Improved energy storage performance of nanocomposites with Bi4.2K0.8Fe2O9+δ nanobelts

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Improved energy storage performance of nanocomposites with Bi4.2K0.8Fe2O9þd nanobelts Chuangming Hou a, 1, Zhiwei Bao a, 1, Haoyang Sun a, Yuewei Yin a, **, Xiaoguang Li a, b, c, * a Hefei National Laboratory for Physical Sciences at the Microscale, Department of Physics, and CAS Key Laboratory of Strongly-coupled Quantum Matter Physics, University of Science and Technology of China, Hefei, 230026, China b School of Physics and Materials Science, Anhui University, Hefei, 230601, China c Collaborative Innovation Center of Advanced Microstructures, Nanjing, 210093, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 March 2019 Received in revised form 8 April 2019 Accepted 16 April 2019 Available online xxx

Modern electronics and electric power grids require high performance polymer-based dielectric nanocomposites. To realize large-scale applications, the energy density of nanocomposites needs to be further increased. Here, we demonstrate a remarkable improvement in energy density of poly(vinylidene fluoride) (PVDF) matrix upon the incorporation of high-k Bi4.2K0.8Fe2O9þd (BKFO) nanobelts. High aspect ratio BKFO nanobelts can enhance the Young's moduli of the nanocomposites and increase the path tortuosity of electrical trees, which are favorable for increasing the breakdown strength of the system. Thus, the dielectric constant and breakdown strength increase simultaneously at a low volume fraction (0.35 vol%) of BKFO nanobelts, and an ultrahigh recoverable energy density of 25.4 J/cm3 is achieved. These results provide a strategy to develop high performance flexible high-energy-density devices. © 2019 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Dielectric nanocomposite Energy storage Nanobelt Breakdown strength

1. Introduction Dielectric materials can provide an extremely high power density [1], thus are highly desired in modern electronics and pulse power devices [2e4]. However, compared with commercial Li-ion batteries and electrochemical supercapacitors, the electrostatic dielectric capacitors have too low energy densities to be widely utilized for practical applications [5]. According to the classical electrodynamics, the relative dielectric constant (εr) and the breakdown strength (Eb) codetermine the maximum recoverable energy density (Umax) as Umax ¼ ε0 εr E2b =2 in linear dielectric materials, where ε0 is the vacuum permittivity. Although inorganic ceramics show very high εr, their Umax is low because of the low Eb [6,7]. Given that the Umax is approximately proportional to the square of Eb, flexible polymers with high Eb have become a very promising candidate of energy storage dielectrics. Thus, the key

* Corresponding author. Hefei National Laboratory for Physical Sciences at the Microscale, Department of Physics, and CAS Key Laboratory of Strongly-coupled Quantum Matter Physics, University of Science and Technology of China, Hefei, 230026, China. ** Corresponding author. E-mail addresses: [email protected] (Y. Yin), [email protected] (X. Li). Peer review under responsibility of The Chinese Ceramic Society. 1 These authors contributed equally to this work.

point to further increase Umax of polymers is to increase εr of polymers. The inclusion of high-k dielectric nanofillers into polymers is considered to be an effective solution to increase εr of nanocomposites [8]. However, it usually takes a large proportion of nanofillers to make an obvious increase of εr, which will always introduce air voids in nanocomposites and in turn lead to a degraded Eb and limit the further increase of Umax [9]. Furthermore, there exists a dramatically increased local electric field near the polymer/nanofiller interface, which will also lead to a reduced Eb [1,10]. Fortunately, the above-mentioned negative effects have little influence when the volume fraction of nanofillers is low, and the morphology and distribution of nanofillers must be rationally designed to achieve an obviously enhanced energy storage performance. As reported by Tang et al. [11], the εr of nanocomposites increases with increasing the aspect ratio of BaTiO3 nanowires even if the volume fraction of BaTiO3 nanowires is not changed. Recently, several experiments showed that Umax can reach 20 J/cm3 in the poly(vinylidene fluoride) (PVDF)-based nanocomposites with only 3 vol% TiO2 nanowires embedded with BaTiO3 nanoparticles [12] or with only 1 wt% Ti0.87O2 nanosheets [13]. Besides the improved εr, the Eb of the nanocomposites with high aspect ratio SrTiO3 nanowires was also proved to be higher than the Eb of nanocomposites with short SrTiO3 nanorods, because the former nanofillers with higher aspect ratio would lead to a higher path tortuosity in the

https://doi.org/10.1016/j.jmat.2019.04.006 2352-8478/© 2019 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article as: Hou C et al., Improved energy storage performance of nanocomposites with Bi4.2K0.8Fe2O9þd nanobelts, Journal of Materiomics, https://doi.org/10.1016/j.jmat.2019.04.006

