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Enhancement of lithium ion conduction in the cubic rare earth oxide
Electrochemistry Communications 9 (2007) 245–248 www.elsevier.com/locate/elecom
Enhancement of lithium ion conduction in the cubic rare earth oxide Shinji Tamura, Akihiro Mori, Nobuhito Imanaka
*
Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Received 23 August 2006; received in revised form 16 September 2006; accepted 19 September 2006 Available online 24 October 2006
Abstract A new type of lithium ion conducting solid electrolyte based on a cubic rare earth oxide was developed by co-doping LiNO3 and KNO3 into a (Gd1 xNdx)2O3 solid, which possesses large interstitial open spaces within the structure. Among the samples prepared, 0.6(Gd0.4Nd0.6)2O3–0.16LiNO3–0.24KNO3 exhibits the highest lithium ion conductivity of 8.05 · 10 2 and 1.35 · 10 3 S cm 1 at 400 and 100 C, respectively, which is comparable to that of the LISICON materials. Pure Li+ ion conduction was successfully demonstrated by the dc electrolysis method. 2006 Elsevier B.V. All rights reserved. Keywords: Solid electrolyte; Lithium ion; Cubic rare earth oxide; Lithium nitrate; Potassium nitrate
1. Introduction In the electronics field, Li+ ion conducting materials are expected to be used as components of rechargeable secondary batteries with high power density and chemical sensors. Therefore, the Li+ ion conduction in crystalline solids, glasses, and polymers has been extensively studied. Amongst these different structures, the crystalline solid (solid electrolyte) is one of the most suitable forms from a practical-use perspective, because there is no possibility of electrolyte leakage even at elevated temperatures. Up until now, many types of highly conducting Li+ ion solid electrolytes, such as Li+–b’’-alumina [1], Li3N [2,3], LISICON (Li+ super ionic conductor) analogues [4–9], and perovskite related materials [10–12] have been reported as high Li+ ion conducting solid electrolytes with high chemical stability in an air atmosphere. Although all of them possess a relatively large conduction pathway through the Li+ ion sites in the structure, the Li+ ion conduction is affected by the structural, chemical, and/or physical properties of the parent crystal lattice, because Li+ ions are also one of the constituent ions in the solid lattice. On the other *
Corresponding author. Tel.: +81 6 6879 7352; fax: +81 6 6879 7354. E-mail address: [email protected] (N. Imanaka).
1388-2481/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.09.016
hand, we have recently developed a new type of Li+ ion conducting solid, a LiNO3-doped cubic rare earth oxide of (Gd0.9La0.1)2O3–LiNO3 [13,14] by following quite a new concept. This new concept involves the insertion of LiNO3 into the relatively large interstitial open spaces present in the cubic (Gd0.9La0.1)2O3 lattice where Li+ ions do not function as components of the parent crystal lattice. High Li+ ion conductivity that enters the range of practical application (r >10 3 S cm 1 above 200 C) was realized by the smooth migration of Li+ ions through the large interstitial open spaces. However, the (Gd0.9La0.1)2O3–LiNO3 system has a disadvantage in that an abrupt reduction in conductivity occurs below 250 C, which is close to the melting point of LiNO3 (ca. 255 C). One effective way to improve the conductivity below 250 C is to lower the melting point of LiNO3. In this work, we selected a LiNO3–KNO3 eutectic mixture in a molar ratio of LiNO3:KNO3 = 2:3 as the nitrate to be doped into the cubic rare earth oxide, because the melting point of the 2LiNO3–3KNO3 eutectic mixture is as low as 125 C. Furthermore, it is necessary to expand the crystal lattice of the parent oxide in order to introduce large K+ cations into the interstitial open spaces. Although there are cubic rare earth oxides such as Eu2O3 and Sm2O3 whose lattice sizes are larger than that of Gd2O3, Eu3+ and Sm3+ ions are easily
