Excellent Energy Storage Properties Achieved in BaTiO3-based Lead

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Functional Inorganic Materials and Devices

Excellent Energy Storage Properties Achieved in BaTiO3-based Lead-Free Relaxor Ferroelectric Ceramics via Domain Engineering on the Nanoscale Ying Lin, Da Li, Miao Zhang, Shili Zhan, Yaodong Yang, Haibo Yang, and Qibin Yuan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10819 • Publication Date (Web): 27 Aug 2019 Downloaded from pubs.acs.org on August 28, 2019

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ACS Applied Materials & Interfaces

Excellent Energy Storage Properties Achieved in BaTiO3-based Lead-Free Relaxor Ferroelectric Ceramics via Domain Engineering on the Nanoscale Ying Lin §, † , Da Li §, † , Miao Zhang † , Shili Zhan ‡ ,Yaodong Yang & , Haibo Yang* , † , Qibin Yuan* , ‡

† School

of Materials Science and Engineering, Shaanxi Key Laboratory of Green Preparation

and Functionalization for Inorganic Materials, Shaanxi University of Science and Technology, Xi’an 710021, China ‡

School of Electronic Information & Artificial Intelligence, Shaanxi University of Science

and Technology, Xi’an 710021, China &

Frontier Institute of Science and Technology, Xian Jiaotong university, Xian 710049, China

§

Authors contributed to this paper equally

*

Corresponding Authors

(Haibo Yang*) Email: [email protected] (Qibin Yuan*) Email: [email protected]

KEYWORDS: barium titanate, lead-free, relaxor ferroelectrics, domain, energy storage properties

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ABSTRACT: Barium titanate-based energy storage dielectric ceramics have caused more and more concern due to their environmental friendliness and outstanding ferroelectric properties. Here we demonstrate that a recoverable energy density of 2.51 J·cm-3 and a giant energy efficiency of 86.89% can be simultaneously achieved in 0.92BaTiO3-0.08K0.73Bi0.09NbO3 ceramics. In addition, the excellent thermal stability (25-100 oC) and superior frequency stability (1-100 Hz) have been obtained under 180 kV·cm-1, respectively. The first order reversal curve (FORC) method and transmission electron microscope (TEM) measurement show that the introduction of K0.73Bi0.09NbO3 (KBN) makes the ferroelectric domains to transform into highly-dynamic polar nanoregions (PNRs), leading to the concurrently enhanced energy storage properties by the transition from ferroelectric (FE) to relaxor ferroelectric (RFE). Furthermore, it is confirmed by the piezoresponse force microscopy (PFM) that the appearance of PNRs break the long range order to some extent and reduce the stability of microstructure, which explains the excellent energy storage performance of RFE ceramics. Therefore, this work has promoted the practical application ability of BaTiO3-based energy storage dielectric ceramics. INTRODUCTION Sustainable and environmentally-safe energy materials cause more and more concern due to the continuous utilization of unsustainable resources and increasingly serious pollution of the environment.1 Ceramic capacitors have tremendous application prospects in the electronic systems due to their advantages of high charge and discharge speed, large dielectric constant, wide application temperature range and long service life.2-5 In general, the values of the recoverable energy density (Wrec) and energy efficiency (η) of energy storage dielectric materials are calculated through the following equations: 𝑃

𝑊𝑟𝑒𝑐 = ∫𝑃𝑚𝑎𝑥𝐸𝑑𝑃 𝑟

𝜂=

𝑊𝑟𝑒𝑐 𝑊

× 100%

(1) (2)

where P, Pmax, Pr, and E mean the polarization (µC·cm-2), maximum polarization (µC·cm-2), remnant polarization (µC·cm-2) and electric field strength (kV·cm-1), respectively. Thus, large Pmax and tremendous dielectric breakdown strength (BDS) as well as small Pr are the keys to bring about outstanding energy storage performance. Generally, ceramic dielectrics for energy storage applications include linear dielectric, ferroelectric (FE), and anti-ferroelectric (AFE).6-8 Among them, the energy storage properties of linear 2 ACS Paragon Plus Environment

