Environmentally Friendly Flux Growth of High-Quality, Idiomorphic

Synopsis. High-quality, idiomorphic Li5La3Nb2O12 crystals with well-developed {211} and {110} crystal faces were successfully grown from LiOH flux at ...
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Environmentally Friendly Flux Growth of High-Quality, Idiomorphic Li5La3Nb2O12 Crystals Yusuke Mizuno,† Hajime Wagata,† Hitoshi Onodera,† Kunio Yubuta,‡ Toetsu Shishido,‡ Shuji Oishi,† and Katsuya Teshima*,† †

Department of Environmental Science and Technology, Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan ‡ Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan S Supporting Information *

ABSTRACT: High-quality, idiomorphic, single-phase Li5La3Nb2O12 crystals were successfully grown using a LiOH flux cooling method at the relatively low temperature of 500 °C at a solute concentration of 5 mol %. The grown Li5La3Nb2O12 crystals had polyhedral shapes with well-developed, flat {211} and {110} faces. Their shapes were relatively uniform, and the average crystal size was approximately 59.2 μm. No aggregation was observed in scanning electron microscopy images. The high crystallinity of the Li5La3Nb2O12 crystals was confirmed by transmission electron microscopy images. Their lattice parameter was determined from the X-ray diffraction pattern to be a = 1.281 nm, which is consistent with the literature value (a = 1.282 nm). Furthermore, the crystal phase, form, size, and crystallinity of the flux-grown Li5La3Nb2O12 crystals were obviously dependent on the growth conditions including the solute concentration and holding temperature.

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INTRODUCTION Green energy technologies, such as small-scale hydro and wind power technologies, geothermal generation, solar cells,1,2 fuel cells,3,4 and lithium-ion batteries (LIBs),5−7 have recently garnered the attention of many researchers because they provide an effectual approach to solve the problem of environmental warming caused by the release of greenhouse gases from vehicles and during the production of electricity. In particular, LIBs have been investigated for use as power sources in electric vehicles, hybrid electric vehicles, and plug-in hybrid electric vehicles because of the high energy density resulting from their high discharge voltages.8,9 Among LIBs, all-solidstate LIBs10−12 that use inorganic oxides as solid-electrolyte materials, such as Li5La3Nb2O12,13,14 Li3xLa2/3−xTiO3 (0.03 < 3x < 0.75),15,16 Li1.5Al0.5Ge1.5(PO4)3,17 and Li7La3Zr2O12,18 have attracted attention because of their safety and stability. Garnet-type Li5La3Nb2O12 exhibits a relatively high lithium-ion conductivity of ∼10−5 S·cm−1.19,20 The crystal structure of Li5La3Nb2O12 is classified as a cubic system with space group Ia3̅ and has a lattice parameter a of 1.282 nm.21 Figure 1 shows a schematic representation of the Li5La3Nb2O12 crystal structure. Li5La3Nb2O12 crystal consists of [La3Nb2O12]5− and Li ions. The framework of [La3Nb2O12]5− has six octahedral and two trigonal prismatic sites per formula unit and can form a three-dimensional passageway for Li ions. A large number of previous studies have been conducted on the preparation of Li5La3Nb2O12 powders using solid-state reaction methods at relatively high temperatures (at 900 °C for 24 h).13,21,22 These methods, however, have several inherent © 2012 American Chemical Society

Figure 1. Schematic representation of the crystal structure of garnettype Li5La3Nb2O12.

disadvantages such as high environmental loads and relatively poor crystallinities of the products. In order for Li5La3Nb2O12 to be used in all-solid-state LIBs, it should have a high crystallinity, because Li ions can move smoothly in the crystal lattice. Therefore, a fabrication method that produces highly crystalline Li5La3Nb2O12 crystals in a simple, inexpensive, and environmentally friendly manner is required. The flux method, which is used for crystal growth from a liquid phase, is simple, inexpensive, and environmentally friendly, and it affords binary or multicomponent crystals at Received: April 9, 2012 Revised: December 5, 2012 Published: December 11, 2012 479

