Novel sodium niobate-based lead-free ceramics as new environment

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Novel sodium niobate-based lead-free ceramics as new environment-friendly energy storage materials with high energy density, high power density, and excellent stability Mingxing Zhou, Ruihong Liang, Zhiyong Zhou, Shiguang Yan, and Xianlin Dong ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01926 • Publication Date (Web): 07 Sep 2018 Downloaded from http://pubs.acs.org on September 7, 2018

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ACS Sustainable Chemistry & Engineering

Novel sodium niobate-based lead-free ceramics as new environment-friendly energy storage materials with high energy density, high power density, and excellent stability Mingxing Zhou,†,‡ Ruihong Liang,*,† Zhiyong Zhou,† Shiguang Yan,† and Xianlin Dong*, †,§

†

Key Laboratory of Inorganic Functional Materials and Devices, Shanghai Institute of

Ceramics, Chinese Academy of Sciences, 588 Heshuo Road, Jiading District, Shanghai 201800, People’s Republic of China ‡

University of Chinese Academy of Sciences, 19 Yuquan Road, Shijinshan District, Beijing

100049, People’s Republic of China §

State Key Laboratory of High Performance Ceramics and Superfine Microstructure,

Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Changning District, Shanghai 200050, People’s Republic of China

Author information Corresponding Authors *E-mail: [email protected] *E-mail: [email protected].

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ABSTRACT: Recently, ceramic capacitors with fast charge-discharge performance and excellent energy storage characteristics have received considerable attention. A novel NaNbO3-based lead-free ceramics (0.80NaNbO3-0.20SrTiO3, abbreviated as 0.80NN-0.20ST) featuring with ultrahigh energy storage density, ultrahigh power density, and ultrafast discharge performance, were designed and prepared in this study. These 0.80NN-0.20ST ceramics exhibited a high breakdown strength of 323 kV/cm, attributable to their small grain size and dense microstructure, a recoverable energy storage density of 3.02 J/cm3, and an energy storage efficiency of 80.7% at an applied electric field of 310 kV/cm. The excellent stability of energy storage properties in frequency (0.1-1000 Hz), temperature (20-120 ℃), and fatigue resistance (cycle number: 105) were also observed in the 0.80NN-0.20ST ceramics. In contrast to recently reported other lead-free ceramic-based dielectric capacitors, the 0.80NN-0.20ST ceramics display a high energy storage efficiency combined with a high recoverable energy storage density, which indicates that they have wide application foreground and potential in the field of energy storage. These ceramics also show a considerable current density of 677 A/cm2, an ultrahigh power density of 23.7 MW/cm3, and a short release duration (~225 ns). This study brings the NaNbO3-based ceramics into a new chapter of research and application of energy storage dielectric capacitors. KEYWORDS:

Energy

storage,

Lead-free,

NaNbO3-based,

Ceramic

capacitors,

Charge-discharge performance INTRODUCTION Ceramic-based dielectric capacitors exhibit advantages such as faster charge-discharge capability, higher power density, higher working voltage, and excellent thermal stability compared with the fuel or solar cells, batteries as well as super-capacitors. Therefore, ceramic-based dielectric capacitors have recently caught great interest because of their potential applications in pulsed power weapons, electric vehicles, and air crafts. 1-5 Generally, charge energy density Wch, recoverable energy storage density Wrec, and energy storage efficiency η of ceramic-based dielectric capacitors can be worked out through from the integral of P-E loops on basis of equations as below:6-7

=

(1) 2


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=

(2)