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dielectrics are limited due to their smaller polarization value. AFE ceramics are beneficial to obtain high energy storage properties owing to their large △P (Pmax-Pr) as well as noteworthy double P-E loops.9-11 However, the abundant lead element in most AFE ceramics is harmful to human and the environment12, such as Pb(Tm1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O313. BaTiO3-based ferroelectrics ceramics, as a representative perovskite structure ferroelectric material, are always arousing interest in progressive energy storage applications.14-16 Unfortunately, the Wrec and η for most reported BaTiO3-based ceramics are still less than 2.5 J·cm-3 and 80%, respectively, due to large Pr and/or small BDS.17-21 Therefore, it is imperative to improve the energy storage performance of BaTiO3-based RFE ceramics. In this work, we utilized the composition-driven structure design strategy to enhance synergistic improvement of energy density and efficiency in a novel (1-x)BaTiO3-x(K0.73Bi0.09)NbO3 (abbreviated as (1-x)BT-xKBN) system based on the dynamic nanoscale domain and refined grain sizes, respectively. A high value of Wrec (2.51 J·cm-3) along with a giant value of η (86.89%) are simultaneously achieved in 0.92BT-0.08KBN ceramic at 327 kV·cm-1. More importantly, the intrinsic origin of outstanding energy-storage performance is investigated systematically by TEM, PFM and the FORC distribution. It is revealed by the FORC distributions and TEM that the introduction of KBN makes the ferroelectric domains of BaTiO3 to transform into highly-dynamic PNRs, and thus leads to a transition from FE to RFE. Moreover, it is confirmed by the PFM that the increase of PNRs reduces the stability on the nanoscale, which explains the excellent energy storage performance of RFE ceramics. EXPERIMENTAL SECTION Materials Preparation. Lead-free (1-x)BaTiO3-xK0.73Bi0.09NbO3 ceramics ((1-x)BT-xKBN; x= 0, 0.02, 0.04, 0.08 and 0.12) were fabricated using the traditional solid-state sintering technique. Highpurity oxide powders of Bi2O3 (>99.9%), TiO2 (>99.9%), K2CO3 (>99.0%), BaCO3 (>99.0%) and Nb2O5 (>99.5%) were weighed based on the formal compositions of (1-x)BT-xKBN and milled for 24 h. These powers were heated at 950 oC for 5 h after drying. The resulting powders were milled again for 12 h with ethanol as the medium. After drying, these powders were pressed into a round block with the diameter of 12 mm using cold isostatic pressing method, then sintered for 4 h at 1180-1300 oC to obtain samples. Characterization. X-ray diffraction meter (XRD) with a CuKa radiation (D8 Advance, Bruker, Germany) has been used to determine the phase structure of (1-x)BT-xKBN ceramics. The scanning electron microscopy (SEM) (JSM-6700, JEOL Ltd., Tokyo, Japan) was used to record the 3 ACS Paragon Plus Environment


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microstructure of the sintered samples polished and thermally etched at 1100 oC for 25 min. The highresolution transmission electron microscopy (HR-TEM) and bright-field transmission electron microscopy (BF-TEM) measurements were measured via TEM (JEM-2100CX, JEOL, Japan). The samples need to be processed using a Gatan PIPS 695 system before TEM testing. In order to get the samples with smooth and parallel surfaces for dielectric measurement, the sintered and polished ceramics were coated with silver paste and then calcinated for 30 min at 850 oC. The temperature dependent dielectric properties were analyzed by a LCR meter (3532-50, Hioki, Ueda, Japan) from 190 oC to 175 oC. The ceramic samples were polished to a thickness of 0.2 mm, then round gold electrodes with a diameter of 2 mm were made on the surfaces of ceramic samples by the ion sputter apparatus for ferroelectric performance test. Then the P-E loops of the samples were measured using a ferroelectric test system (Premier II, Radiant, USA). During the measurement, the samples were immersed in silicone oil. The samples were treated in the similar way for testing the FORC distribution, except that the thickness of the sample was 0.3 mm. The FORC distribution of the ceramics were obtained by a sets of FORC loops measured by an aixACCT system (TF analyzer 2000, Germany). The PFM measurement was implemented under alternating voltage (10-30 V) and at a fixed frequency (50 Hz) by an atomic force microscopy (AFM, MFP-3D, Asylum Research, USA). RESULTS AND DISCUSSION The XRD spectra of the (1-x)BT-xKBN ceramics at room temperature are illustrated in Fig. 1a. It can be found that all the ceramic samples have a typical perovskite phase, suggesting that KBN entirely enters into BT lattices without generating impurities. Moreover, as shown in Fig. 1b, it is obvious that the (200) peaks slightly move to lower degrees and the (002)/(200) peaks gradually merge with increasing the content of KBN, suggesting the increase of cell volume and the transition from a tetragonal phase (x≤0.02) to a pseudo-cubic phase (0.4≤x≤0.12).22 The lattice fringes can be observed from the HR-TEM images in Fig. 1c-e, the interplanar spacing of the (100) crystal planes increases from 0.399(8) nm for x=0 to 0.401(1) nm for x=0.08, which is consistent with the XRD result. The selected area electron diffraction (SAED) patterns were utilized to further ensure the phase structure of the samples, as illustrated in Fig. 1f-h. The SAED patterns of 0.98BT-0.02KBN and 0.92BT-0.08KBN bulk ceramics viewed along the [110] zone axes are shown in Fig. 1g-h. In addition, all the elements of 0.98BT-0.02KBN ceramic have been also confirmed to be evenly distributed by the element-mapping analysis in Fig. 1k-p, indicating a highly chemical uniformity in this system. 4 ACS Paragon Plus Environment