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electrical treeing process [14]. Furthermore, high-throughput phase-field simulations showed that nanosheets with a high aspect ratio parallelly aligned in nanocomposites can lead to the simultaneous enhancements of εr and Eb [15]. These results demonstrate that a significantly enhanced energy storage performance is more likely to be achieved in nanocomposites with low volume fraction of high aspect ratio nanofillers. These inspire us to utilize high aspect ratio Bi4.2K0.8Fe2O9þd (BKFO) nanobelts as nanofillers. According to earlier researches [16,17], BKFO nanobelts synthesized by a hydrothermal method have a high aspect ratio. Besides, BKFO nanobelts host regular stacking of the BiFeO3-like perovskite blocks and the rock salt slabs along the c axis of the crystal, and show a very high εr of about 200 at room temperature [16]. These features shall be very favorable for improving the energy storage performance of nanocomposites. Here, we studied the energy storage properties of PVDF-based nanocomposites with high aspect ratio BKFO nanobelts. Benefitting from simultaneous improvements of εr and Eb, an ultrahigh Umax of 25.4 J/cm3 is obtained in the nanocomposite with only 0.35 vol% BKFO nanobelts. 2. Material and methods The BKFO nanobelts were fabricated by a hydrothermal method [16,17]. First, an aqueous solution (5 mL) was prepared by dissolving Bi(NO3)3$5H2O (0.4 mmol), Fe(NO3)3$9H2O (0.19 mmol), and nitric acid (1 mL) in distilled water. The mixture was dropped into KOH solution (15 mL) in a stainless-steel Teflon-lined autoclave (25 mL). The reaction was performed at 190  C for 90 min. The final products were centrifugated and washed. 0.2 g PVDF powder was dissolved into 5 mL N,N-Dimethylformamide and stirred for 10 h. Then, the PVDF solution was mixed with BKFO nanobelts suspension by stirring and sonicating. The mixture was cast onto a clean glass and dried at 110  C for 10 h in an oven. Then, the nanocomposites were uniaxially stretched at 80  C, and their lengths increased from 3 cm to 12 cm within 10 s. The thicknesses of nanocomposites after stretching were about 12 mm. Finally, circular Au electrodes (2 mm in diameter) were sputtered on nanocomposites via shadow masks. The structures of the BKFO nanobelts as well as the nanocomposites were determined by X-ray diffraction (XRD) using Cu Ka1 radiation (l ¼ 1.540598 Å) (Panalytical X'pert). The morphology and energy dispersive spectrum (EDS) of BKFO nanobelts as well as the cross-sectional images of nanocomposites were measured by a scanning electron microscope (SEM) (Zeiss GeminiSEM 500). The infrared spectra of nanocomposites were measured by a Fourier Transform Infrared Spectrometer (Nicolet 8700). The dielectric properties of nanocomposites were measured by an LCR meter (Agilent 4294A) from 100 Hz to 10 MHz. The polarization-electric field (PeE) curves of nanocomposites were measured at a frequency of 10 Hz using monopolar mode by a Radiant Technologies Precision Premier II tester equipped with a high voltage amplifier (TREK MODEL 609B). The Young's moduli of the nanocomposites were measured by a dynamic thermomechanical analysis meter (DMA Q800). 3. Results and discussion The XRD pattern of the ground BKFO nanobelts is shown in Fig. 1a, and the lattice parameters are about a ¼ 5.470 Å, b ¼ 5.468 Å, c ¼ 32.592 Å (space group Fmmm) [16]. As illustrated by the SEM image in Fig. 1b, the width of BKFO nanobelts is 40e70 nm. The length of nanobelts can be several micrometers, much larger than their width and demonstrating a high aspect ratio. The corresponding EDS of BKFO nanobelts is given in the inset

