Synthesis, crystal structure and conductive properties of garnet-type lithium ion conductor Al-free Li 7 x La 3 Zr 2 x Ta x O 12 (0 x 0.

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1 Full paper Synthesis, crystal structure and conductive properties of garnet-type lithium ion conductor Al-free Li 7 x La 3 Zr 2 x Ta x O 12 (0 x 0.6) Naoki HAMAO, Kunimitsu KATAOKA, Norihito KIJIMA and Junji AKIMOTO ³ National Institute of Advanced Industrial Science and Technology (AIST), Higashi, Tsukuba, Ibaraki , Japan We investigated the phase formation and the conductive properties of the Al-free Li 7¹x La 3 Zr 2¹x Ta x O 12 (0 x 0.6) samples. The X-ray diffraction patterns of the Li 7 La 3 Zr 2 O 12 (x = 0) and Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 (x = 0.6) samples were assigned to be single phases of tetragonal (space group: I4 1 /acd) and cubic (space group: Ia-3d) structures, respectively. On the other hand, the intermediate compositional samples of Li 7¹x La 3 Zr 2¹x Ta x O 12 (0.2 x 0.5) showed a coexistence of both the tetragonal and cubic phases. To investigate the conductive property of the prepared samples, the Li-ion conductivity was measured in a temperature range from 253 to 313 K by AC impedance method. All of the Al-free Li 7¹x La 3 Zr 2¹x Ta x O 12 (0 x 0.6) samples exhibited relatively high conductivity of ³10 ¹4 Scm ¹1 at room temperature, and the Li 6.5 La 3 Zr 1.5 Ta 0.5 O 12 (x = 0.5) sample showed the highest Li-ion conductivity of ¹4 Scm ¹1 at room temperature. In order to clarify the relationship between the Li-ion conductivity and the Li-ion arrangement, the crystal structure analysis of Li 7¹x La 3 Zr 2¹x Ta x O 12 (0 x 0.6) was performed by Rietveld analysis using powder X-ray diffraction data. The Li(2) atom at 96h site was gradually shifted together with increasing Ta-content from x = 0.2 to 0.5 resulting the shorter Li Li distance in the loop structure of the cubic garnet-type framework structure The Ceramic Society of Japan. All rights reserved. Key-words : Solid electrolyte, Garnet oxide, Li-ion conductor, Crystal structure [Received January 25, 2016; Accepted April 14, 2016] ³ Corresponding author: J. Akimoto; j.akimoto@aist.go.jp Preface for this article: DOI Introduction The garnet-type Li-ion conductor has been expected as solid electrolyte for all-solid-state Li-ion batteries. In this decade, the garnet-related type Li 7 La 3 Zr 2 O 12, which was reported by R. Murugan et al. for first time, 1) has been studied from view points of the chemical stability, the conductive properties and the crystal structure in detail. In order to increase the Li-ion conductivity of the garnet-type Li 7 La 3 Zr 2 O 12, a large number of studies on the chemical substitution using various cation species has been reported. 2) 8) Among them, Li 7¹x La 3 Zr 2¹x Ta x O 12 with the cubic garnet-related type structure shows a highest Li-ion conductivity (³10 ¹4 S cm at 300 K). 9) 12) For this reason, the chemical and structural properties of Li 7¹x La 3 Zr 2¹x Ta x O 12 were widely investigated in the literature. However, it is difficult to prepare the high-quality Li 7¹x La 3 Zr 2¹x Ta x O 12 samples because aluminum is easily contaminated from the crucible material during the hightemperature heating. Recently, the garnet-type Al-free Li 7¹x La 3 Zr 2¹x Ta x O 12 samples were synthesized by some research groups. 