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Fig. 1. (a) XRD patterns of (1-x)BT-xKBN ceramics and (b) the enlarged (200) peaks. (c-e) HR-TEM images of (1-x)BT-xKBN ceramics. Insets are the corresponding inverse fast Fourier transform (IFFT) images. (f-h) SAED patterns of (1-x)BT-xKBN ceramics. (i) Cross-sectional SEM micrograph of x=0.02 sample. (j) Overlay of various elements on the image (i). EDX element mapping images of x=0.02 sample for (k) Blue-Ba, (l) Yellow-Ti, (m) Cyan-K, (n) Purple-Bi, (o) Orange-Nb, and (p) Green-O. The temperature dependent dielectric properties for (1-x)BT-xKBN ceramics from -190 oC to 175 oC

are shown in Fig. 2a and Fig. S2a-e. It can be found that the maximum value of ε′ (εm′) decreases

and Tm at the frequency of 1 Hz gradually shifts toward lower temperatures with increasing the content of KBN. To further investigate the dielectric dispersion and diffuseness for (1-x)BT-xKBN samples, the following equation was used:23 1 ε′

1

(T ― Tm)γ

m

C

―ε ′=

(3)

where εm′ is the permittivity at Tm, C and γ are the constant and diffuseness degree with the value ranging from 1 for typical FE behaviors to 2 for ideal RFE behaviors, respectively.24 Fig. S2f exhibits the plots of ln(1/ε′-1/εm′) as a function of ln(T-Tm) for the (1-x)BT-xKBN ceramics at 1 MHz. It is obvious that ln(T-Tm) and ln(1/ε′-1/εm′) exhibit almost a linear relationship and the value of γ increases 5 ACS Paragon Plus Environment