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reduced in the cubic oxides. This introduces the possibility of electronic conduction due to their valence change, which is an undesirable characteristic for a solid electrolyte. Therefore, these oxides are not appropriate for use as the parent oxide. Alternatively, the cubic crystal lattice can be expanded by the partial substitution of the Gd3+ (0.1078 nm; CN = 6) [15] site in Gd2O3 with larger trivalent cations such as La3+ (0.1172 nm; CN = 6) [15]. However, for substitution with La3+, cubic crystal lattices larger than that of (Gd0.9La0.1)2O3 cannot be obtained, because this composition has a solid solution limit where the cubic phase is formed [13,14]. Therefore, the Gd3+ sites in Gd2O3 were partially substituted with Nd3+ ions (0.1123 nm; CN = 6) [15] in an attempt to obtain a large cubic crystal lattice. Furthermore, since it has been reported that only the hexagonal and monoclinic structures are formed in the Gd2O3–Nd2O3 system when prepared by solid-state reaction [16], we applied the co-precipitation method to obtain a cubic Gd2O3–Nd2O3 solid solution. In this communication, we report the development of (1 y z)(Gd1 xNdx)2O3–yLiNO3–zKNO3 solids in an attempt to realize high Li+ ion conduction in a cubic rare earth oxide even at temperatures below 250 C. The Li+ ion conduction properties of the prepared (1 y z) (Gd1 xNdx)2O3–yLiNO3–zKNO3 solids were investigated. 2. Experimental (Gd1 xNdx)2O3 solid solutions were prepared by the coprecipitation method from 1.0 mol L 1 aqueous solutions of Gd(NO3)3 and Nd(NO3)3. A stoichiometric mixture of these solutions was poured into 0.5 mol L 1 oxalic acid and the precipitate obtained was heated at 1000 C for 12 h. The (Gd1 xNdx)2O3 solid solution obtained was then mixed with LiNO3 and KNO3 in a appropriate ratio and the mixed powder was calcined at 400 C for 12 h in air. The ratio of LiNO3 and KNO3 was maintained at 2:3. After X-ray powder diffraction (XRD) analysis of the samples using a Cu Ka radiation (Rigaku, Multiflex), the powder was pelletized and sintered at 400 C for 12 h in air.
Fourier-transform infrared (FT-IR) spectra of the sample powders were measured with an FT-IR spectrometer (Bruker, TENSOR 27). Ac conductivity was measured using the sintered sample pellets with Au sputtered electrodes in the center of both surfaces. Dc electrolysis of the 0.6(Gd0.4Nd0.6)2O3–0.16LiNO3–0.24KNO3 pellet was conducted by applying a dc voltage of 3 V at 200 C for 72 h in air. 3. Results and discussion Fig. 1 displays the XRD patterns and the lattice volume variation for the (Gd1 xNdx)2O3 (0.1 6 x 6 0.7) solids with the corresponding data for pure cubic Gd2O3. The samples with x 6 0.6 were found to have a cubic phase (space group: Ia3, similar to that for pure Gd2O3, while the sample with x = 0.7 was a two phase mixture of cubic Gd2O3 and monoclinic NdGdO3. The lattice volume of the sample with single cubic phase (x 6 0.6) linearly increases in accordance with Vegard’s Law. These results indicate that (Gd1 xNdx)2O3 clearly forms a solid solution with cubic symmetry in the composition range of x 6 0.6. This is the first report of a cubic phase for the Nd2O3–Gd2O3 system, although the reason for the formation of the cubic phase, neither hexagonal nor monoclinic phase, is uncertain and requires further consideration. The XRD patterns of (1 y z)(Gd0.4Nd0.6)2O3–yLiNO3– zKNO3 are shown in Fig. 2. The LiNO3:KNO3 ratio was fixed at 2:3 on the basis that the lowest melting point was obtained at this ratio. For those samples with y + z 6 0.4, a single cubic phase was obtained. Additional XRD peaks identified as KNO3 were observed when the total nitrate content of y + z was higher than 0.4. The reason for only KNO3 appearing as the secondary phase in the samples with y + z > 0.4 is thought to be caused by the large ionic size of K+ (0.138 nm [15]) compared with that of Li+ (0.076 nm [15]). Although both lithium and potassium nitrates can be fully dissolved up to their solubility limit (y + z = 0.4), LiNO3 may preferentially penetrate into the interstitial open spaces present in the (Gd0.4Nd0.6)2O3 lattice. Therefore,
Fig. 1. (a) XRD profiles and (b) the lattice volume of the (Gd1 xNdx)2O3 (0.1 6 x 6 0.7) solids (¤) with the corresponding datum for cubic Gd2O3 (s).
S. Tamura et al. / Electrochemistry Communications 9 (2007) 245–248
Fig. 2. XRD patterns of the (1 y z)(Gd0.4Nd0.6)2O3–yLiNO3–zKNO3 (y:z = 2:3; 0.2 6 y + z 6 0.5) solids and the parent solvent phase of (Gd0.4Nd0.6)2O3. (y + z = (a) 0.2, (b) 0.3, (c) 0.4 and (d) 0.5).