13) 16) Y. Wang et al. reported the existence of two phases of tetragonal and cubic structures in the compositional range of 0.1 x 0.4 for the Al-free Li 7¹x La 3 Zr 2¹x Ta x O 12 samples. 16) However, the effect of only Ta substitution on the structural property and the Li-ion conductivity has not been clarified yet. In the present study, we synthesized the garnet-type Li 7¹x - La 3 Zr 2¹x Ta x O 12 (0 x 0.6) by the conventional solid-state synthesis method under Al-free conditions. In addition, we investigated the influence of Ta substitution on the crystal phase formation and the Li-ion conductive properties. 2. Experimental The garnet-type oxide Li 7¹x La 3 Zr 2¹x Ta x O 12 (0 x 0.6) samples were synthesized by solid state reaction method. Li 2 CO 3 (99.99%, Rare Metallic Co., Ltd.), La 2 O 3 (99.99%, Rare Metallic Co., Ltd., dried at 1173 K for 12 h), ZrO 2 (99.99%, Rare Metallic Co., Ltd.) and Ta 2 O 5 (99.99%, Rare Metallic Co., Ltd.) were used as starting materials. An excess Li source was added to compensate for the volatilization of lithium during the high temperature calcination. The starting materials were mixed by ball milling (Pulverisette, Fritsch, Germany) for 2 h at 350 rpm in ethanol using zirconia balls. The mixed powder was calcined at 1127 K for 3 h in air after pressing into pellet at 60 MPa. Finally, the pressed pellets were sintered at 1373 K for 4 h in air using YSZ crucibles. At sintering step, the pellets were covered with the mother powder of the same composition to prevent the Li loss. The phase identification of prepared samples was performed by powder X-ray diffraction (Rigaku, SmartLab) using Cu K radiation with 2ª range from 10 to 90 at an interval step of Chemical composition of the Li 7¹x La 3 Zr 2¹x Ta x O 12 samples was measured by inductively coupled plasma optical emission spectroscopy (ICP-OES). The electrical conductivity was measured by AC impedance method using a Solartron 1260 impedance analyzer. The measurement was carried out in a temperature range from 253 to 313 K at 100 mv applied AC amplitude, at 32 MHz to 100 Hz frequencies. The both sides of measured pellet were coated by Au as Li-ion blocking electrodes using Au spattering. The crystal structure of the obtained Li 7¹x La 3 Zr 2¹x Ta x O 12 (0 x 0.6) samples was analyzed by Rietveld method using 2016 The Ceramic Society of Japan DOI

2 JCS-Japan XRD data. For the Rietveld analysis of collected XRD patterns, the space groups of I4 1 /acd and Ia-3d were used for the tetragonal and cubic phases, respectively. 3. Results and discussion 3.1 Sample characterizations Figure 1(a) shows the X-ray diffraction patterns of Li 7¹x - La 3 Zr 2¹x Ta x O 12 (0 x 0.6). The patterns of Li 7 La 3 Zr 2 O 12 (x = 0) and Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 (x = 0.6) were assigned to be single phases of tetragonal (S.G.: I4 1 /acd) and cubic (S.G.: Ia-3d) structures, respectively. On the other hand, the diffraction patterns of the Li 7¹x La 3 Zr 2¹x Ta x O 12 (0.2 x 0.5) samples showed both tetragonal and cubic phases. Additionally, some unknown peaks were detected in the samples with x = 0.2 and 0.3, as shown in Fig. 1(b). Figure 2 shows the Ta-compositional dependence of lattice parameters calculated from powder XRD data. For the Li 7¹x La 3 Zr 2¹x Ta x O 12 (0.2 x 0.6), the lattice parameter of the cubic phase decreased together with increasing Ta-content according to the differences in ionic radii of Zr 4+ and Ta 5+. Therefore, this fact suggested that the Zr 4+ site in the octahedral coordination for