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Table 1. Growth Conditions for Li5La3Nb2O12 Crystals flux

solute run

holding temperature/°C

LiOH·H2O/g

La2O3/g

Nb2O5/g

LiOH·H2O/g

1 2 3 4 5 6 7 8

500 500 500 500 900 400 900 900

1.209 0.421 1.738 2.104 1.209 1.209 1.081 2.530

3.005 0.981 4.049 4.902 3.005 3.005 2.518 5.894

1.634 0.533 2.202 2.666 1.634 1.634 1.370 3.201

4.902 8.336 3.129 1.683 4.902 4.902

temperatures below the melting points of the solutes.23−27 Furthermore, this method allows for the growth of crystals in an unconstrained fashion; that is, the crystals can grow without mechanical or thermal constraints in the solution and therefore develop facets. In our previous research, high-quality Li ion conductive crystals such as LiCoO29 and Li4Ti5O1226 were successfully grown from a NaCl flux. NaCl flux is very inexpensive and environmentally friendly (e.g., harmless to human beings and the environment), although a relatively hightemperature treatment is needed to grow crystals from this flux because its melting point is 801 °C. In contrast, the use of LiOH flux is expected to allow high-quality Li5La3Nb2O12 crystals to be grown at a relatively low temperature, because it has a melting point of 477 °C. Furthermore, Li5La3Nb2O12 would dissolve more easily in LiOH than in NaCl or other hydroxides because of the common ion (Li ions) effect in the former case. Herein, we report the low-temperature growth of high-quality, idiomorphic Li5La3Nb2O12 crystals with welldeveloped crystal faces obtained by cooling a LiOH flux. Furthermore, the effects of the solute concentration and holding temperature on the crystal phase, form, size uniformity, and crystallinity of the grown Li5La3Nb2O12 crystals were also studied. To our knowledge, this is the first report of the successful growth of high-quality, idiomorphic Li5La3Nb2O12 crystals at the low temperature of 500 °C.

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NaCl/g

solute concentration (mol %)

5.727

5 1 10 20 5 5 5

EM-002B) at 200 kV. The crystal phases were identified using X-ray diffraction (XRD, RIGAKU, MiniflexII) with Cu Kα radiation (λ = 0.154 nm). The X-ray diffractometer was operated at 30 kV and 20 mA in the 2θ range from 5° to 80°. The average sizes of the grown crystals were measured using a laser diffraction particle size analyzer (SHIMADZU, SALD-7100) with a 375 nm ultraviolet (UV) semiconductor laser in the measurement range from 10 nm to 300 μm.

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RESULTS AND DISCUSSION Highly crystalline, idiomorphic Li5La3Nb2O12 crystals were successfully grown using a LiOH flux cooling method at a temperature of 500 °C and at a solute concentration of 5 mol % (run 1). Figure 2a,b shows SEM images of Li5La3Nb2O12 the

EXPERIMENTAL SECTION

Li5La3Nb2O12 crystals were grown by a cooling of LiOH flux. Reagentgrade LiOH·H2O, La2O3, and Nb2O5 powders (Wako Pure Chemical Industries, Ltd.) were used as starting materials. Typical growth conditions are summarized in Table 1. In this study, the solute is considered to be a mixture of LiOH·H2O, La2O3, and Nb2O5 powders with stoichiometric Li5La3Nb2O12 abundances, and the flux is generated from oversupplied LiOH·H2O, as shown in Table 1. The solute concentration was varied from 1 to 20 mol %, and the mass of the reagent was maintained at approximately 10 g for all growth conditions (in fact, the mass of grown crystals and flux was kept at just 10 g). Each mixture was placed in an alumina crucible with a capacity of 30 cm3. After the lids were loosely closed, the crucibles were placed in an electric furnace, heated to temperatures of 400, 500, or 900 °C at a rate of 500 °C·h−1, and held at this temperature for 10 h. After heating, the crucibles were cooled to 300 °C at a rate of 200 °C·h−1 using a cooling program and then cooled to room temperature in the furnace. The crystal products were separated from the remaining flux in warm water. In addition, to confirm the effect of the LiOH flux, the growth of Li5La3Nb2O12 crystals following a flux method using NaCl (Reagent-grade, Wako Pure Chemical Industries, Ltd.) and using a solid-state reaction method (i.e., flux-free) was attempted. The holding temperature was set at 900 °C for both these syntheses. The obtained crystal products were studied using scanning electron microscopy (SEM, JEOL, JCM-5700) at an acceleration voltage of 15 kV and using transmission electron microscopy (TEM, TOPCON,