KNO3 was deposited as a secondary phase when the amount of alkaline metal nitrates exceeded the solubility limit. In order to identify whether nitrate ions (NO3 ) were present in the sample, FT-IR measurements were performed for the sintered samples of (1 y z)(Gd0.4Nd0.6)2O3– yLiNO3–zKNO3 (y:z = 2:3; 0 6 y + z 6 0.5). All the samples prepared showed two bands in the IR spectra at approximately 825 and 1384 cm 1, which were not observed in (Gd0.4Nd0.6)2O3 (Fig. 3). These bands are ascribed to the out-of-plane m2 deformation mode and the m3 asymmetric stretching mode of the NO3 ion, respectively. This result indicates that NO3 ions remain in the sample. Fig. 4 shows the compositional dependence of the conductivity for the (1 y z)(Gd0.4Nd0.6)2O3–yLiNO3– zKNO3 (y:z = 2:3; 0 6 y + z 6 0.5) solids at 400 C. The conductivity increased linearly with (y + z), and the highest conductivity (8.05 · 10 2 S cm 1) was obtained for the solubility limit composition of y + z = 0.4 among the single phase samples (y + z 6 0.4). Although the conductivity also slightly increased in the two phase mixture range (y + z > 0.4), the degree of increase in the slope is quite different from that observed for the single phase region (y + z 6 0.4). The increase in conductivity for the two phase mixture region may be caused by K+ ion conduction in the nitrates that appeared as a secondary phase. In order to directly demonstrate Li+ ion conduction in the 0.6(Gd0.4Nd0.6)2O3–0.16LiNO3–0.24KNO3 solid solution, dc electrolysis was performed at 200 C by applying a dc voltage of 3 V which is higher than the decomposition voltage of ca. 1.0 V. Li-containing compounds, such as Li3AuO3 and Li5AuO4, were recognized only at the cathodic surface of the electrolyzed pellet, whereas such compounds were not at all observed at the anodic side, indicating that only Li+ ions are conducted in the sample
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Fig. 3. FT-IR spectra for the (1 y z)(Gd0.4Nd0.6)2O3–yLiNO3–zKNO3 (y:z = 2:3; 0.2 6 y + z 6 0.5) solids and the parent solvent phase of (Gd0.4Nd0.6)2O3. (y + z = (a) 0.2, (b) 0.3, (c) 0.4 and (d) 0.5).
Fig. 4. Compositional dependence of (1 y z)(Gd0.4Nd0.6)2O3–yLiNO3–zKNO3 solids at 400 C.
the conductivity for the (y:z = 2:3; 0 6 y + z 6 0.5)
(Detailed discussions of the dc electrolysis procedure is described in previous papers [13,14]). Fig. 5 presents the Li+ ion conductivity of the 0.6(Gd0.4Nd0.6)2O3–0.16LiNO3–0.24KNO3 solid solution with the corresponding data for the 0.48(Gd0.9La0.1)2 O3–0.52LiNO3 solid that we have reported previously [13,14], in addition to representative Li+ ion conducting solid electrolytes with high chemical stability in atmospheric air. Although the Li+ ion conductivity of 0.6(Gd0.4Nd0.6)2O3–0.16LiNO3–0.24KNO3 is lower than that of 0.48(Gd0.9La0.1)2O3–0.52LiNO3 at temperatures above 250 C, due to the low concentration of conducting Li+ ions, the conductivity at temperatures below 250 C is greatly improved. The Li+ ion conductivity for the 0.6(Gd0.4Nd0.6)2O3–0.16LiNO3–0.24KNO3 solid at 100 C (1.35 · 10 3 S cm 1) is more than four orders of magnitude higher than that for the 0.48(Gd0.9La0.1)2O3–0.52LiNO3
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by doping a eutectic mixture of LiNO3 and KNO3, with a melting point as low as 125 C, into the interstitial open spaces of the cubic (Gd0.4Nd0.6)2O3 lattice that has been intentionally expanded compared with that of (Gd0.9La0.1)2O3. Both the lowering of the melting point of the nitrates and expansion of the cubic lattice of the rare earth oxide promote Li+ ion conduction and the measured conductivity is comparable to that for well-known high Li+ ion conductors such as Li+–b-alumina and the LISICON series over the temperature range of 100–400 C. Acknowledgements
Fig. 5. Temperature dependence of the conductivity for the 0.6(Gd0.4Nd0.6)2O3–0.16LiNO3–0.24KNO3 solid solution (d) with the corresponding data for 0.48(Gd0.9La0.1)2O3–0.52LiNO3 (h), Li+–b-alumina (—), Li3N (- - -), and the LISICON series (shaded area).