=

× 100%

(3)

where E, P, Pr, and Pmax are the applied electric field, polarization, remnant polarization, and maximum polarization, respectively. Among the various ceramic-based dielectric capacitors developing at present, the Wrec of anti-ferroelectric ceramics is superior to those of linear and ferroelectric ceramics as a result of their high Pmax, low Pr, and moderate breakdown strength. For instance, (Pb,La)ZrO3, (Pb,La)(Zr,Ti)O3, and Pb(Zr,Sn,Ti)O3 systems.8-11 However, several hundred charge-discharge cycles are too much for anti-ferroelectric to hold due to the cracks in their transitions between the anti-ferroelectric and ferroelectric phases during the charge–discharge cycles.12-13 Furthermore, the great majority of the anti-ferroelectric ceramics are lead-based materials with Pb contents over 60 wt.%, which presents a serious drawback, as the toxicity of Pb can cause environmental deterioration and human health hazards. Therefore, development of lead-free ceramic-based dielectric capacitors with fast charge-discharge performance and high energy storage capacity is urgently needed. Low electric breakdown strength, small Pmax, and large Pr are the three major constraints to the improvement of Wrec of the lead-free ceramics.1, 14-15 Another important parameter of energy storage ceramic-based dielectric capacitors is η. Lower values of η imply that more energy may be lost in form of heat during the discharge process, resulting in degrading of capacitor’s properties or even break down the capacitors.16 According to the eq 2, high values of Wrec can only be realized by combining small Pr, high Pmax, and a large breakdown strength. The key to solve this problem is to find a suitable material system and adjust its properties by modifying its composition design. In the past decade, NaNbO3, as a critical end-member of (K0.5Na0.5)NbO3-based (KNN) piezoelectric ceramics, has gained substantial attention because of the comparatively outstanding piezoelectric properties.17-18 A great deal of research has been concerned on enhancing the piezoelectric properties and temperature stability of KNN-based piezoelectric ceramics. Nevertheless, only little attention has been paid to applying NaNbO3 ceramics to energy storage applications. NaNbO3 ceramics manifest a square P-E loop with relatively large polarization (30 µC/cm2 at 40 kV/cm) due to an electric field-induced metastable 3



ferroelectric phase at room temperature.19 Considerably large remnant polarization and low breakdown strength downgrade their use in energy storage devices. According to the existing results, it is well-marked that, adding certain compounds can improve the breakdown strength and reduce the remnant polarization of lead-free ceramics, which will improve their energy storage properties. For example, 0.95(Bi0.5Na0.5)TiO3-0.05Ba(Al0.5Nb0.5)O3 ceramics with SrTiO3 modifier exhibited a high Wrec (1.89 J/cm3) at 190 kV/cm, accompanied by a η value of 77%.16 Liu et al. reported 0.80(Bi0.5Na0.5)TiO3-0.20(Bi0.5K0.5)TiO3 ceramics doped with SrTiO3 (25 mol%), which showed a relatively high Wrec (0.97 J/cm3) while electric field is 130 kV/cm.20 The special electric properties of SrTiO3 may result in the enhancement of energy storage performance. As is generally known, SrTiO3 is a typical linear dielectric and displays special electrical properties, for example ultralow dielectric loss (< 1%), an appropriate dielectric constant (~300), and a relatively high breakdown strength (> 200 kV/cm).21 The introduction of SrTiO3 into the ceramic matrix will provide more active dipoles and larger endurance of electric field, which is propitious to obtain high values of Wrec, as they possess small Pr, large Pmax, and a high breakdown strength synchronously. On the basis of the above considerations, we applied the following strategy in the present study: the modifier SrTiO3 is introduced to control the dielectric characteristics (i.e., weak dielectric nonlinearly, ultralow dielectric loss, and appropriate dielectric constant) to improve the breakdown strength, and we kept the NaNbO3 matrix at a relatively high maximum polarization to strength the energy storage properties of this NaNbO3-based ceramics. Therefore, a novel NaNbO3-based lead-free ceramics, with the composition of 0.80NaNbO3-0.20SrTiO3 (denoted as 0.80NN-0.20ST), were chosen and prepared for achieving relatively higher breakdown strength, large Pmax, and large energy storage density in this study. Outstanding energy storage capability with a high breakdown strength of 323 kV/cm, an ultrahigh Wrec of 3.02 J/cm3 and a relatively high η of 80.7% were attained at 310 kV/cm. On the other hand, we studied the power density and discharge power performance of the 0.80NN-0.20ST ceramics, revealing a large current density of 677A/cm2 and ultrahigh power density of 23.7 MW/cm3, and a release of the stored energy in sub-microsecond duration (~225 ns). This study exploits a new application area for environment-friendly NaNbO3-based ceramics, as well as to provide a new way to acquire high energy storage 4