Figure 2. (a) Low- and (b) high-magnification SEM images of Li5La3Nb2O12 crystals grown at a holding temperature of 500 °C and a solute concentration of 5 mol %. (c) Schematic illustration of a Li5La3Nb2O12 crystal dominantly surrounded by {211} and {110} faces.

crystals grown in run 1. The Li5La3Nb2O12 crystals were observed to be relatively uniform in size and shape and poorly aggregated (Figure 2a). The high-magnification SEM image (Figure 2b) shows that the crystals are polyhedral with welldeveloped, relatively flat faces and an average size of approximately 60 μm. Figure 2c is a schematic illustration of 480

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the basis of the SAED pattern in Figure 4a, the d22̅0 and d211̅ spacings were determined to be 0.452 and 0.517 nm, respectively. These values are in good agreement with the theoretical values21 of d22̅0 = 0.455 and d211̅ = 0.525 nm. Figure 4c shows a lattice image of the flux-grown Li5La3Nb2O12 crystal that was taken with the incident beam along the [113] direction. No obvious defect was observed in these images, which further confirms the high crystallinity of the obtained sample. From these results, we conclude that high-quality Li5La3Nb2O12 crystals were successfully grown at the relatively low temperature of 500 °C by cooling the LiOH flux. Next, we investigated the effect of the solute concentration on the crystal phase, crystallinity, shape, and size of the Li5La3Nb2O12 crystals. Figure 5 shows SEM images of

a Li5La3Nb2O12 crystal dominantly surrounded by {211} and {110} faces; this figure was drawn using crystallographic data from the literature.21 The illustration is in excellent agreement with the observed structure of the flux-grown Li5La3Nb2O12 crystals (Figure 2b). The face angle between the a-face and the b-face in Figure 2b was determined to be about 32° ± 1°, which is in good agreement with the face angle between the {211} and {110} faces of the Li5La3Nb2O12 crystal (θ{211}∧{110} = 30.00°). Figure 3 shows the size distribution of the

Figure 3. Particle size distribution for Li5La3Nb2O12 crystals grown at a holding temperature of 500 °C and a solute concentration of 5 mol % measured using a laser diffraction method.

Li5La3Nb2O12 crystals grown in run 1. The average diameter was found to be 59.2 μm, which is in good agreement with the average size (about 60 μm) measured from the SEM image. Figure 4a,b shows the selected area electron diffraction (SAED) patterns of a Li5La3Nb2O12 crystal grown from the LiOH flux in run 1. Highly ordered SAED spots were clearly observed, indicating the high crystallinity of the Li5La3Nb2O12 crystal. On

Figure 5. SEM images of Li5La3Nb2O12 crystals grown at solute concentrations of (a, b) 1, (c, d) 10, and (e, f) 20 mol %.