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gradually from 1.08 to 1.80 with increasing the content of KBN by calculating and fitting with Equation (3). The γ value of 0.92BT-0.08KBN ceramic is 1.80, which is greatly close to 2, indicates that the ceramic has a strong relaxation behavior. As shown in Fig. 2b and Fig. S1a-f, the introduction of KBN is found to promote the refinement of grain size compared with the pure BT ceramics, the average grain size of ceramics dramatically decreases from 12.69 μm at x=0 to 0.2 μm at x=0.08. It may be due to the fact that the dopant hinders grain boundary motion and reduces interface energy during sintering, which inhibits the grain growth. And the results are consistent with the reports of recent literatures such as KNN-BMN1, BNT-BKT-BT25 and BT-BNT-BMT26. The refined grain sizes and enhanced BDS are in favor of high energy storage properties.1, 27-30 The date of BDS of (1-x)BTxKBN ceramics are depicted in Fig. S3 through linear fitting, and all the data points are in good agreement with the Weibull distribution and the  values are greater than 17. The P-E loops of the (1-x)BT-xKBN ceramics under the critical electric fields at ambient temperature are described to evaluate the energy storage properties of dielectric ceramics in Fig. 2c. The pure BT ceramic reveals a fat loop, which is the typical feature of ferroelectrics but adverse to the energy storage performances. With in introduction of KBN, the loops become thinner and thinner, which contributes to the improvement of energy storage characteristics. The values of Wrec and η of (1x)BT-xKBN ceramics are depicted in Fig. 2e. It can be recognized that the Wrec and η values of the (1x)BT-xKBN ceramics gradually increases from 0.45 to 2.51 J·cm-3 and from 28.23% to 87.15% with increasing the content of KBN, respectively. Meanwhile, for the composition of x=0.08, which exhibits an excellent Wrec of 2.51 J·cm-3 and a giant η of 86.89% under 327 kV·cm-1. Moreover, Fig. 2g displays a comparison of the Wrec and η between the 0.92BT-0.08KBN ceramic and other previously reported bulk ceramics to assess the energy storage performance of this contribution.31-39 From which it is obvious that the Wrec of greater than 2.5 J·cm-3 and η of larger than 80% can hardly be simultaneously achieved for most of the dielectric ceramics. For instance, AgNbO3-based ceramics acquire an outstanding Wrec value (3.30 J·cm-3) but a unsatisfactory η value (50%).38 By contrast, Sr0.3(Bi0.7 Na0.67Li0.03)0.5TiO3 ceramics show a great η value of 87.2% but a low Wrec value of 1.7 J·cm-3.39 To our joy, the as-prepared dielectric ceramic of 0.92BT-0.08KBN in this work has an outstanding Wrec value of 2.51 J·cm-3 and a giant η value of 86.89% simultaneously, superior to the previous reported lead-free bulk ceramics in terms of overall performances.


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Fig. 2. (a) Temperature dependence of dielectric constant of the (1-x)BT-xKBN ceramics at the frequency of 1 kHz. (b) BDS and average grain size of (1-x)BT-xKBN ceramics. Insets are the microstructure of BT and 0.92BT-0.08KBN ceramics. (c) P-E loops of (1-x)BT-xKBN ceramics. (d) PE loops of 0.92BT-0.08KBN ceramic at different temperatures under 180 kV·cm-1. (e) Energy storage properties of (1-x)BT-xKBN ceramics. (f) Energy storage properties of 0.92BT-0.08KBN ceramics with different temperatures. (g) Comparison of Wrec and η between 0.92BT-0.08KBN ceramic and some recently reported lead-free ceramics. More importantly, the pulsed charge-discharge measurements are closer to the actual performance of practical application.40, 41 The underdamped discharge waveforms and current peak (Imax), current 7 ACS Paragon Plus Environment


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density (CD) as well as power density (PD) depending on the electric field at room temperature for 0.92BT-0.08KBN ceramic are shown in Fig. S4. It can be found a series of typical underdamped discharge waveform, as shown in Fig. S4a. In addition, it can be recognized from Fig. S4b that the Imax, CD and PD values gradually increases from 21.34 to 106.34 A, from 169.9 to 846.66 A·cm-2 and from 1.70 to 76.20 WM·cm-3 with increasing electric field from 20 to 180 kV·cm-1, respectively. This novel BaTiO3-based lead-free relaxor ferroelectric ceramics with performances of ultrahigh CD and PD deserve more attention in the application of high power system. Thus, the (1-x)BT-xKBN ceramics as environmentally-friendly materials shows great potential in future applications of electronic systems.

Fig. 3. (a) BF-TEM micrographs of the BT-KBN ceramics. (b) FORC loops of the BT-KBN ceramics (Only 6 out of the total 60 loops are shown for clarity). (c) Evolution of FORC distribution of the BTKBN ceramics. 8 ACS Paragon Plus Environment