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properties energy storage materials. EXPERIMENTAL SECTION Sample preparation. The traditional solid-state reaction technique was applied to prepare the 0.80NN-0.20ST ceramics. These raw materials (Na2CO3 (99.8%), Nb2O5 (99.93%), SrCO3 (99.0%), and TiO2 (99.8%)) were mixed and ball milled for 24 hours with using ethanol. Then the mixture was calcined at 1050 ℃ in air for 2 hours after drying. The calcined powder was pressed (120 MPa) into 13-mm diameter pellets after re-milled, and dried. The samples were sintered at 1175, 1200, 1225, 1250, 1275, and 1300 ℃ for 2 hours, respectively. Meanwhile, these samples were covered by the corresponding calcined powders, during sintering procedure and double-crucible method was used to realize minimization of evaporation of elemental Na. In this study, we have adopted two sample sizes for the electrical measurements due to the different requirements of measurement equipment and test conditions. For the dielectric breakdown strength and pulsed charge-discharge measurements. After burnishing to sintered pellets with the thinness of 0.4 mm, they were coated with silver paste (electrode area: 12.25 mm2) to characterize their electrical properties. Firing at 750 ℃ for 30 min in air renders a silver electrode. Additionally, to prevent breakdown of the edges of the specimen and achieve a high electric field strength during the P-E hysteresis loops tests. The specimens were polished and thinned to 0.2 ± 0.02 mm. Point electrodes (1.5 mm ×1.5 mm) were obtained at room temperature by RF magnetron sputtering using a 4-inch silver target. Material characterization. The density of ceramic samples was performed by means of Archimedes method. The surface morphology of the sample was investigated by using a field emission

scanning

electron

microscope

(FESEM,

Magellan400,

FEI

Company).

Energy-dispersive X-ray spectroscopy (EDX) was gathered from the attachment to the Magellan-400. Using X-ray diffraction (XRD, D/MAX-2550V; Rigaku, Tokyo, Japan) with Cu Kα radiation to analyze the phase structure. Dielectric properties were measured by Novocontrol turnkey dielectric spectrometer (Concept 80, Germany). Employing a ferroelectric analyzer (TF Analyzer 2000, aixACCT, Aachen, Germany) to obtain capacitance-voltage and unipolar P-E loops. Using a voltage-withstand test equipment to determine the break strength. Through a charge-discharge equipment of special design, 5



high-speed discharge capacitance, inductance, and resistance load circuit (RLC), the properties of energy release of the ceramic capacitors were probed.22 RESULTS AND DISCUSSION Variations of real and relative densities as functions of different sintering temperatures are shown in Figure 1a. The rising of the sintering temperature causes the real and relative densities of 0.80NN-0.20ST ceramics which was at first rose to a maximum value of 4.55 g/cm3 (98%) at a sintering temperature of 1250 ℃, and then decreased when the sintering temperature is further increased. The density decrease of 0.80NN-0.20ST ceramics at higher sintering temperatures may be due to the unavoidable volatilization of Na element which has a relatively low melting point.23 Figure 1b shows the XRD data of 0.80NN-0.20ST ceramics. Inset of Figure 1b shows the amplified XRD patterns for ranges of 2θ = 32-46°, revealing a pure perovskite structure and no clear impurity phase. This suggests the SrTiO3 can diffuse into NaNbO3 host lattice. At room temperature, the 0.80NN-0.20ST ceramics exist in a pseudo-cubic phase, which is featured of a single peak of (110) and (200) at around 2θ = 32.3 and 46.3°, respectively. The SEM micrograph of the 0.80NN-0.20ST ceramics is shown in Figure 1c. The specimen displays uniform distributed fine-grain. Barely any pores were visible on the fracture surface of the 0.80NN-0.20ST ceramics (Figure S1 of the Supporting information). Figure 1c also represents the elemental mapping of the 0.80NN-0.20ST ceramics, which can be seen a uniform distribution of elements in the 0.80NN-0.20ST ceramics. An analytical software (Nano measurer) was used to determine the grain size distribution of the 0.80NN-0.20ST ceramics. Statistical analysis reveals that the grain size distribution (Figure 1d) follows a Gaussian distribution with an average grain size of only 1.86 µm. Previous studies indicated that high breakdown strengths can be generated by small grain size and a dense microstructure.2, 24-25