Li5La3Nb2O12 crystals grown at solute concentrations of 1 (run 2), 10 (run 3), and 20 mol % (run 4) at a holding temperature of 500 °C. As shown in Figure 5a,b, the Li5La3Nb2O12 crystals grown at 1 mol % were relatively uniform in size (about 30 μm) and shape (a polyhedral shape). They had well-developed faces, as did those grown at 5 mol % shown in Figure 2b. The crystals grown at the solute concentration of 10 mol % (Figure 5c,d) were also relatively uniform in size (about 50 μm); however, their crystal faces were irregular compared to those of the crystals grown at 5 mol %. When the crystals were grown at 20 mol %, aggregates of small particles were obtained (Figure 5e,f). Figure 6 shows the powder XRD profiles of the pulverized crystals grown at solute concentrations of 1, 5, 10, and 20 mol %. The crystals grown at 1, 5, and 10 mol % were found to consist of a homogeneous Li5La3Nb2O12 phase (peaks marked with ●) by comparing their XRD patterns (Figure 6a−c). The lattice parameter of the

Figure 4. (a, b) SAED patterns and (c) lattice image of a Li5La3Nb2O12 crystal grown at a holding temperature of 500 °C and a solute concentration of 5 mol %. 481

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grown at 900 °C was approximately 60 μm. They had irregular shapes and many pores. When the crystals were grown at 400 °C, aggregates of small particles were produced (Figure 7b). Figure 8 shows the powder XRD profiles for the pulverized

Figure 6. XRD profiles of pulverized crystallites grown at solute concentrations of (a) 1, (b) 5, (c) 10, and (d) 20 mol %.

Li5La3Nb2O12 crystals was determined to be a = 1.281 nm from the XRD pattern in Figure 6b, which is consistent with the literature value21 of a = 1.282 nm. When the crystals were grown at a solute concentration of 20 mol %, diffraction lines from the Li5La3Nb2O12 crystals as well as the La(OH)3 powder (peaks marked with ★) were observed, as shown in Figure 6d. The SEM and XRD results demonstrate that the solute concentration played a crucial role in the production of Li5La3Nb2O12 crystals. At the solute concentrations below 5 mol %, high-quality Li5La3Nb2O12 crystals were grown successfully by cooling the LiOH flux at the relatively low temperature of 500 °C. These different results can presumably be attributed to the solubility of the solute in the LiOH flux. When the solute concentration is below 5 mol %, the solute is presumably completely soluble in the flux at 500 °C, and thus the Li5La3Nb2O12 crystals grow gradually through a process involving formation of a nucleus and gradual crystal growth during the cooling of the solution due to the comparatively low amount of supersaturation. As a result, the crystals grown at these concentrations had well-developed faces, and their sizes increased with increasing solute concentration. In contrast, the solute might not completely dissolve in the flux at over 10 mol %. Therefore, crystals with irregular faces and byproducts were obtained. Next, we also investigated the effect of the holding temperature on the crystal phase, crystallinity, shape, and size of the Li5La3Nb2O12 crystals. Figure 7 shows SEM images of Li5La3Nb2O12 crystals grown at holding temperatures of 900 (run 5) and 400 °C (run 6) at a solute concentration of 5 mol %. As shown in Figure 7a, the size of the Li5La3Nb2O12 crystals

Figure 8. XRD profiles of pulverized crystallites grown at holding temperatures of (a) 900 and (b) 400 °C.

