of composites. The nanocomposite films were prepared as follows. Firstly, the fillers were dispersed into DMF and PVDF were dissolved into DMF, mechanical stirred for 24 h to form a homogeneous solution. The solution was followed by coating on a glass, then the as-cast films was put into vacuum-dried at 50 °C for 8 h to remove the solvent. To obtain a nonpolar γ phase and a flat film, the temperature was raised to 200 °C for 7 min and then the film was rapidly put into ice water for quenching. After the quenching process, the film was put into the oven at 50 °C for 10 h to remove the surface water. Finally, the thickness of the obtained film was about 15 μm.

Characterization. The crystalline structures of Al2O3 NFs, CCTO NFs and CCTO@Al2O3 NFs, as well as the PVDF-based composites, were performed using an X-ray diffractometer (PANalytical Empyrean XRD). The core-shell structure of the CCTO@Al2O3 NFs was observed by Transmission electron microscopy (TEM). The size of nanofibers and the distribution of nanofibers in the matrix were observed by 7



Scanning electron microscopy (SEM). The dielectric properties of nanocomposites in the frequency range of 100 Hz-1 MHz were measured using a dielectric spectroscopy (Novocontrol Alpha-A). The DC breakdown strength of nanocomposites was tested using experimental transformers. Using a plate electrode, the voltage was raised at a rate of 200 V/s and performed in a silicone oil bath at room temperature. Each group of composites tested 10 effective points and the thickness of the composites was about 15 μm. The ferroelectric performance test of nanocomposites was carried out by using ferroelectric comprehensive test system.



RESULTS AND DISCUSSION

The crystalline structures of inorganic filler phase and the PVDF-based composites were shown in Fig. 1. Both of α and γ phases appear when the Al2O3 NFs was annealed at a temperature of 1000 °C. The crystal planes, as presented in Fig. 1 (a), have been indexed according to the standard PDF cards (No. 01-077-039 and No. 01-077-2135). The diffraction peaks appear at 2θ of 25.6°, 35.2°, 43.4°, 52.6°, 57.6° corresponding to α-phase, and the diffraction peaks index at 2θ of 19.4°, 32.0°, 37.7°, 39.4°, 45.9°, 60.8°, 66.9°, 85.0°corresponding to γ-phase. The XRD diffraction peaks of CCTO NFs can be found at the 2θ position of 2θ=29.5°, 34.2°, 38.4°, 42.2°, 45.8°, 49.2°, 61.3°, 72.1°, 82.3°, which are indexed by standard PDF card (No. 96-153-2159). According to results of XRD diffraction for CCTO@Al2O3 NFs, the diffraction peaks of Al2O3 and CCTO NFs can be found, which indicate that there is no other impurity and chemical reaction between Al2O3 and CCTO NFs. The Fig. 1 (b) shows the XRD diffraction peaks of CCTO@Al2O3 NFs/PVDF nanocomposites with 8


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different doping contents. It was reported that β and γ phases of PVDF were observed by stretching and quenching process.31-33 The diffraction peak of α phase is at 2θ=17.8°, and the β phase at 2θ=20.7°, 20.8°, as well as γ phase at 2θ=18.3°, 20.0° and 26.5°. At the same time, the XRD diffraction peaks of CCTO@Al2O3 NFs were found in PVDF composites and the diffraction peaks became more and more obvious as the content increased. The result indicates that the addition of inorganic filler does not destroy the structure of the PVDF matrix and does not react with the matrix, but the physical complex.

Fig. 1 The crystalline structures of inorganic filler phase and the PVDF-based composites. (a) Al2O3 NFs, CCTO NFs and CCTO@Al2O3 NFs, (b) CCTO@Al2O3 NFs /PVDF nanocomposites with different volume fractions.

In order to further investigate the microstructure of inorganic filler, especially for the CCTO@Al2O3 NFs with core-shell structure, transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) were performed, as shown in Fig. 2 (a) and (b), respectively. It can be clearly seen that nanofibers were consist with CCTO core-layer and Al2O3 shell-layer structure, in which the thickness of the CCTO core-layer and the Al2O3 shell-layer was about 200 nm and 50 nm, respectively, as shown in Fig. 2 (a). The EDS analysis was employed to identify the elemental 9



distribution, as exhibited in Fig. 2 (b). The A point shows that the CCTO core-layer is dominated by Al, O, Ca, Cu and Ti elements, while B point represents that the Al2O3 shell-layer is mainly composed by Al and O elements. The TEM and EDS results indicate that the CCTO@Al2O3 NFs with core-shell structure have been prepared successfully.