6


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Figure 1. (a) Real and relative density of the 0.80NN-0.20ST ceramics as functions of the sintering temperature. (b) XRD spectra of the 0.80NN-0.20ST ceramics. (c) Elemental mapping and SEM surface micrographs of the 0.80NN-0.20ST ceramics (sintered at 1250 ℃

for 2 h and thermally etched at 1000 ℃ for 0.5 h). (d) Particle size distribution of the 0.80NN-0.20ST ceramics. The dielectric-temperature spectrum of the 0.80NN-0.20ST ceramics are displayed in Figure 2a. The Tc of the 0.80NN-0.20ST ceramics is below room temperature, which reveals that the 0.80NN-0.20ST ceramics are presented with para-electric pseudo-cubic phase at room temperature, which is consistent with the XRD results that are displayed in Figure 1b. According to the previous research result of Fletcher: energy storage materials operate in paraelectric-state can obtain the maximum energy storage density due to the dielectric constant has a relatively weak dependence on applied electric field.26 Obviously, the 0.80NN-0.20ST ceramics, which are in the paraelectric-state at room temperature, are expected to have a high energy storage density. Furthermore, at room temperature, the tanδ and dielectric constant value of the 0.80NN-0.20ST ceramics are 0.15% and 1450, respectively. The ultralow tanδ and moderate dielectric constant are also favorable for a high 7



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breakdown strength. Figure 2b presents the dielectric constant and tanδ of the 0.80NN-0.20ST ceramics as functions of the frequency at room temperature. At the measured frequency range between 100 Hz and 1 MHz, dielectric constant and tanδ of the 0.80NN-0.20ST ceramics show an excellent stability. Moreover, the tanδ value exhibits a fairly low value (at 10-3 order magnitude) in the whole frequency range. Generally, the high dielectric nonlinearity restricts the enhancement of the energy storage density and the application of the capacitors under a high electric field. The electric field dependence of the dielectric constant was measured at 1 kHz under DC bias over 0-80 kV/cm at room temperature. As shown in Figure 2c, the dielectric constant as a function of the applied electric field for the 0.80NN-0.20ST ceramics displays a horizontal line, which indicates that almost no energy loss occurs during the process of charging and discharging.27 The dielectric constant change (%) with respect to electric field can be defined as follows: Dielectric constant change (%) =

- (.)/- () - ()

× 100%

(4)

where εr(0) and εr(E) present respectively the dielectric constant under the 0 bias field and applied bias field E. When the value of the E increases up to ± 80 kV/cm, the dielectric constant changes only about 11.5%, exhibiting weaker dielectric nonlinearity. In terms of energy storage capacitors, high breakdown strength is an important guarantee for obtaining high operative electric field and energy storage density. Weibull plots are employed to achieve breakdown strength, which requires at least 8-10 samples to be checked out for the measurements.24 The credible values of breakdown strength can be worked out by the following equations:28 12 = ln(2 )

(5) 2

32 = ln 4ln 51/ 41 − 89:;

Novel sodium niobate-based lead-free ceramics as new environment

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