crystals grown at holding temperatures of 900 and 400 °C. The crystals grown at 900 °C were found to consist of a homogeneous Li5La3Nb2O12 phase (peaks marked with ●), as shown in Figure 8a. By contrast, the products obtained at 400 °C did not contain Li5La3Nb2O12, and their diffraction lines were attributable to LiNbO3 (peaks marked with ◆), Nb2O5 (peaks marked with ■), and Li(OH)3 (peaks marked with ★), as shown in Figure 8b. The SEM and XRD results confirm that a holding temperature of 500 °C is proper for the growth of high-quality Li5La3Nb2O12 crystals. When the holding temperature was higher, at 900 °C, relatively poor crystals with irregular shapes and many pores were produced. The poor quality is considered to result from gas emitted as a result of evaporation of the flux at high temperatures. Note that the evaporative rates of the flux at 500 and 900 °C are 0.93% and 31.24%, respectively. (The calculation method is shown in Supporting Information.) By contrast, Li5La3Nb2O12 could not be obtained at 400 °C because the temperature was higher than the melting point of LiOH (477 °C), and thus, LiOH did not act as the flux. Finally, to confirm the effectiveness of the LiOH flux, the synthesis of Li5La3Nb2O12 crystals using the NaCl flux method (run 7) and a solid-state reaction method (run 8) was attempted. Figure 9a,b shows an SEM image and the XRD profile of the products obtained using the NaCl flux method. Aggregates of small particles were produced, and the XRD profile included not only Li5La3Nb2O12 peaks (marked with ●) but also LiLa2NbO6 peaks (marked with ▲) and La(OH)3 peaks (marked with ★). The products synthesized using solidstate reaction were also aggregates of small particles (Figure 9c) and consisted of Li5La3Nb2O12 (peaks marked with ●), LiLa2NbO6 (peaks marked with ▲), and La3O4 (peaks marked with ☆), as shown in their XRD profile (Figure 9d). It was clear that the shape and phase of the particles obtained by using NaCl flux were quite similar to those obtained by solid-state reaction. They had small sizes and poor crystal faces in comparison with those grown from LiOH flux. These results

Figure 7. SEM images of Li5La3Nb2O12 crystals grown at holding temperatures of (a) 900 and (b) 400 °C. 482

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Figure 9. SEM images and XRD profiles of products synthesized by (a and b) NaCl flux method and (c and d) solid-state reaction method.

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indicated that NaCl did not sufficiently act as a flux for growth of Li5La3Nb2O12 crystals and clearly demonstrated that LiOH flux was effective in promoting the growth of high-quality, idiomorphic Li5La3Nb2O12 crystals. Note that the La(OH)3 phase was observed in the XRD pattern of the particles obtained by using NaCl flux, while the La2O3 phase was observed in the case of the solid-state reaction. We assumed that the difference of the obtained impurity phases was caused by aqueous chemical reaction. In the case of NaCl flux, the remaining La2O3 reacted to form La(OH)3 during the procedure to remove the flux in warm water. Actually, La2O3 was converted to La(OH)3 in water at approximately 80 °C for 1 h (Supporting Information). The existence of lanthanum compounds indicated that NaCl flux did not dissolve them effectively and was not suitable for the growth of Li5La3Nb2O12.

ASSOCIATED CONTENT

S Supporting Information *

Formula for calculating the evaporative rates and XRD patterns of La2O3 powders. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) from the Japan Science and Technology Agency and Research and Development Program for Innovative Energy Efficiency Technology in 2011 (23-0712004) from New Energy and Industrial Technology Development Organization (NEDO) of Japan.

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CONCLUSIONS High-quality, idiomorphic Li5La3Nb2O12 crystals were successfully grown using the LiOH flux cooling method at a relatively low temperature of 500 °C. The Li5La3Nb2O12 crystals grown at a solute concentration of 5 mol % had a polyhedral shapes with well-developed {211} and {110} faces. The average crystal size measured using a laser diffraction particle size analyzer was about 59.2 μm. The TEM images indicated that the flux-grown Li5La3Nb2O12 crystals had very good crystallinity. On the basis of the powder XRD pattern, their lattice parameter was determined to be a = 1.281 nm. The crystal phase, crystallinity, shape, and size of the flux-grown Li5La3Nb2O12 crystals were obviously dependent on the solute concentration and the holding temperature. Comparison of these flux-grown Li5La3Nb2O12 crystals with those synthesized using the NaCl flux method and a solid-state reaction method showed that the LiOH flux was effective in promoting the growth of high-quality Li5La3Nb2O12 crystals. Finally, we believe that our growth technique for Li5La3Nb2O12 crystals using a LiOH flux can reduce the amount of environmental damage involved in their production and reduce the production cost because it enables us to grow Li5La3Nb2O12 crystals at a much lower temperature than that used for the conventional solid-state reaction. In the future, it will become more and more important to use environmentally friendly (low-temperature) processes to fabricate various functional materials.

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