Fig. 2 (a) The TEM patterns of CCTO@Al2O3 NFs, (b) EDS analysis corresponding (a).

The SEM patterns of inorganic fillers phase and the PVDF-based composites are shown in Fig. 3. The Fig. 3 (a1), 3 (b1), 3 (c1) shows SEM results of Al2O3 NFs, CCTO NFs and CCTO@Al2O3 NFs, respectively. We can see that the diameter of Al2O3 NFs with smooth surface is about 200-300 nm as shown in Fig. 3 (a1). The surface of CCTO NFs looks like more rough, the diameter of CCTO NFs is about 200-300 nm as shown in Fig. 3 (b1). The surface of the CCTO@Al2O3 nanofibers is more smoother compared to that of CCTO NFs, the diameter is about 300 nm as shown in Fig. 3 (c1), which is consistent with the TEM results. The schematic diagraphs of Al2O3, CCTO, and CCTO@Al2O3 nanofibers and composites are given as Fig. 3(a2), 3(b2), 3(c2), respectively. The cross-section SEM results for the Al2O3

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NFs/PVDF nanocomposites, CCTO NFs/PVDF nanocomposites, and CCTO@Al2O3 NFs/PVDF nanocomposites are given as Fig. 3 (a3), 3 (b3), 3 (c3), respectively, the content of inorganic filler in Fig. 3 (a3) (b3) (c3) is 6 vol.%. It can be found that the nanofibers are uniformly dispersed in the PVDF matrix, which may be benefit from the surface modification for the inorganic NFs with PDA.

Fig. 3 The SEM patterns of inorganic fillers (a1) Al2O3 NFs, (b1) CCTO NFs, (c1) CCTO@Al2O3 NFs, the schematic diagraphs of fibers and nanocomposites (a2) Al2O3 NFs/PVDF, (b2) CCTO NFs/PVDF, (c2) CCTO@Al2O3 NFs/PVDF, the cross-section SEM patterns of the PVDF-based nanocomposites with an inorganic filler content of 6 vol.%, (a3) Al2O3 NFs/PVDF, (b3) CCTO NFs/PVDF, (c3) CCTO@Al2O3 NFs/PVDF.

Dielectric properties of the CCTO@Al2O3 NFs/PVDF nanocomposites with different volume fractions at room temperature in the frequency range of 100 Hz-1 MHz are shown in Fig. 4 (a). The dielectric constant of the CCTO@Al2O3 NFs/PVDF nanocomposites decreases with increasing frequency, because the dipole mobility of

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the polymer matrix is limited at high frequencies, resulting in a decreased dielectric constant. The dielectric constant of nanocomposites increases gradually with the increase of the content of Al2O3 NFs, CCTO NFs and CCTO@Al2O3 NFs as shown in Fig. S2 in the Supporting Information. The dielectric constant of 6 vol.% CCTO@Al2O3 NFs/PVDF nanocomposites reaches 18.4 at 100 Hz. The dielectric loss of the CCTO@Al2O3 NFs/PVDF nanocomposites decreases slightly at low frequency ranged from 102 to 103 Hz, which is attributed to the Maxwell-Wagner-Sillars interfacial polarization. Little variation in the frequency range from 103 to 105 Hz, and then an increase in the dielectric loss tangent at high-frequency (105~106 Hz) is attributed to the α relaxation, which is associated with the glass transition of the pure PVDF polymer.34-36 It is noted that all the composites have a lower loss tangent at high frequency, overall less than 0.13. In order to further stress the effect of CCTO@Al2O3 NFs core-shell structure on the dielectric properties, the dielectric properties of Al2O3 NFs/PVDF, CCTO NFs/PVDF and CCTO@Al2O3 NFs/PVDF composites have been investigated and shown in Fig. 4 (c). It is obvious that the CCTO NFs play an important role in enhancing the dielectric constant of PVDF, which is attributed to its giant dielectric constant, however, the dielectric loss of CCTO NFs/PVDF is higher than that of Al2O3 NFs/PVDF, CCTO@Al2O3 NFs/PVDF, and this phenomenon may be caused by the weak ability to bind charge. Besides, although the Al2O3 NFs possesses well ability to bind charge induced by its wide band-gap,37 leading to the lower dielectric loss in Al2O3 NFs/PVDF, the dielectric constant increases slightly. What is most important is that both of increased dielectric