the cubic phase was successfully substituted by Ta 5+. It should be noted that the two-phase coexistence was observed in the Li 7¹x La 3 Zr 2¹x Ta x O 12 (0.2 x 0.5) samples. A similar two-phase coexistence was previously reported by Y. Wang et al., 16) due to the order disorder of Li defects. However, the lattice parameters for both tetragonal and cubic phases strangely changed with Ta-content in the present samples, as shown in Fig. 2. This fact can be explained by the heterogeneous distribution of Ta in the present samples, in spite of high-temperature heating. It maybe also mean the structural change in the solid solution compound, as observed additional unknown peaks [Fig. 1(b)]. In the present study, the reason of two-phase coexistence is not clear. We have a plan to examine the Ta distribution by microscopic observation method. Table 1 shows the results of chemical compositional analysis by ICP-OES. The atomic ratio of La/Zr/Ta for Li 7¹x La 3 Zr 2¹x - Ta x O 12 (0 x 0.5) was close to the expected compositional ratio. Although the Li content was also analyzed by ICP, the resultant Li-content was significantly shifted to larger values because of an excess Li-source in the preparation. Furthermore, Al was not detected (<5 ppm) since all samples were synthesized in Al-free atmosphere. 3.2 Conductive properties Figure 3(a) shows the Cole-Cole plots of Li 7¹x La 3 Zr 2¹x Ta x O 12 with x = 0.3 and 0.5. The Li-ion conductivity measurement was performed by AC impedance method. The Nyquist plot of the AC impedance spectra for all sintering samples was observed one semicircle. Since the conductivity could not be separated into the bulk and grain boundary resistance element, the Li-ion conductivity of Li 7¹x La 3 Zr 2¹x Ta x O 12 (0.2 x 0.6) was calculated from the one semicircle as a total conductivity. Additionally, it was assumed that the grain-boundary resistances were not affected by the amount of Ta substitution. Figure 3(b) shows the temperature dependence of total Li-ion Fig. 2. Lattice parameters of Li 7¹x La 3 Zr 2¹x Ta x O 12 (0 x 0.6). Fig. 1. a) Powder X-ray patterns of Li 7¹x La 3 Zr 2¹x Ta x O 12 (0 x 0.6), b) Zoomed-in the patterns. Table 1. Results of chemical composition analysis for Li 7¹x La 3 Zr 2¹x - Ta x O 12 (0 x 0.5) Sample Atomic ratio of La:Zr:Ta Li 6.8 La 3 Zr 1.8 Ta 0.2 O 12 3:1.80:0.19 Li 6.7 La 3 Zr 1.7 Ta 0.3 O 12 3:1.70:0.28 Li 6.6 La 3 Zr 1.6 Ta 0.4 O 12 3:1.59:0.38 Li 6.5 La 3 Zr 1.5 Ta 0.5 O 12 3:1.49:

3 JCS-Japan Hamao et al.: Synthesis, crystal structure and conductive properties of garnet-type lithium ion conductor Al-free Li 7 x La 3 Zr 2 x Ta x O 12 (0 : x : 0.6) conductivity for the Li 7¹x La 3 Zr 2¹x Ta x O 12 (0.2 x 0.6). The total conductivity is expressed by the Arrhenius law [ = A/T exp(¹ea/k B T)], where A is frequency factor, Ea is activation energy of Li-ion conductivity, k B is Boltzmann constant ( ¹23 J/K) and T is absolute temperature. Since the total Li-ion conductivity of the all samples was linear against 1/T function, the structure transition was not occurred in the measured temperature range. The Li-ion conductivity increased with the Ta-content. Among all the prepared samples, the Li 6.5 La 3 Zr Ta 0.5 O 12 (x = 0.5) shows the highest Li-ion conductivity ( ¹4 Scm ¹1 ) at room temperature in the present study. Figure 4 shows the activation energy and the room temperature conductivity of all the prepared samples as a function of Ta-content. In these sample, the