Structure Analysis of Beta-alumina Synthesized by SolidStateReaction①

JI Guang-Hui ZHU Cheng-Fei CHANG Feng-Zhen

?

Structure Analysis of Beta-alumina Synthesized by SolidStateReaction①

JI Guang-Hui ZHU Cheng-Fei②CHANG Feng-Zhen

(210009)

beta-Al2O3, structure analysis, solid state reaction

1 INTRODUCTION

Sodium-sulfur battery separator material with beta-alumina is a new type of high-energy solid elec-trolyte battery. It has been studied in the developed countries since the mid-1960s with wide application prospects. In recent years, many scientists have made researches on it in China. The operating tem-perature of sodium-sulfur battery is 300～350 ℃[1-5].The requirement of beta-Al2O3solid electrolyte is very strict for it needs insulation on working. So, the relative studies become more and more[6], concer-ning the preparation and properties of beta-Al2O3electrolyte for sodium sulfur battery.

There are two phases in the beta-Al2O3solid elec-trolyte, including??-Al2O3and-Al2O3. In most cases,?? and-phases coexist at the same time,and the conductivity of??-phase is 3～4 times more than that of the-phase[7]. The stoichiometry of??-Al2O3and-Al2O3is respectively Na2O·5.33Al2O3and Na2O·11Al2O3. During the actual preparation,??-Al2O3phase is slightly lack of Na2O content, while-Al2O3generally contains the excess Na2O content. Therefore, the actual composition of them are generally between Na2O·5.3A12O3and Na2O·8.5Al2O3, which shows the obviousnon-stoichio-metric composition[8].

In the preparation process, such factors as dif-ferent ratios of raw material, different preparation process and volatilization of Na2O cause the ratio to change between A12O3and Na2O.The addition of Li and Mg oxide, as stabilizer and impurity ions (e.g., Ca, Fe, Cr, Ni, Ti, etc.)[9-15], not only causes the changes of ionic conductivity, bulk density and microstructure, but also changes the crystal structure. If the relation between the crystal structure change and performance was found[16], it can be helpful to the mechanism research of the performance changes and finding theadding elements which meet the performance requirements by theoretical calcula-tions. So, it becomes more important to build the crystal structures of??-Al2O3and-Al2O3under difficult conditions.He Weichun.[17]resolved the crystal structure of ferrate salt K3Na(FeO4)2by using the Rietveld method, but reports on the beta-alumina structure analysis are rare nowadays.

In this paper, Beta-Al2O3solid electrolyte sample without stabilizers was prepared by using solid phase synthesis method and its crystal structure model was established to explore and analyze the structure parameters.

2 EXPERIMENTAL

2.1 Reagents and instruments

Nanometer-Al2O3, model VK-L30, purchased in Jing Rui New Material Co.Ltd.in Xuancheng, Anhui. Na2C2O4and AgNO3were all analytically pure, along with PVA, purchased from Sinopharm Chemical Reagent Co.Ltd.Ethanol, purchased in September Biological Engineering Co.Ltd. in Yang-zhou City.Water used in experiment was deionized water.

DMax/rB X-ray diffraction analyzer (Rigaku companies). ADVANTXP X-ray fluorescence spec- trometer (Thermoelectric Group of Swiss ARLCompany in the USA). SI1287 electrochemical Tester (Solartron company in the UK).

2.2 Experiment process

2.2.1 Sample preparation

Beta alumina was fabricated from powder mixture of high-purity-Al2O3and reagent-grade Na2C2O4, and sample with Na2O/Al2O3= 1:5 mole ratio (Na2O content by weight percent: 10.84%) was prepared. The mixed powders were ball-milled with ethanol for 10 h, dried at 80 ℃, and then calcined at 1250 ℃ for 2 h. These products were the precursor powders of beta-Al2O3. The precursor powders were ball-milled with ethanol for another 10 h and then dried. Some polyvinyl alcohol was added as a binder. Pellet specimens with a diameter of 20 mm were uniaxially pressed at about 10 Mpa. The green ceramics were covered with powders of the same compositions and fired at 1600 ℃ for 10 minutes.