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constant and decreased dielectric loss have been obtained in CCTO@Al2O3 NFs/PVDF, the CCTO NFs with high dielectric constant can improve the dielectric constant of the composites due to the interface polarization, meanwhile, the Al2O3 NFs shell-layer and barrier effect between the Al2O3 NFs shell-layer and CCTO NFs core-layer impede carriers migration and decrease dielectric loss. The electric-field dependence of the leakage current density of the CCTO@Al2O3 NFs/PVDF nanocomposites with different volume fractions is shown in Fig. 4 (b). The leakage current density of CCTO@Al2O3 NFs/PVDF is higher than that of pure PVDF, which may be caused by the percolation effect and trapped charge at the interface between the inorganic fillers and matrix. With the increase of CCTO@Al2O3 NFs volume fractions, all the composites can also maintain a lower leakage current density at high field strength, the 4 vol.% CCTO@Al2O3 NFs/PVDF exhibits the lower leakage current density. Besides, the leakage current densities measured at 100 kV/mm are compared with the three composites as shown in Fig. 4 (d). The leakage current density of the CCTO@Al2O3 NFs/PVDF nanocomposites is significantly lower than that of CCTO NFs/PVDF nanocomposites as shown in Fig. S3 in the Supporting Information, this phenomenon may be attributed to suppressed carrier migration induced by the core-shell structure of Al2O3 insulation shell-layer and CCTO core-layer. The low leakage current density is beneficial for enhancing the breakdown field strength and energy storage density of the composites.

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Fig. 4 (a) Frequency-dependence of the dielectric constant (εr) and dielectric loss (tan δ), (b) Electric-field dependence of the leakage current density of the CCTO@Al2O3 NFs/PVDF nanocomposites, (c) εr, tan δ, and (d) leakage current density of Al2O3 NFs/PVDF nanocomposites, CCTO NFs/PVDF nanocomposites, CCTO@Al2O3 NFs/PVDF nanocomposites with different volume fractions.

For ferroelectric materials, the polarization of the materials is not linear dependent on the applied electric field. Thus, the energy density is also related to the polarization with respect to the applied electric field. Generally, the Sawyer-Tower circuit is employed to determine the relationship between the polarization and electric field. On the basis of this approach, the energy storage capability of the dielectric materials can be evaluated from the D-E loops.38 The D-E loops measured at 100 Hz with varying electric field for different nanocomposites are presented in Fig. S4 in the Supporting Information, it can be found that the electrical displacement of

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CCTO@Al2O3 NFs/PVDF nanocomposites is significantly higher at the same volume fraction. Fig. 5 (a) shows the electric-field dependence of the energy density and efficiency of different volume fractions of the CCTO@Al2O3 NFs/PVDF nanocomposites. The energy density of 4 vol.% CCTO@Al2O3 NFs/PVDF nanocomposites reaches 8.46 J/cm3 at 340kV/mm, which is 230% over than pure PVDF (3.68 J/cm3 at 330 kV/mm), and the efficiency is 0.54, which may be attributed to the enhancement of electric displacement for CCTO@Al2O3 NFs/PVDF. Fig. 5 (b) shows the electric-field dependence of the energy density and efficiency of different nanocomposites at the same volume fractions. Due to the large dielectric difference between CCTO fillers and PVDF matrix, this leads to the serious distortion of electric field between the interface of CCTO fillers and PVDF matrix, resulting in the lower breakdown field strength and lower energy density of CCTO NFs/PVDF nanocomposites. Although the Al2O3 NFs/PVDF nanocomposites can endure a higher electric field, the lower electric displacement leads to lower energy density as shown in Fig. S5 in the Supporting Information. The CCTO@Al2O3 NFs/PVDF nanocomposites can endure a higher electric field and have a higher electric displacement, which improve the energy density.

Fig. 5 (a) Energy Density and Efficiency of different volume fractions of the CCTO@Al2O3 15



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NFs/PVDF nanocomposites, (b) Energy Density and Efficiency of different nanocomposites at the same volume fractions.