Li 7¹x La 3 Zr 2¹x Ta x O 12 samples with x = 0.4 and 0.5 show the highest Li-ion conductivity and low activation energy. On the other hand, the samples with the low Ta-content with 0.2 x 0.4 also has a relatively high Liion conductivity and low activation energy despite a mixture of tetragonal and cubic phases. These results can be understood that the Li-ion conductivity for the prepared Li 7¹x La 3 Zr 2¹x Ta x O 12 (0.2 x 0.4) samples was mainly affected by the contribution of the cubic phase, and/or that the tetragonal phase has high conductivity such as that for the cubic phase. 3.3 Crystal structure analysis In order to reveal the origin of the improved Li-ion conductivity for Li 7¹x La 3 Zr 2¹x Ta x O 12 (0.2 x 0.6), the crystal structure analysis was performed by the Rietveld analysis using powder XRD data. The structure refinements for the Li 7 La 3 Zr 2 O 12 (x = 0) and Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 (x = 0.6) were performed as a single-phase model using the reported structural parameters for tetragonal (I4 1 /acd, no. 142) 17) and cubic (Ia-3d, no. 230) 18) structures. On the other hand, the crystal structures for the intermediate Li 7¹x La 3 Zr 2¹x Ta x O 12 (0.2 x 0.5) were refined using a two-phase model of the tetragonal and cubic structures. Figures 5 and 6 show the Rietveld refinement patterns of Li 7¹x La 3 Zr 2¹x - Ta x O 12 with x = 0.3 and 0.5, and the refined parameters are given in Tables 2 and 3, respectively. By assuming coexistence of tetragonal and cubic phases, the calculated diffraction patterns were in good agreement with the observed ones. Accordingly, the Li 7¹x La 3 Zr 2¹x Ta x O 12 (0.2 x 0.5) samples synthesized under Al-free experimental condition showed two-phase coexistence in the present study from the results of the crystal structure analysis. This fact is in contrast with the previous result that the Alcontaining garnet-type Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 had a single phase cubic structure. Moreover, the lattice parameters of the tetragonal phase observed in the present Li 7¹x La 3 Zr 2¹x Ta x O 12 (0.2 x 0.5) samples were gradually changed together with Ta-content from the original values for the tetragonal Li 7 La 3 Zr 2 O 12, 17) as mentioned previously. This fact maybe suggest the structural difference such as Li-ion arrangement and/or site occupation in the present tetragonal phases and Li 7 La 3 Zr 2 O 12. In fact, the difference of crystal structure in the tetragonal and cubic garnettype structure framework is due to the Li atomic arrangement. 18) However, it is difficult to investigate the Li atomic arrangement in detail the structure analysis using the laboratory system X-ray diffraction. We are now attempting to examine the phase transition of this tetragonal phase using the neutron beam and the transmission electron microscope in polycrystalline, and the results will be published in the near future. As previously mentioned from the results of conductive property measurement, we can speculate that the Li-ion conductivity of Li 7¹x La 3 Zr 2¹x Ta x O 12 (0.2 x 0.6) is mainly affected by the contribution of the cubic phase. In order to examine the effect of Ta substitution to the cubic phase for Li 7¹x La 3 Zr 2¹x Ta x O 12 (0.2 x 0.6), we compared the Li site distributions in the cubic structure of Li 7¹x La 3 Zr 2¹x Ta x O 12 (0.2 x 0.6). The local Li-ion arrangements in the garnet type framework structure of Fig. 3. (a) Cole-Cole plots of Li 7¹x La 3 Zr 2¹x Ta x O 12 (x = 0.3, 0.5) at 297 K. (b) Arrhenius plots of total conductivities for Li 7¹x La 3 Zr 2¹x Ta x O 12 (0 x 0.6). Fig. 4. Activation energy and total conductivities at 297 K for Li 7¹x La 3 Zr 2¹x Ta x O 12 (0.2 x 0.6). 680