solid (9.55 · 10 8 S cm 1). The increase in conductivity at temperatures below 250 C is thought to be caused by the co-doping effect of KNO3 on the formation of an ion conducting passage; while LiNO3 and KNO3 are tightly held in the interstitial open spaces in the cubic rare earth oxide at low temperature, the nitrate may be in a liquid-like phase which is partially dissociated to cations and NO3 anions at elevated temperatures over the melting point of the mixed nitrate. However, since the large K+ and NO3 ions are still held in the interstitial open spaces, only the smallest ion, Li+, can migrate smoothly in the solid. The 0.6(Gd0.4Nd0.6)2O3–0.16LiNO3–0.24KNO3 solid exhibits conductivity that is comparable to that observed for Li+– b-alumina and the LISICON series over the temperature range from 100 to 400 C. 4. Conclusion A new type of Li+ ion conducting solid electrolyte of 0.6(Gd0.4Nd0.6)2O3–0.16LiNO3–0.24KNO3 was developed
This work was partially supported by the Industrial Technology Research Grant Program in ’05 (Project ID: 05A18011d) and in ’02 (Project ID: 02A27004c) from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References [1] G.C. Farrington, B.S. Dunn, J.L. Briant, Solid State Ionics 3/4 (1981) 405. [2] B.A. Boukamp, R.A. Huggins, Phys. Lett. A 58 (1976) 231. [3] R.A. Huggins, Electrochim. Acta 22 (1977) 773. [4] H.Y.-P. Hong, Mater. Res. Bull. 13 (1978) 117. [5] J. Kuwano, A.R. West, Mater. Res. Bull. 15 (1980) 1661. [6] A.R. Rodger, J. Kuwano, A.R. West, Solid State Ionics 15 (1985) 185. [7] R. Kanno, T. Hata, Y. Kawamoto, M. Irie, Solid State Ionics 130 (2000) 97. [8] R. Kanno, M. Murayama, J. Electrochem. Soc. 148 (2001) A742. [9] M. Murayama, N. Sonoyama, A. Yamada, R. Kanno, Solid State Ionics 170 (2004) 173. [10] A. Rabenau, Solid State Ionics 6 (1982) 277. [11] Y. Inaguma, C. Liquan, M. Ito, T. Nakamura, Solid State Commun. 86 (1993) 689. [12] Y. Inaguma, J. Yu, Y.J. Shan, M. Ito, T. Nakamura, J. Electrochem. Soc. 142 (1995) L8. [13] S. Tamura, A. Mori, N. Imanaka, Solid State Ionics 175 (2004) 467. [14] N. Imanaka, S. Tamura, A. Mori, Electrochemistry 71 (2003) 1039. [15] R.D. Shannon, Acta Cryst. A32 (1976) 751. [16] S.J. Schneider, R.S. Roth, J. Res. Natl. Bur. Stand., Sect. A 64 (1960) 317.
Enhancement of lithium ion conduction in the cubic rare earth oxide Shinji Tamura, Akihiro Mori, Nobuhito Imanaka
*
Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Received 23 August 2006; received in revised form 16 September 2006; accepted 19 September 2006 Available online 24 October 2006
Abstract A new type of lithium ion conducting solid electrolyte based on a cubic rare earth oxide was developed by co-doping LiNO3 and KNO3 into a (Gd1 xNdx)2O3 solid, which possesses large interstitial open spaces within the structure. Among the samples prepared, 0.6(Gd0.4Nd0.6)2O3–0.16LiNO3–0.24KNO3 exhibits the highest lithium ion conductivity of 8.05 · 10 2 and 1.35 · 10 3 S cm 1 at 400 and 100 C, respectively, which is comparable to that of the LISICON materials. Pure Li+ ion conduction was successfully demonstrated by the dc electrolysis method. 2006 Elsevier B.V. All rights reserved. Keywords: Solid electrolyte; Lithium ion; Cubic rare earth oxide; Lithium nitrate; Potassium nitrate
1. Introduction In the electronics field, Li+ ion conducting materials are expected to be used as components of rechargeable secondary batteries with high power density and chemical sensors. Therefore, the Li+ ion conduction in crystalline solids, glasses, and polymers has been extensively studied. Amongst these different structures, the crystalline solid (solid electrolyte) is one of the most suitable forms from a practical-use perspective, because there is no possibility of electrolyte leakage even at elevated temperatures. Up until now, many types of highly conducting Li+ ion solid electrolytes, such as Li+–b’’-alumina [1], Li3N [2,3], LISICON (Li+ super ionic conductor) analogues [4–9], and perovskite related materials [10–12] have been reported as high Li+ ion conducting solid electrolytes with high chemical stability in an air atmosphere. Although all of them possess a relatively large conduction pathway through the Li+ ion sites in the structure, the Li+ ion conduction is affected by the structural, chemical, and/or physical properties of the parent crystal lattice, because Li+ ions are also one of the constituent ions in the solid lattice. On the other *
Corresponding author. Tel.: +81 6 6879 7352; fax: +81 6 6879 7354. E-mail address: [email protected] (N. Imanaka).