Table.1 shows the comparison of electric field strength (E) and discharged energy density (Ue) in this work and the related dielectric composites reported in previous literature studies. It is noteworthy that the designed and prepared PVDF-based composites of CCTO@Al2O3 NFs with core-shell structures have a better energy storage density than many composites reported previously.25, 39-46 Table.1 Comparison of electric field strength (E) and discharged energy density (Ue) in this work and the related dielectric composites reported in previous literature studies

Sample E(kV/mm) Ue(J/cm3) 8 vol.% BT@AO-DA NFs/PVDF 300 6.5 2.1 vol.% BCZT NFs/PVDF 340 5.9 2 wt.% c-PVDF/PDA@BT 275 2.9 12 wt.% BNNS/PMMA 400 3.5 2.5 vol.% BT@Ag NFs/ PVDF 350 7.75 5 vol.% PVP modified ST NP/PVDF 270 5.1 2.7 vol.% h-BN-RGO/epoxy 100 0.2 3 vol.% BT@SiO2/PI 346 2.13 3 vol.% BT/PI 270 1.7 2 vol.% CCTO NFs/PVDF 180 2.8 4 vol.% Al2O3 NFs/PVDF 380 5.1 4 vol.% CCTO@Al2O3 NFs/PVDF 340 8.5

η 0.53 0.53 0.75 0.85 0.56 0.65 0.77 / 0.55 0.63 0.54 0.54

Ref. 2017 (ref. 25) 2017 (ref. 39) 2017 (ref. 40) 2017 (ref. 41) 2017 (ref. 42) 2017 (ref. 43) 2017 (ref. 44) 2017 (ref. 45) 2017 (ref. 46) this work this work this work

Increasing the breakdown strength is another way to improve the discharge energy density. Fig. 6 (a) shows the Weibull breakdown of different contents of the CCTO@Al2O3 NFs/PVDF nanocomposites. According to Weibull distribution formula: P(𝐸) = 1 − exp⁡(−(𝐸 ⁄𝐸b )

𝛽

(3)

where P(E) represents the probability of failure, E represents the breakdown field strength of the experiment, 𝐸b is characteristic breakdown strength at the cumulative 16


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failure probability of 63.28%, and β is the shape parameter associated with linear regressive fit of the data distribution. Formula deformation, taking both sides of the logarithm can be linear formula: ln⁡(− ln(1 − P)) = 𝛽(ln𝐸 − ln𝐸b )

(4)

ln⁡(− ln(1 − P)) and ln𝐸 are plotted in Fig. 6 (a). The results of the linear fitting are shown in Table.2. Different contents of the CCTO@Al2O3 NFs/PVDF nanocomposites have higher shape parameters, indicating that the breakdown strength is more concentrated. Compared with pure PVDF, the breakdown strength of 2 vol.% CCTO@Al2O3 NFs/PVDF nanocomposites increases from 331.51 kV/mm to 345.30 kV/mm, and 4 vol.% CCTO@Al2O3 NFs/PVDF nanocomposites increases from 331.51 kV/mm to 353.77 kV/mm. This may be attributed to the following reasons: firstly, CCTO@Al2O3 NFs uniformly disperse in the PVDF matrix, and at the interface between CCTO@Al2O3 NFs and PVDF, the polymer chains are tightly bonded to the CCTO@Al2O3 NFs and within these regions the mobility of the polymer chains is thus decreased. The transfer of charge carriers through the loose polymer chains not bonded to the nanofibers is suppressed, giving rise to higher breakdown strength.47 Secondly, CCTO@Al2O3 NFs have a large aspect ratio and tend to be oriented in the in-plane direction during solution casting, resulting in anisotropy of the composites, which can effectively reduce the electric field applied in the out-of-plane direction.17 Besides, large aspect ratio nanofibers may restrain the growth of electrical treeing by bringing up twisted pathways for treeing within the nanocomposites.38, 48 As the filling content increases, the breakdown field decreases, it

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is possible that more voids and cracks may be introduced. As shown in Fig. 6 (b), the breakdown strength of CCTO NFs/PVDF nanocomposites is much lower than that of CCTO@Al2O3 NFs/PVDF and Al2O3 NFs/PVDF (more detailed in Fig. S6, Table S1-S2 in the Supporting Information). The local electric field distortion can be induced by large dielectric difference between CCTO and PVDF, resulting in the lowest breakdown strength.