4 JCS-Japan Fig. 5. Observed (+), calculated (green line), and difference (bottom) pattern for the Rietveld refinement using the X-ray diffraction data of Li 6.7 La 3 Zr 1.7 Ta 0.3 O 12. The short vertical lines below the diffraction pattern of all possible Bragg reflections for the cubic phase (upper) and the tetragonal phase (lower). Fig. 6. Observed (+), calculated (green line), and difference (bottom) pattern for the Rietveld refinement using the X-ray diffraction data of Li 6.5 La 3 Zr 1.5 Ta 0.5 O 12. The short vertical lines below the diffraction pattern of all possible Bragg reflections for the cubic phase (upper) and the tetragonal phase (lower). Li 7¹x La 3 Zr 2¹x Ta x O 12 (x = 0.3 and 0.5) were shown in Fig. 7. The Li(1) and Li(2) atoms are situated at 24d and 96h sites forming the tetrahedral Li(1)O 4 and octahedral Li(2)O 6 coordination, respectively. In Li 6.5 La 3 Zr 1.5 Ta 0.5 O 12, the Li(2) atom was situated on a straight line between two Li(1) atoms which may be the Liion conduction pathway [Fig. 7(b)]. In the case of Li 6.7 La 3 Zr Ta 0.3 O 12 with x = 0.3, on the contrary, the Li(2) atom was located at the perpendicular position to the conduction path, as shown in Fig. 7(a). Accordingly, the Li(1) Li(2) distance [1.50(6) ] for Li 6.5 La 3 Zr 1.5 Ta 0.5 O 12 was shorter than that [1.73(2) ] for Li 6.7 La 3 Zr 1.7 Ta 0.3 O 12. The shorter Li Li distance constructing the loop structure 18) within the Li-ion conduction pathway would contribute to the improved conductive properties of Li 6.5 La 3 Zr Ta 0.5 O 12. From this speculation, the position of the Li(2) atom in the loop structure may be a key role in the Li-ion conductivity and the low activation energy. 4. Conclusions We synthesized the garnet-type Li 7¹x La 3 Zr 2¹x Ta x O 12 (0 x 0.6) under Al-free conditions by solid-state method. The X-ray diffraction patterns of the Li 7 La 3 Zr 2 O 12 (x = 0) and Li 6.4 La 3 Zr Ta 0.6 O 12 (x = 0.6) samples were assigned to be single phases of tetragonal (space group: I4 1 /acd) and cubic (space group: Ia-3d) structures, respectively. On the other hand, the intermediate compositional samples of Li 7¹x La 3 Zr 2¹x Ta x O 12 (0.2 x 0.5) showed a coexistence of both the tetragonal and cubic phases. These samples exhibited a relatively high Li-ion conductivity of ³10 ¹4 Scm ¹1, and the Li 6.5 La 3 Zr 1.5 Ta 0.5 O 12 (x = 0.5) sample showed the highest Li-ion conductivity of ¹4 Scm ¹1 at 297 K, even the sample contained a small amount of tetragonal phase. Furthermore, the Li(2) atom at 96h site was gradually shifted together with increasing Ta-content from x = 0.2 to 0.5 resulting the shorter Li Li distance in the loop structure of the 681