1388-2481/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.09.016
hand, we have recently developed a new type of Li+ ion conducting solid, a LiNO3-doped cubic rare earth oxide of (Gd0.9La0.1)2O3–LiNO3 [13,14] by following quite a new concept. This new concept involves the insertion of LiNO3 into the relatively large interstitial open spaces present in the cubic (Gd0.9La0.1)2O3 lattice where Li+ ions do not function as components of the parent crystal lattice. High Li+ ion conductivity that enters the range of practical application (r >10 3 S cm 1 above 200 C) was realized by the smooth migration of Li+ ions through the large interstitial open spaces. However, the (Gd0.9La0.1)2O3–LiNO3 system has a disadvantage in that an abrupt reduction in conductivity occurs below 250 C, which is close to the melting point of LiNO3 (ca. 255 C). One effective way to improve the conductivity below 250 C is to lower the melting point of LiNO3. In this work, we selected a LiNO3–KNO3 eutectic mixture in a molar ratio of LiNO3:KNO3 = 2:3 as the nitrate to be doped into the cubic rare earth oxide, because the melting point of the 2LiNO3–3KNO3 eutectic mixture is as low as 125 C. Furthermore, it is necessary to expand the crystal lattice of the parent oxide in order to introduce large K+ cations into the interstitial open spaces. Although there are cubic rare earth oxides such as Eu2O3 and Sm2O3 whose lattice sizes are larger than that of Gd2O3, Eu3+ and Sm3+ ions are easily
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reduced in the cubic oxides. This introduces the possibility of electronic conduction due to their valence change, which is an undesirable characteristic for a solid electrolyte. Therefore, these oxides are not appropriate for use as the parent oxide. Alternatively, the cubic crystal lattice can be expanded by the partial substitution of the Gd3+ (0.1078 nm; CN = 6) [15] site in Gd2O3 with larger trivalent cations such as La3+ (0.1172 nm; CN = 6) [15]. However, for substitution with La3+, cubic crystal lattices larger than that of (Gd0.9La0.1)2O3 cannot be obtained, because this composition has a solid solution limit where the cubic phase is formed [13,14]. Therefore, the Gd3+ sites in Gd2O3 were partially substituted with Nd3+ ions (0.1123 nm; CN = 6) [15] in an attempt to obtain a large cubic crystal lattice. Furthermore, since it has been reported that only the hexagonal and monoclinic structures are formed in the Gd2O3–Nd2O3 system when prepared by solid-state reaction [16], we applied the co-precipitation method to obtain a cubic Gd2O3–Nd2O3 solid solution. In this communication, we report the development of (1 y z)(Gd1 xNdx)2O3–yLiNO3–zKNO3 solids in an attempt to realize high Li+ ion conduction in a cubic rare earth oxide even at temperatures below 250 C. The Li+ ion conduction properties of the prepared (1 y z) (Gd1 xNdx)2O3–yLiNO3–zKNO3 solids were investigated. 2. Experimental (Gd1 xNdx)2O3 solid solutions were prepared by the coprecipitation method from 1.0 mol L 1 aqueous solutions of Gd(NO3)3 and Nd(NO3)3. A stoichiometric mixture of these solutions was poured into 0.5 mol L 1 oxalic acid and the precipitate obtained was heated at 1000 C for 12 h. The (Gd1 xNdx)2O3 solid solution obtained was then mixed with LiNO3 and KNO3 in a appropriate ratio and the mixed powder was calcined at 400 C for 12 h in air. The ratio of LiNO3 and KNO3 was maintained at 2:3. After X-ray powder diffraction (XRD) analysis of the samples using a Cu Ka radiation (Rigaku, Multiflex), the powder was pelletized and sintered at 400 C for 12 h in air.