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Furthermore, the outstanding thermal stability of materials in energy storage is also highly important for the work of electronic equipment under extreme conditions. Thus, we plotted the P-E loops of 0.92BT-0.08KBN ceramic under 180 kV·cm-1 and different temperatures, as shown in Fig. 2d. It can be found that the 0.92BT-0.08KBN ceramic exhibits slim P-E loops. The maximum polarization Pmax and the remnant polarization Pr slightly decrease and increase with increasing the measuring temperature, respectively. The Wrec and η values of 0.92BT-0.08KBN ceramic at 10 Hz and elevated temperatures are described in Fig. 2f. It can be seen that the Wrec for 0.92BT-0.08KBN ceramic slightly fluctuate in the measured temperature range from 1.09 to 1.13 J·cm-3, possibly attributed to their improved electrical conductivity at rising temperatures.22 Moreover, the P-E loops of 0.92BT-0.08KBN ceramic is also measured over the frequency range of 1-100 Hz, as shown in Fig. S5a. The results show that outstanding frequency stabilities with Wrec of 1.13-1.19 J·cm-3 and η of 81.61-86.87% have been obtained over a wide frequency range, as shown in Fig. S5b. Therefore, 0.92BT-0.08KBN ceramic exhibits an excellent thermal stability and a superior frequency stability for energy storage properties, which are beneficial to practical application for pulsed power system. The excellent energy storage properties of the BT-KBN system should be linked to the construction of domain structure via composition design. In order to have a deeply understanding of the contribution of FE-to-RFE transformation to energy storage properties at a microscope perspective, the BF-TEM measurements and FORC distribution were performed in the representative ceramic compositions (x=0, 0.02, 0.08), as shown in Fig. 3. It can be found from Fig. 3a that domain structures evolve gradually from lamellar shape with a width of about 100 nm for x=0 to a diverse topography with a lamellar shape and smaller width of about 30 nm in the core and unfeatured nanodomains (PNRs) for x=0.02, and ultimately form a homogeneous state dominated by PNRs for x=0.08. Such mottled pattern with numerous nanodomains is consistent with the previouly reported observations of BaTiO3 ceramics.42 Notably, the possible inducement of chemical inhomogeneity to the formation of diverse domain topography for x=0.02 has been excluded by homogenous element distribution based on the EDX mapping in Fig. 1i-p, which is a different kind of scenarios compared with other ferroelectrics including bismuth element.43 The polarization properties by composition-driven variation characteristics in domain structure can be investigated via the FORC distribution in Fig. 3c evaluated according to the asymmetric P-E loops in Fig. 3b. For pure BT sample, its FORC contour diagram exhibits a distinctive image feature of highly concentered zone near the origin in Fig. 3c, accompanied 9 ACS Paragon Plus Environment


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by the highest rectangular factor (Pr/Ps=0.571) in Fig. 3b, implying that the larger polarization originates from the intensively domain switching and domain wall motion at relatively lower electric field, which is a response to the larger Pmax of the pure BaTiO3 ceramic.44 The background intensity beyond this region becomes more suppressed due to the adequate saturation of domain switching at higher electric fields. Such heterogeneous distribution reflects that the typical FE state and strongly nonlinear polarization of pure BT. With increasing the value of x (x=0.02 and 0.08), the intensity of whole diagram territory is further suppressed and the highly concentered zone near the origin become more dispersed, accompanied by the rectangular factor being on the order of 0.210 at x=0.02 and 0.203 at x=0.08, which indicates the weakened nonlinear polarization typical of RFE state and a shift toward RFE state, since RFE is characterized with small energy barrier for switching and ultralow coercivity.45

Fig. 4. Poling behaviors of (1-x)BT-xKBN (x=0, 0.02, and 0.08) ceramics. Out-of-plane PFM phase images after poling treatment with different electrical voltages and relaxation durations: (a) x=0, (b) x=0.02 and (c) x= 0.08. 10 ACS Paragon Plus Environment