2.2.2 Ionic conductivity test

The ionic conductivity of beta alumina was exa-mined using a frequency response analyzer (Solar-tron1260) and an electrochemical interface (SI 1287) at 50～500 ℃ in the air[18]. The frequency range applied was from 1 kHz to 1 Hz with signal amp-litude of 20 mV. The sintered specimens for ionic conductivity measurements were disc-shape of about 16 mm diameter and thickness of about 2 mm. Both faces of the pellets were carefully polished, covered with silver paste and fired afterwards to obtain silver electrode as the current collector. Then platinum lead was well attached to the silver electrode surfaces, and impedance measurements were carried out in these two-electrode symmetric cells. The impedance spectra were analyzed by the equivalent circuit of the program Zview.

2.2.3 X-ray diffraction and structure analysis

The phase composition analysis of the precursor powders and sinters was carried out on the X-ray diffraction analyzer (ARL, X? TRA) using Curadiation over the range of 5～80° (2). The scanning speed of the goniometer was 1.2° (2) per minute. GSAS software[19]was usedinRietveld refinement and analysis structure, and identifythe chemical formula initially. Diamond software was used to draw the crystal structure, and analyze the structural parameters, bond lengths,bond angles and so on[20]by the data of the refinement.

2.2.4 Authentication of the formula

The surface of the sintered samples was polished.The samples were weighted (denoted byG) after ultrasonic cleaning and drying. The samples were put into silver nitrate and heated to 300 ℃ for 24h.Na+was exchangedby Ag+to Ag+-beta-Al2O3. Then the nitric acid was used todissolve Ag of the surface. And the sampleswere weightedagain (denoted byG) after washing and dryingout to calculate the mass fraction of sodium atoms (denoted byW)[21].

The same sintered sampleswere milled into powder to test by X-ray fluorescence spectrometer so as to get the mass fraction of sodium atoms (denoted byW).

Fig. 1. Ionic conductivity of electrolytes

3 RESULTS AND DISCUSSION

3.1 Ionic conductivity

Fig.1shows the temperature variation curve for ionic conductivity of electrolytes, obtained from fitting of the electrochemical impedance spec- troscopy. As can be seen, solid electrolyte’s con- ductivity was 0.0066S/cm at 300 ℃, much closeto the conductivity of-Al2O3electrolyte according to Ref. 22.The major phase of electrolyte was-Al2O3at the experimental condition with less-Al2O3, which consisted with the later analysis.

Fig. 2. XRD patterns of calcinations

GSAS software was used to analyze structures and revise the above XRD patterns. The structure of??-Al2O3was referred to Al11Na1.71O17(Na2O·6.44 Al2O3) (ISCD201178) and-Al2O3to Al21.66Na3O34(Na2O·7.22Al2O3) (ISCD 60636). The crystal struc- ture diagram of??-Al2O3was drawn by Diamond software based on the refinement data, as shown in Fig.3.

Fig. 3. Structure of??-Al2O3(a)Unit cell (b)Sketch map of layer structure (c)Plane diagram of Na-O layer

The unit cell of??-Al2O3is shown in Fig. 3(a), with= 5.6017 ? and= 33.6219 ?. There are 33 Al atoms and 51 O atoms in every cell. For the Na atom, 18 atomic site occupations are observed in each unit cell, with the atomic occupancy to be 0.295. It shows that each unit cell contains 5.34 Na atoms indeed, agreeable with the atom amount ratio of Na1.78Al10.74O17(Na2O·6.03Al2O3). The schematic diagram of Na–O bonds is shown in the magnified part of Fig. 3(a). Four O atoms make up a triangular pyramid whose edge side length is 4.9179 ? and the bottom side length 2.7419 ?. Its three top Na–O bond lengths are 2.5692 ? and the angle is 16.177°. The six below Na–O bond lengths are 2.5535 ? and the angle is 64.945° (Na as the apex). Concrete parameters are shown in Table 1.