Fig. 6 (a) Weibull breakdown strength of different volume fractions of the CCTO@Al2O3 NFs/PVDF nanocomposites, (b) Breakdown strength of different nanocomposites. Table.2 Linear fitting results and Weibull parameters of the CCTO@Al2O3 NFs/PVDF nanocomposites.

linear ﬁtting results CCTO@Al2O3 NFs/PVDF slope ln(−ln(1−P(E)) R intercept 0 14.67 -85.14 0.99 2 vol.% 16.97 -99.18 0.98 4 vol.% 17.28 -101.41 0.98 6 vol.% 12.61 -71.89 0.96

Weibull parameters S 0.56 0.75 0.81 0.88

β

Eb

14.67 16.97 17.28 12.61

331.51 345.30 353.77 299.18

It is well known that except the influence of dielectric difference between the fillers and matrix, the agglomeration of fillers also play an important role on the breakdown strength, because all of fillers in this study have been modified by

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dopamine, so the inorganic fillers disperse uniformly, and the influence of electric breakdown strength of composites is mainly determined by the dielectric difference. In order to further study the influence of the CCTO@Al2O3 NFs with core-shell structure on the local electric field of the composite, the electric field of the composite with Al2O3 NFs, CCTO NFs and CCTO@Al2O3 NFs as filler phases has been analyzed by finite element method. The inorganic fillers with high dielectric constant became the local electric field defect center in the matrix, which give rise to the local electric field distortion, resulting in the decreased electric breakdown strength. We used COMSOL Multiphysics to simulate the electric field of the composites, the selected module is the electrostatic field under steady state. In the simulation, the dielectric constant of CCTO was set to 105,49 Al2O3 was 10,50 and PVDF was 8.5.13 An electric field of 200 kV/mm was applied. Fig. 7 shows the finite element simulation of the electric field distribution in the composite. The colors in the figure reflect the magnitude of the local electric field strength. It can be seen that the field strength of Al2O3 NFs/PVDF does not change significantly, while the local field strength of CCTO NFs/PVDF varies greatly, indicating that the large dielectric difference between CCTO and PVDF results in serious distortion of the electric field and reduces the breakdown field strength of the composites. In order to relieve the local electric field distortion and improve the breakdown field strength of composites, a layer of Al2O3 is coated to reduce the dielectric constant difference of CCTO and PVDF. As shown in Fig. 7 (c1), the local electric field distortion of CCTO@Al2O3 NFs/PVDF nanocomposites is effectively alleviated. It is worth noting that the

19



distortion of the field strength is more serious at the interface between the inorganic fillers and the matrix as shown in Fig. 7 (b2). It shows that the charge accumulates more in these places. Al2O3 as an insulating layer can reduce the charge accumulation as shown in Fig. 7 (c2). The results show that CCTO@Al2O3 NFs/PVDF nanocomposites have an excellent breakdown performance.

Fig. 7 Finite element simulation of electric field distribution. (a1) Al2O3 NFs/PVDF nanocomposites, (b1) CCTO NFs/PVDF nanocomposites, (c1) CCTO@Al2O3 NFs/PVDF nanocomposites. (a2), (b2) and (c2) local magnification of (a1), (b1) and (c1), respectively.



CONLUSIONS

In this study, the 1D core-shell CCTO@Al2O3 NFs have been prepared via coaxial electrospinning technique and high-temperature calcination. The Al2O3 NFs, CCTO NFs and core-shell CCTO@Al2O3 NFs are chosen as inorganic fillers, the microstructures and electrical performance of CCTO@Al2O3/PVDF have been investigated systematically. Compared with that of CCTO NFs/PVDF composites, the Al2O3 as insulation layer can better impede the carrier migration in the polymer body 20


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and reduce the leakage current density, meanwhile, Al2O3 layer acts as an interfacial buffer layer in CCTO@Al2O3 NFs/PVDF to relieve the dielectric difference between the CCTO and PVDF, leading to a higher electric breakdown strength compared to that of CCTO/PVDF composites. In addition, the improved electric displacement of CCTO@Al2O3 NFs/PVDF has been obtained compared to that of pure PVDF, which may be attributed to doping the CCTO NFs with high dielectric constant and the enhanced interface polarization. The energy density of 4 vol.% CCTO@Al2O3 NFs/PVDF nanocomposites reaches 8.64 J/cm3 at 340 kV/mm, which is 230% over than pure PVDF(3.68 J/cm3 at 330 kV/mm), and the efficiency is 0.54. Moreover, the breakdown strength of 4 vol.% CCTO@Al2O3 NFs/PVDF nanocomposites reaches to 353.77 kV/mm. The constructed 1D core-shell CCTO@Al2O3 NFs as inorganic fillers has been an effective way to improve the energy storage density and discharge efficiency of polymer-based capacitors. 