Fourier-transform infrared (FT-IR) spectra of the sample powders were measured with an FT-IR spectrometer (Bruker, TENSOR 27). Ac conductivity was measured using the sintered sample pellets with Au sputtered electrodes in the center of both surfaces. Dc electrolysis of the 0.6(Gd0.4Nd0.6)2O3–0.16LiNO3–0.24KNO3 pellet was conducted by applying a dc voltage of 3 V at 200 C for 72 h in air. 3. Results and discussion Fig. 1 displays the XRD patterns and the lattice volume variation for the (Gd1 xNdx)2O3 (0.1 6 x 6 0.7) solids with the corresponding data for pure cubic Gd2O3. The samples with x 6 0.6 were found to have a cubic phase (space group: Ia3, similar to that for pure Gd2O3, while the sample with x = 0.7 was a two phase mixture of cubic Gd2O3 and monoclinic NdGdO3. The lattice volume of the sample with single cubic phase (x 6 0.6) linearly increases in accordance with Vegard’s Law. These results indicate that (Gd1 xNdx)2O3 clearly forms a solid solution with cubic symmetry in the composition range of x 6 0.6. This is the first report of a cubic phase for the Nd2O3–Gd2O3 system, although the reason for the formation of the cubic phase, neither hexagonal nor monoclinic phase, is uncertain and requires further consideration. The XRD patterns of (1 y z)(Gd0.4Nd0.6)2O3–yLiNO3– zKNO3 are shown in Fig. 2. The LiNO3:KNO3 ratio was fixed at 2:3 on the basis that the lowest melting point was obtained at this ratio. For those samples with y + z 6 0.4, a single cubic phase was obtained. Additional XRD peaks identified as KNO3 were observed when the total nitrate content of y + z was higher than 0.4. The reason for only KNO3 appearing as the secondary phase in the samples with y + z > 0.4 is thought to be caused by the large ionic size of K+ (0.138 nm [15]) compared with that of Li+ (0.076 nm [15]). Although both lithium and potassium nitrates can be fully dissolved up to their solubility limit (y + z = 0.4), LiNO3 may preferentially penetrate into the interstitial open spaces present in the (Gd0.4Nd0.6)2O3 lattice. Therefore,
Fig. 1. (a) XRD profiles and (b) the lattice volume of the (Gd1 xNdx)2O3 (0.1 6 x 6 0.7) solids (¤) with the corresponding datum for cubic Gd2O3 (s).
S. Tamura et al. / Electrochemistry Communications 9 (2007) 245–248
Fig. 2. XRD patterns of the (1 y z)(Gd0.4Nd0.6)2O3–yLiNO3–zKNO3 (y:z = 2:3; 0.2 6 y + z 6 0.5) solids and the parent solvent phase of (Gd0.4Nd0.6)2O3. (y + z = (a) 0.2, (b) 0.3, (c) 0.4 and (d) 0.5).