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To have a distinct insight to energy storage properties based on dynamic respond of domain to external electric field, the PFM method was used to investigate the variation of microcosmic domain. Apply a negative voltage over an area of 3×3 µm2 and a same-amplitude positive voltage over an region of 1×1 µm2 to check for transformation in amplitude and direction of domain. Fig. 4 shows the PFM phase patterns applied at different applied electrical voltages of ±10 V and ±20 V, with the relaxation durations of 0 min and 30 min. It can be found that the microscopic 180o domains for pure BT ceramics were richly induced at a lower voltage (10 V) and the domain basically dose not change after the relaxation time of 30 minutes from Fig. 4a and Fig. S6, indicating that pristine BT ceramics have strong ferroelectric properties. As shown in Fig. 4b-c, the feedback signal become weaker and weaker in the entire electrically excited regions with content of KBN, which means polarization is less sensitive to external voltage, resulting in a macroscopically reduced remnant polarization of the sample of x=0.08. In addition, it can be observed after being relaxed for 30 min that the switched domains in the PFM signal suffer dramatically loss with increasing the content of KBN, which means the relaxation properties of the ceramics are increased. The fast overturn of nanoscale domains such as PNRs is beneficial to the improved energy storage performances. CONCLUSIONS In summary, a recoverable energy density (2.51 J·cm-3) and a giant energy efficiency (86.89%) can be simultaneously obtained in 0.92BT-0.08KBN RFE ceramics. The average grain size of (1-x)BTxKBN ceramics sharply declines from 12.69 µm for x=0 to 0.20 µm for x=0.08, which is beneficial to improve BDS. In addition, the excellent thermal stability (25-100 oC) and superior frequency stability (1-100 Hz) have also been achieved under 180 kV·cm-1, respectively. Moreover, it is revealed by the FORC method and TEM measurement that the introduction of KBN transforms the ferroelectric domains into highly-dynamic PNRs, resulting in a FE to RFE transition. Furthermore, the PFM results confirm that the increase of PNRs reduces the stability on the nanoscale and explains that the relaxor ferroelectric has a smaller Pr of the macroscopic polarization. Therefore, this work has promoted the practical application ability of BaTiO3-based lead-free relaxor ferroelectric ceramics for pulsed capacitors.



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ASSOCIATED CONTENT Supporting Information. SEM, average grain sizes, temperature dependent of dielectric properties, Weibull distribution, PFM amplitude images and P-E loops. “This material is available free of charge via the Internet at http://pubs.acs.org.” NOTES The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant No. 51572159), the Shaanxi Science & Technology Co-ordination & Innovation Project of China (Grant No. 2017TSCXL-GY-08-05) and the Science Fund for Distinguished Young Scholars of Shaanxi Province (Grant No. 2018JC-029). REFERENCES (1) Shao, T. Q.; Du, H. L.; Ma, H.; Qu, S. B.; Wang, J.; Wang, J. F.; Wei, X. Y.; Xu, Z. PotassiumSodium Niobate Based Lead-Free Ceramics: Novel Electrical Energy Storage Materials. J. Mater. Chem. A 2017, 5, 554−563. (2) Seongtae, K.; Wesley, H.; Edward, A.; Eugene, F.; Michael, L. Nonlinear Dielectric Ceramics and Their Applications to Capacitors and Tunable Dielectrics. IEEE Electr. Insul. Mag. 2011, 27, 43−55. (3) Yao, K.; Chen, S. T.; Mojtaba, R.; Sharifzadeh, M. M.; Yu, S. H.; Francis Eng Hock, T.; Thirumany, S.; Lu, L. Nonlinear Dielectric Thin Films for High-Power Electric Storage with Energy Density Comparable with Electrochemical Supercapacitors. IEEE Trans. Ultrason. Eng. 2011, 58, 1968−1974. (4) Correia, T. M.; McMillen, M.; Rokosz, M. K.; Weaver, P. M.; Gregg, J. M.; Viola, G.; Cain, M. G. A Lead-Free and High-Energy Density Ceramic for Energy Storage Applications. J. Am. Ceram. Soc. 2013, 96, 2699−2702. (5) Yang, H. B.; Yan, F.; Lin, Y.; Wang, T.; Wang, F.; Wang, Y. L.; Guo, L. N.; Tai, W. D.; Wei, H. Lead-Free BaTiO3-Bi0.5Na0.5TiO3-Na0.73Bi0.09NbO3 Relaxor Ferroelectric Ceramics for High Energy Storage. J. Eur. Ceram. Soc. 2017, 37, 3303−3311. (6) Fletcher, N. H.; Hilton A. D.; Ricketts, B. W. Optimization of Energy Storage Density in Ceramic Capacitors. J. Phys. D: Appl. Phys. 1996, 29, 253−258. (7) Burn, I.; Smyth, D. M. Energy Storage in Ceramic Dielectrics. J. Mater. Sci. 1972, 7, 339−343. 12 ACS Paragon Plus Environment

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Anti-Ferroelectric

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Table of Contents Graphic


Excellent Energy Storage Properties Achieved in BaTiO3-based Lead

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