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental methods, supplemental results, and additional figures including dielectric constant and dielectric loss, leakage current density, D-E hysteresis loops, energy density and efficiency, Weibull breakdown strength and linear fitting results and Weibull parameters (PDF). 

AUTHOR INFORMATION

21



Page 22 of 38

Corresponding Author *Changhai Zhang. E-mail: [email protected].

*Tiandong Zhang. E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. 

ACKNOWLEDGMENTS

The authors thanks the support of the National Natural Science Foundation of China (61640019), the Youth Innovative Talents Training Plan of Ordinary Undergraduate Colleges in Heilongjiang (UNPYSCT-2016157), the Open Foundation of State Key Laboratory of Electronic Thin Films and Integrated Devices (KFJJ201601), Natural Science Foundation of Heilongjiang Province of China (B201408), Science Funds for the Young Innovative Talents of HUST (201102). 

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Synopsis: Environment-friendly poly(vinylidene fluoride) composites possesses high energy storage by introduced large aspect ratio core-shell CaCu3Ti4O12@Al2O3 nanofibers with low doping content.

31



Fig. 1 The crystalline structures of inorganic filler phase and the PVDF-based composites. (a) Al2O3 NFs, CCTO NFs and CCTO@Al2O3 NFs, (b) CCTO@Al2O3 NFs /PVDF nanocomposites with different volume fractions. 46x15mm (300 x 300 DPI)


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Fig. 2 (a) The TEM patterns of CCTO@Al2O3 NFs, (b) EDS analysis corresponding (a). 62x28mm (300 x 300 DPI)



The SEM patterns of inorganic fillers (a1) Al2O3 NFs, (b1) CCTO NFs, (c1) CCTO@Al2O3 NFs, the schematic diagraphs of fibers and nanocomposites (a2) Al2O3 NFs/PVDF, (b2) CCTO NFs/PVDF, (c2) CCTO@Al2O3 NFs/PVDF, the cross-section SEM patterns of the PVDF-based nanocomposites with an inorganic filler content of 6 vol.%, (a3) Al2O3 NFs/PVDF, (b3) CCTO NFs/PVDF, (c3) CCTO@Al2O3 NFs/PVDF. 103x89mm (300 x 300 DPI)


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Fig. 4 (a) Frequency-dependence of the dielectric constant (εr) and dielectric loss (tan δ), (b) Electric-field dependence of the leakage current density of the CCTO@Al2O3 NFs/PVDF nanocomposites, (c) εr, tan δ, and (d) leakage current density of Al2O3 NFs/PVDF nanocomposites, CCTO NFs/PVDF nanocomposites, CCTO@Al2O3 NFs/PVDF nanocomposites with different volume fractions. 94x63mm (300 x 300 DPI)



Fig. 5 (a) Energy Density and Efficiency of different volume fractions of the CCTO@Al2O3 NFs/PVDF nanocomposites, (b) Energy Density and Efficiency of different nanocomposites at the same volume fractions. 47x16mm (300 x 300 DPI)


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Fig. 6 (a) Weibull breakdown strength of different volume fractions of the CCTO@Al2O3 NFs/PVDF nanocomposites, (b) Breakdown strength of different nanocomposites. 53x20mm (300 x 300 DPI)



Fig. 7Finite element simulation of electric field distribution. (a1) Al2O3 NFs/PVDF nanocomposites, (b1) CCTO NFs/PVDF nanocomposites, (c1) CCTO@Al2O3 NFs/PVDF nanocomposites. (a2), (b2) and (c2) local magnification of (a1), (b1) and (c1), respectively. 75x40mm (300 x 300 DPI)


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High Energy Storage Density for Poly(vinylidene fluoride) Composites

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