of Fig. 1b, indicating that the BKFO nanobelts are composed of Bi, K, Fe, and O elements. Fig. 1c and d show the XRD patterns and the infrared spectra of both pure PVDF and nanocomposites. As shown in Fig. 1c, PVDF matrix is mainly of polar b-phase. Compared with non-polar a-phase PVDF, polar b-phase PVDF shows a much higher polarization [18,19], and the stretched polymer is proved to have higher breakdown strength [20]. These can induce a much higher energy density in PVDF-based nanocomposites as well. It is noted that after adding BKFO nanobelts, the intensity of a(120) peak of aPVDF appears. This is consistent with the infrared spectra shown in Fig. 1d, in which the characteristic bands at 614 cm1, 764 cm1, and 975 cm1 of a-PVDF appear after adding BKFO nanobelts [21]. These results demonstrate that fractions of a-PVDF in nanocomposites are higher than that in pure PVDF. The reason may be that the BKFO nanobelts partially break the interchain coupling in ferroelectric b-PVDF matrix, and distort the crystalline ordering of PVDF matrix [19]. It should be noted that, as shown in Fig. S1 (Supplementary materials), the BKFO still exhibits as nanobelts with length of about 1e2 mm in both pristine and stretched nanocomposites. Before stretching, the orientations of BKFO nanobelts are random in pristine nanocomposites, as shown in Fig. S1a-c. According to phase field simulations, nanocomposites with nanobelts perpendicular to the electric field will show enhanced energy storage performance [15]. Thus, our nanocomposites were uniaxially stretched to align the BKFO nanobelts as parallel as possible to the surface of nanocomposites, as shown in Fig. S1d-f. Fig. 2a shows the frequency dependencies of εr of nanocomposites with different volume fractions of BKFO nanobelts. Due to the dielectric relaxation of PVDF, the εr of nanocomposites decreases rapidly with increasing frequency [22]. As shown in Fig. 2b, because of the inclusion of high-k BKFO nanobelts, the values of εr at 1 kHz increase rapidly from 7.9 of the PVDF matrix to 11.2 of the nanocomposite with 0.35 vol% BKFO nanobelts, showing an enhancement of 41.8% as compared with the pure PVDF matrix. Then, the εr of the nanocomposite increases slowly to 12.4 with further increasing volume fraction of BKFO nanobelts to 0.9 vol%, and the reason may be that air voids begin to emerge in the nanocomposites [9]. Fig. 2c shows the dielectric loss tangent (tand) of nanocomposites. It should be noted that at about 10 kHz, the frequency of interest for common power conditioning [2], the tand is only 0.043 to 0.053. Below 1 kHz, the tand values of nanocomposites are lower than that of PVDF matrix, which may arise from the tiny crystal structure change of PVDF matrix (see Fig. 1d). At high frequencies, the increased tand with increasing frequency is probably related to the aa relaxation of main chains in PVDF matrix [22]. Besides the increased εr, an enhanced Eb is also highly desired for improving energy storage performance. According to earlier reports, enhanced mechanical properties can lead to higher Eb [4,23]. Usually, rigid nanofillers will lead to enhanced mechanical strength of nanocomposites and a decline in the elongation at break [24]. Fig. 3 shows the room temperature Young's moduli of nanocomposites with different additions of BKFO nanobelts. The Young's moduli of nanocomposites quickly increase from 696 MPa of PVDF matrix to 1539 MPa when the volume fraction of BKFO nanobelts increases to 0.35 vol%, and then increase slowly to 2105 MPa with further increasing BKFO nanobelts to 0.9 vol%. The enhanced Young's modulus means nanocomposites can withstand a higher Coulomb force, and protect the nanocomposites from an electromechanical failure [23]. And as shown in Fig. S2 (Supplementary materials), although the elongation of nanocomposites decreases with increasing volume fraction of BKFO nanobelts, nanocomposites still show good flexibility. Furthermore, it should be noticed that in our stretched nanocomposites, the orientations of

Please cite this article as: Hou C et al., Improved energy storage performance of nanocomposites with Bi4.2K0.8Fe2O9þd nanobelts, Journal of Materiomics, https://doi.org/10.1016/j.jmat.2019.04.006

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Fig. 1. (a) XRD pattern of BKFO nanobelts. (b) SEM image of BKFO nanobelts. The inset is the corresponding EDS of BKFO nanobelts on a silicon substrate. (c) XRD patterns and (d) infrared spectra of the pure PVDF and nanocomposites.

Fig. 2. (a) Frequency-dependent εr of nanocomposites. (b) Variation of εr of nanocomposites with different volume fractions of BKFO nanobelts. The solid line is a guide for eyes. (c) Frequency-dependent tand of nanocomposites.