Table 1. Crystal Structure Data of β??-Al2O3

A mirror plane exists in each Na-O layer in the??-Al2O3crystal structure[25]. Therefore, each unit cell contains three similar interlamination cons- tructions. Fig.3(b) showsthe structural diagram of each layer. It can be calculated that the distance from atom center to centeris=4.6562? between two sides of the O atom layers in the Na–O layers. Fig.3(c)is a plane graph of the Na–O layer. The atomic migration span of Na is decided by the distance of its atomic site occupations. As shown in the figure, if considering the three Na atoms (=0.7229?) in each triangular pyramid as a unit, six units could constitute a regular hexagon with the side length of 3.2629?and the maximum atomic migration span of Nato be 2.4283?.

Fig.4(a) shows the unit cell of-Al2O3, with=5.5941 and= 22.5300 ?.The atomic site occu-pations of Al, O and Na in each unit cell are 22,34and 12, respectively. The parameters are shown in Table2. Depending on the valence balance calcula-tion,the amounts of Al, O and Na atoms contained in each unit cell are 21.81,34 and 2.58 in turn. It conformed to the chemical formula Na2.57Al21.81O34(Na2.57O·8.52Al2O3). In the-Al2O3crystal structure, the Na-O layer isn’t a mirror plane. As is shown in the diagram of single-layer structure, the distancer from atom center to centeris=4.6562? in two sides of the O atom layers in Na-O layers. Fig.4(c)depicts that the distance between three Na atoms in the middle positions and O atoms on two sides is 2.7358 ?. About the other three Na atoms, the distance between O atom and the Na atom at 1# position is 2.7156 ?, and that between O atom and the Na atom at 2# and 3# positionsis 2.9599 ?. Meanwhile, maximum atomic migration span of Na between adjacent unit cells is 6.9037 ? (from the 1# occupied site to the 3# of adjacent unit cell).

3.3 Chemical formula validation

By Ag ion exchange method, we get G= 6.3485 g andG= 7.5540 g. It is known that the rate of Na ion replaced by the Ag ion is achieved above 99%[8], so we could consider that the Na ions are completely replaced by the Ag ion. The quality of the Na element is figured out by formula 1,w=0.3262g, and Na element content isW=w/G=5.138wt%.

(1)

The content of Na element isW=5.589wt% tested by XRF at the same time.The measurementchemical formula of??-Al2O3is Na2O·6.03Al2O3and that for-Al2O3is Na2O·8.52Al2O3, which have been studied by the Rietveld structure analysis and refinement. Meanwhileit could be calculated thatthe Na atom content is 5.329wt% according to the content of??-phase to be 21wt%.With the results of Rietveld structure analysis and refinement method as benchmark,the errors of each method are 3.58% and 4.88%, respectively. These errors are mainly due to the few unsubstituted ionsduring ion exchange and fine distinctions of Na element between each sample.The small error proved that the crystal structure analysis and chemical type determination are correct.

Fig. 4. Structure of-Al2O3(a)Unit cell (b)Sketch map of layer structure (c)Planediagram of the Na–O layer

Table 2. Crystal Structure Data of β-Al2O3

3.4 Analysis of the relationship between conductivity and structure

It could be found that there is more Na ato-mic site occupation in the??-Al2O3phase than in-Al2O3, which led to more sites to move for the Na atoms. At the same time, the distribution of Na atoms in??-Al2O3is more homogeneous, causing smaller migration distances for Na atoms. Although the width of sodium channels is a little narrower in??-Al2O3, energy barrierEismainly determined by the migration distances according to the ion mobility theory. The maximum span in the sodion migration of??-phase (2.4283?) is only about a third of the maximum span of??-phase (6.9037 ?), so the??-phase ionic conductivity is 3～4 times of thephase.

4 CONCLUSION

(1) Wen, Z. Y.; Cao, J. D.; Du,Z. H.Research on sodium sulfur battery for energy storage.2008,179, 1697-1701.

(2) Akihiko, I.; Yu, Y.; Hirokazu,K.Growth and microstructure of Ba-alumina films by laser chemical vapor deposition.2013,33, 2655-2661.

(3) Natalia, A.C.; Kei, H.; Brigitta, P. Investigations of lithium-sulfur batteries using electrochemical impedance spectroscopy.2013,97, 42-51.

(4) Manickam, M.; Danielle, M. Electrochemical energy storage device for securing future renewable energy.2013,101, 66-70.