5 JCS-Japan Hamao et al.: Synthesis, crystal structure and conductive properties of garnet-type lithium ion conductor Al-free Li 7 x La 3 Zr 2 x Ta x O 12 (0 : x : 0.6) Table 2. Refined structure parameters for Li 6.7 La 3 Zr 1.7 Ta 0.3 O 12 at 298 K using X-ray diffraction data. R-factors were R wp = 11.98%, R p = 9.05% and R e = 6.56%. Refined lattice parameters of tetragonal phase (I4 1 /acd) and cubic phase (Ia-3d) were a = (6), c = (8) and a = (2), respectively. The weight ratio of tetragonal phase and cubic phase was 0.20:0.80 cubic phase La 24c 1 1/8 0 1/ (4) Zr 16a 0.838(3) (5) Ta 16a =x (Zr) =y (Zr) =z (Zr) =U eq (Zr) Li(1) 24d 0.49(3) 3/8 0 1/4 0.03(2) Li(2) 96h (2) 0.672(2) 0.572(2) 0.02(1) O 96h 1 ¹0.0356(3) (3) (3) 0.009(2) tetragonal phase La(1) 8b 1 0 1/4 1/ (2) La(2) 16e (4) 0 1/ (2) Zr 16c (2) Li(1) 8a 1 0 1/4 3/ (2) Li(2) 16f (3) 0.524(3) 1/ (1) Li(3) 32g (6) 0.054(4) 0.783(5) 0.009(1) O(1) 32g 1 ¹0.032(2) 0.056(2) 0.145(2) 0.003(2) O(2) 32g (2) 0.854(2) 0.533(2) 0.006(2) O(3) 32g (2) 0.045(2) 0.441(2) 0.005(2) Table 3. Refined structure parameters for Li 6.5 La 3 Zr 1.5 Ta 0.5 O 12 at 298 K using X-ray diffraction data. R-factors were R wp = 8.52%, R p = 6.38% and R e = 4.66%. Refined lattice parameters of tetragonal phase (I4 1 /acd) and cubic phase (Ia-3d) were a = (2), c = (5) and a = (1), respectively. The weight ratio of tetragonal phase and cubic phase was 0.02:0.98 cubic phase La 24c 1 1/8 0 1/ (4) Zr 16a 0.759(2) (5) Ta 16a =x (Zr) =y (Zr) =z (Zr) =U eq (Zr) Li(1) 24d 0.62(2) 3/8 0 1/4 0.03(2) Li(2) 96h (5) 0.688(4) 0.593(5) 0.019(3) O 96h 1 ¹0.0312(3) (4) (4) 0.009(2) tetragonal phase La(1) 8b 1 0 1/4 1/ (2) La(2) 16e (5) 0 1/ (1) Zr 16c (2) Li(1) 8a 1 0 1/4 3/ (2) Li(2) 16f (3) / (3) Li(3) 32g (3) 0.12(9) 0.81(5) 0.003(4) O(1) 32g 1 ¹0.069(8) 0.063(9) 0.18(1) 0.014(5) O(2) 32g (1) 0.85(1) 0.56(1) 0.006(5) O(3) 32g (9) 0.048(9) 0.46(1) 0.005(3) Fig. 7. The coordination polyhedral around Li site of Li 7¹x La 3 Zr 2¹x Ta x O 12.a)x = 0.3, b) x =

6 JCS-Japan cubic garnet-type framework structure, from the results of the present powder XRD structure analysis. Acknowledgements This work was supported by the Advanced Low Carbon Technology Research and Development Program (ALCA-SPRING) from Japan Science and Technology Agency (JST). References 1) R. Murugan, V. Thangadurai and W. Weppner, Angew. Chem., 119, (2007). 2) L. Dhivya, N. Janani, B. Palanivel and R. Murugan, AIP Adv., 3, (2013). 3) S. Ohta, T. Kobayashi and T. Asaoka, J. Power Sources, 196, (2011). 4) R. Murugan, S. Ramakumar and N. Janani, Electrochem. Commun., 13, (2011). 5) J. Wolfenstine, J. Ratchford, E. Rangasamy, J. Sakamoto and J. L. Allen, Mater. Chem. Phys., 134, (2012). 6) C. Deviannapoorani, L. Dhivya, S. Ramakumar and R. Murugan, J. Power Sources, 240, (2013). 7) S. Ramakumar, L. Satyanarayana, S. V. Manorama and R. Murugan, Phys. Chem. Chem. Phys., 15, (2013). 8) G. T. Hitz, E. D. Wachsman and V. Thangadurai, J. Electrochem. Soc., 160, A1248 A1255 (2013). 9) Y. Li, C. Wang, H. Xie, J. Cheng and J. B. Goodenough, Electrochem. Commun., 13, (2011). 10) J. L. Allen, J. Wolfenstine, E. Rangasamy and J. Sakamoto, J. Power Sources, 206, (2012). 11) H. Buschmann, S. Berendts, B. Nogwaits and J. Jenek, J. Power Sources, 206, (2012). 12) Y. Wang and W. Lai, Electrochem. Solid-State Lett., 15, A68 A71 (2012). 13) A. Logéat, T. Köhler, U. Eisele, B. Stiaszny, A. Harzer, M. Tovar, A. Senyshyn, H. Ehrenberg and B. Kozinsky, Solid State Ionics, 206, (2012). 14) T. Thompson, J. Wolfenstine, J. L. Allen, M. Johannes, A. Huq, I. N. David and J. Sakamoto, J. Mater. Chem. A Mater. Energy Sustain., 2, (2014). 15) R. Inada, K. Kusakabe, T. Tanaka, S. Kudo and Y. Sakurai, Solid State Ionics, 262, (2014). 16) Y. Wang and W. Lai, J. Power Sources, 275, (2015). 17) J. Awaka, N. Kijima, H. Hayakawa and J. Akimoto, J. Solid State Chem., 182, (2009). 18) J. Awaka, A. Takashima, K. Kataoka, N. Kijima, Y. Idemoto and J. Akimoto, Chem. Lett., 40, (2011). 683