BKFO nanobelts will be aligned perpendicular to that of electric fields, and a higher electric field is required to let the electric tree develop through the nanobelts [15]. Compared with the situation for low aspect ratio nanofillers, the high aspect ratio BKFO nanobelts will lead to the increased path tortuosity of the electrical tree, which shall be favorable for increasing Eb [14]. Fig. 4a shows that the distribution of the measured Eb follows the Weibull distribution function [10]:

PðEÞ ¼ 1  exp½1  ðEi =Eb Þb 

Fig. 3. Young's moduli of nanocomposites as a function of volume fraction of BKFO nanobelts.

(1)

where P(E) is the probability of electrical breakdown. Ei is the value of measured breakdown strength and is sorted from the smallest to the biggest. Here, i and n denote the sequence number of Ei and the number of total experiments, respectively, and P(E) is equal to i=ðn þ 1Þ. Eb refers to the breakdown strength of samples. b is the Weibull modulus, negatively related to the width of distribution and positively related to the homogeneity of the sample [10,12]. The b of the nanocomposites range from 13 to 40, which are comparable to

Please cite this article as: Hou C et al., Improved energy storage performance of nanocomposites with Bi4.2K0.8Fe2O9þd nanobelts, Journal of Materiomics, https://doi.org/10.1016/j.jmat.2019.04.006

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Fig. 4. (a) Weibull distribution of Eb of the nanocomposites. (b) Characteristic Eb of nanocomposites extracted from the fitting results of Weibull distribution.

previous reports [12,25e27]. As shown in Fig. 4b, the Eb value of 622 MV/m for the pure PVDF matrix is consistent with the best data reported previously [12], while the Eb of nanocomposites increases remarkably to 675 MV/m with increasing volume fraction of BKFO nanobelts to 0.35 vol%. This enhanced Eb will give rise to a higher energy density of the nanocomposite. With further increasing BKFO nanobelts to 0.9 vol%, the Eb decreases to 589 MV/m. This variation is similar to earlier reports that the Eb of nanocomposites will decrease when additive proportion of nanofillers is high, because a conductive path may form through the interfaces of nanofillers and matrix at high electric fields [10]. Besides, the decreased Eb may also be ascribed to the emergent air voids at high volume fractions of BKFO nanobelts [9]. The electric field dependencies of recoverable energy densities (U) and total stored energy densities (Us) can be calculated by U ¼ R Pmax RP EdP and Us ¼ 0 max EdP [28], respectively. Here, Pmax and Pr pr refer to the maximum and remnant polarizations, respectively. In addition, the energy storage efficiency (h) is equal to U=Us . To calculate the values of U and h of nanocomposites, we measured the hysteresis PeE curves of nanocomposites at 10 Hz, as shown in Fig. 5. It can be seen that Pmax of 11.7 mC/cm2 in PVDF matrix is consistent with the data of ferroelectric b-phase PVDF reported previously [18,19,29,30]. After adding high-k BKFO nanobelts, the

Pmax of nanocomposites is higher than that of the PVDF matrix because of the improved εr and Eb. For example, Pmax of the nanocomposite with 0.35 vol% BKFO nanobelts is about 13.0 mC/cm2, showing an enhancement of 11.1% as compared with the pure PVDF matrix. Meanwhile, the nanocomposites with BKFO nanobelts show a relatively lower Pr as compared with the pure PVDF matrix, which may be due to the appearance of the non-polar a-phase PVDF in nanocomposites with BKFO nanobelts. Fig. 6 shows the electric-field-dependent U and h for the nanocomposites with different volume fractions of BKFO nanobelts. Generally, because of the higher εr and lower Pr, the values of U and h of nanocomposites are higher than those of PVDF matrix under the same electric field. This result demonstrates the energy storage performance of nanocomposites is effectively improved with the inclusion of BKFO nanobelts. Fig. 7 shows that the Umax of nanocomposites begins to increase with increasing volume fraction of BKFO nanobelts owing to the simultaneously increased Eb and εr, and then decreases with further increasing volume fraction of BKFO nanobelts due to the decreased Eb. The nanocomposite with 0.35 vol% BKFO nanobelts has the highest Umax of 25.4 J/cm3, showing an enhancement of 40.3% as compared with the pure PVDF matrix of 18.1 J/cm3. Table 1 summarizes the recoverable energy density and energy storage

Fig. 5. (a-f) Monopolar P-E curves of nanocomposites with different volume fractions of BKFO nanobelts. The insets show the P-E curves with a maximum electric field of 100 MV/m.