(5) Goodenough, J. Evolution of strategies for modern rechargeable batteries.2013,46, 1053-1061.

(6) Hu, Y. Y.; Wen, Z. Y.; Kun, R. Stateoftheart research and development status of sodium batteries.2013,2, 81-90.

(7) Liu, J.; Zhang, J. G.; Yang, Z. G. Materials science and materials chemistry for large scale electrochemical energy storage: from transportation to electrical Grid.2013,23, 929-946.

(8) Wen, Z. Y.; Hu, Y. Y.; Wu, X. W. Research activities in Shanghai institute of ceramics. Chinese academy of sciences on the solid electrolytes for sodium sulfur batteries.2008, 184, 641-645.

(9) Sun, C. W.; Deng, Q.; Cao, J. D. Effects of calciumpurities on the degradation of″-alumina utilized in sodium-sulfur cells.1992,7, 79-85.

(10) Shan, S. J.; Yang, L. P.; Liu, X. M. Preparation and characterization of TiO2doped and MgO stabilized Na-″-Al2O3electrolyte via a citrate sol-gel method.2013,563, 176-179.

(11) Yang, M.; Zhou, D. F.; Lin, R.Effects of Fe2O3-dopant on microstructure and electrical conductivity of Ce0.8Nd0.2O1.9.2012,30, 93-96.

(12) Wang, L. H.; Gan, G. Y.; Sun, J. L. Research on the influencing mechanism of Fe on the electrical characteristics of ZnO grain boundary.2009,23, 28-30.

(13) Li, X. L.; Chen, J. J. First principle study on conductivity of Cr-doped LiFePO4.2011,18, 44-48.

(14) Wang, N.; Fu, L. X.; Zhang, J. Effect of Ni-doping on the framework structure of mesoporous si1ica.2006, 23, 453-468.

(15) Lu, P. X.; Xu, D. H.; Ma, Q. H. XRD andRamanscattering analysis on chromium doped 0.2PZN-0.8PZT piezocerami.2007, 26, 70-80.

(16) Sun, Y.; Zhao, L.; Pan, H. L. Direct atomic-scale confirmation of three-phase storage mechanism in Li4Ti5O12anodes for room-temperature sodium-ion batteries.2013, DOI:10.1038/ncomms2878.

(17) He, W. C.; Hu, X. R.; Shen, B. C.Electrosynthesis and crystal structure of K3Na(FeO4)2.2007, 4, 655-658.

(18) Cao, C. N.; Zhang, J. Q.Science Press: Beijing2002, 152-153.

(19) Toby, B.H.EXPGUI, a graphical user interface for GSAS.2001,34, 210-213.

(20) Wu, P. W.; Zhu, Z. P.; Dai, J. H. Diamond software application in the teaching of crystal symmetrythe establishment of 32 points group of crystal model library.2008, 3, 236-238.

(21) Tang, S. B.; Yang, H. P.; Peng, D. K. Synthesis and study on the stability of Ag+-″-Alumina.2000,30, 477-481.

(22) Park, J.H.; Kim, K.H.; Lim, S.K. Influence of stabilizers on Na-beta''-Al2O3phase formation in Li2O (MgO)-Na2O-Al2O3ternary systems.1998, 33, 5671-5675.

(23) Youngblood, G.; Virkar, A.V.;Cannon, W.R. Sintering processes and heat treatment schedules for conductive, Lithiastabilized''-Al2O3.1977, 56, 206-210.

(24) Moseley,P.T.; Soudworth, J.L.; Tilley, A.R. Sodium sulfur battery.1985, 23.

(25) Wen,Z. Y.; Hu, Y. Y.; Wu, X. W. Main challenges for high performance NAS battery: materials and interfaces.2013, 23, 1005-1018.

(26) Lu, X. C.; Xia, G. G.; John, P.L.Advanced materials for sodium-beta-alumina batteries: status,challenges and perspectives.2010,195, 2431-2442.

23 January 2014;

23 April 2014

① This project was supported by NSFC projects (21203095),China Postdoctoral Science Foundation Projects (2012M511261) and Jiangsu Colleges and Universities Advantage Discipline Construction projects

. E-mail: zhucf@njtech.edu.cn