Please cite this article as: Hou C et al., Improved energy storage performance of nanocomposites with Bi4.2K0.8Fe2O9þd nanobelts, Journal of Materiomics, https://doi.org/10.1016/j.jmat.2019.04.006

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Fig. 6. (a) Recoverable energy density and (b) energy storage efficiency of nanocomposites as a function of electric field.

recoverable energy density in solution-cast PVDF-based nanocomposites with high-k BKFO nanobelts. A breakdown strength of 675 MV/m much higher than that of PVDF matrix is achieved, and the nanocomposites finally attain an ultrahigh recoverable energy density of 25.4 J/cm3. These results evidence the excellent energy storage performance of nanocomposites with nanobelts, which provides an approach to further improve the energy storage performance of polymer-based nanocomposites. Conflicts of interest The authors declare that there are no conflicts of interest. Acknowledgements

Fig. 7. The maximum recoverable energy density of nanocomposites as a function of volume fraction of BKFO nanobelts.

Table 1 The maximum recoverable energy density and energy storage efficiency of PVDFbased nanocomposites. Material

Umax (J/cm3)

h (%)

Eb (MV/m)

Reference

PVDF with BKFO nanobelts PVDF with Ti0.87O2 nanosheets Trilayered PVDF with BST nanowires and BN nanosheets PVDF with BaTiO3@TiO2 Nanofibers Sandwich-Structured BaTiO3/PVDF PVDF with BT@BN Microhybrids PVDF with Ba0.2Sr0.8TiO3 Nanowires Graphene/BST nanofiber/PVDF nanocomposites PVDF with Ti3C2Tx nanosheets PVDF with Nd-BaTiO3 nanoparticles

25.4 21.1 20.5

63 65 47

675 648 588

Our work [13] [31]

20 18.8 17.6 14.8 14.6

68 60 53 74 66

650 470 580 450 450

[12] [27] [33] [36] [35]

12.5 12.5

64 50

350 420

[34] [32]

This work was supported by the National Natural Science Foundation of China (51790491, 51622209, and 21521001) and the National Key Research and Development Program of China (2016YFA0300103 and 2015CB921201), and this work was partially carried out at the USTC Center for Micro and Nanoscale Research and Fabrication. Chuangming Hou and Zhiwei Bao contributed equally to this work. Appendix A. Supplementary data

efficiency of PVDF-based nanocomposites filled with different nanofillers [12,13,27,31e36]. The Umax and Eb of the nanocomposite with 0.35 vol% BKFO nanobelts are higher than the earlier reported PVDF-based nanocomposites. Because of the ferroelectric nature of b-PVDF matrix, the h of our nanocomposites is not very high but comparable to other PVDF-based nanocomposites. 4. Conclusions In summary, we demonstrate the simultaneously enhanced breakdown strength and dielectric constant, as well as the high

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Chuang-Ming Hou is a Ph.D. student in Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China. His research focuses on fabrication and the energy storage performance o f i n o r g a n i c d i e l e c t r i c fi l m s a n d p o ly m e r- b a s e d nanocomposites.

Zhi-Wei Bao is a M.D. student in Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China. His research focuses on fabrication and the energy storage performance of polymer-based nanocomposites.

Hao-Yang Sun is a Ph.D. student in Department of Physics, University of Science and Technology of China. His research focuses on fabrication and resistive switching of flexible inorganic functional oxides.

Dr. Yue-Wei Yin received his B.S in 2007 and his PhD in 2012, all from the Department of Physics at the University of Science and technology of China. He was a postdoc at the Pennsylvania State University from 2013 to 2015 and University of Nebraska-Lincoln from 2015 to 2017. He is currently a professor at Department of Physics in University of Science and Technology of China. His current research interests include fundamental and applied aspects of perovskite films and heterostructures and prototype multiferroic based electronic devices.

Dr. Xiao-Guang Li is a professor of materials physics in Hefei National Laboratory for Physical Sciences at the Microscale and Department of Physics, University of Science and Technology of China. His research interests include the synthesis, microstructure, physical properties, and prototype devices of transitional metal oxides.

Please cite this article as: Hou C et al., Improved energy storage performance of nanocomposites with Bi4.2K0.8Fe2O9þd nanobelts, Journal of Materiomics, https://doi.org/10.1016/j.jmat.2019.04.006