KNO3 was deposited as a secondary phase when the amount of alkaline metal nitrates exceeded the solubility limit. In order to identify whether nitrate ions (NO3 ) were present in the sample, FT-IR measurements were performed for the sintered samples of (1 y z)(Gd0.4Nd0.6)2O3– yLiNO3–zKNO3 (y:z = 2:3; 0 6 y + z 6 0.5). All the samples prepared showed two bands in the IR spectra at approximately 825 and 1384 cm 1, which were not observed in (Gd0.4Nd0.6)2O3 (Fig. 3). These bands are ascribed to the out-of-plane m2 deformation mode and the m3 asymmetric stretching mode of the NO3 ion, respectively. This result indicates that NO3 ions remain in the sample. Fig. 4 shows the compositional dependence of the conductivity for the (1 y z)(Gd0.4Nd0.6)2O3–yLiNO3– zKNO3 (y:z = 2:3; 0 6 y + z 6 0.5) solids at 400 C. The conductivity increased linearly with (y + z), and the highest conductivity (8.05 · 10 2 S cm 1) was obtained for the solubility limit composition of y + z = 0.4 among the single phase samples (y + z 6 0.4). Although the conductivity also slightly increased in the two phase mixture range (y + z > 0.4), the degree of increase in the slope is quite different from that observed for the single phase region (y + z 6 0.4). The increase in conductivity for the two phase mixture region may be caused by K+ ion conduction in the nitrates that appeared as a secondary phase. In order to directly demonstrate Li+ ion conduction in the 0.6(Gd0.4Nd0.6)2O3–0.16LiNO3–0.24KNO3 solid solution, dc electrolysis was performed at 200 C by applying a dc voltage of 3 V which is higher than the decomposition voltage of ca. 1.0 V. Li-containing compounds, such as Li3AuO3 and Li5AuO4, were recognized only at the cathodic surface of the electrolyzed pellet, whereas such compounds were not at all observed at the anodic side, indicating that only Li+ ions are conducted in the sample
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Fig. 3. FT-IR spectra for the (1 y z)(Gd0.4Nd0.6)2O3–yLiNO3–zKNO3 (y:z = 2:3; 0.2 6 y + z 6 0.5) solids and the parent solvent phase of (Gd0.4Nd0.6)2O3. (y + z = (a) 0.2, (b) 0.3, (c) 0.4 and (d) 0.5).
Fig. 4. Compositional dependence of (1 y z)(Gd0.4Nd0.6)2O3–yLiNO3–zKNO3 solids at 400 C.
the conductivity for the (y:z = 2:3; 0 6 y + z 6 0.5)
(Detailed discussions of the dc electrolysis procedure is described in previous papers [13,14]). Fig. 5 presents the Li+ ion conductivity of the 0.6(Gd0.4Nd0.6)2O3–0.16LiNO3–0.24KNO3 solid solution with the corresponding data for the 0.48(Gd0.9La0.1)2 O3–0.52LiNO3 solid that we have reported previously [13,14], in addition to representative Li+ ion conducting solid electrolytes with high chemical stability in atmospheric air. Although the Li+ ion conductivity of 0.6(Gd0.4Nd0.6)2O3–0.16LiNO3–0.24KNO3 is lower than that of 0.48(Gd0.9La0.1)2O3–0.52LiNO3 at temperatures above 250 C, due to the low concentration of conducting Li+ ions, the conductivity at temperatures below 250 C is greatly improved. The Li+ ion conductivity for the 0.6(Gd0.4Nd0.6)2O3–0.16LiNO3–0.24KNO3 solid at 100 C (1.35 · 10 3 S cm 1) is more than four orders of magnitude higher than that for the 0.48(Gd0.9La0.1)2O3–0.52LiNO3
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by doping a eutectic mixture of LiNO3 and KNO3, with a melting point as low as 125 C, into the interstitial open spaces of the cubic (Gd0.4Nd0.6)2O3 lattice that has been intentionally expanded compared with that of (Gd0.9La0.1)2O3. Both the lowering of the melting point of the nitrates and expansion of the cubic lattice of the rare earth oxide promote Li+ ion conduction and the measured conductivity is comparable to that for well-known high Li+ ion conductors such as Li+–b-alumina and the LISICON series over the temperature range of 100–400 C. Acknowledgements
Fig. 5. Temperature dependence of the conductivity for the 0.6(Gd0.4Nd0.6)2O3–0.16LiNO3–0.24KNO3 solid solution (d) with the corresponding data for 0.48(Gd0.9La0.1)2O3–0.52LiNO3 (h), Li+–b-alumina (—), Li3N (- - -), and the LISICON series (shaded area).
solid (9.55 · 10 8 S cm 1). The increase in conductivity at temperatures below 250 C is thought to be caused by the co-doping effect of KNO3 on the formation of an ion conducting passage; while LiNO3 and KNO3 are tightly held in the interstitial open spaces in the cubic rare earth oxide at low temperature, the nitrate may be in a liquid-like phase which is partially dissociated to cations and NO3 anions at elevated temperatures over the melting point of the mixed nitrate. However, since the large K+ and NO3 ions are still held in the interstitial open spaces, only the smallest ion, Li+, can migrate smoothly in the solid. The 0.6(Gd0.4Nd0.6)2O3–0.16LiNO3–0.24KNO3 solid exhibits conductivity that is comparable to that observed for Li+– b-alumina and the LISICON series over the temperature range from 100 to 400 C. 4. Conclusion A new type of Li+ ion conducting solid electrolyte of 0.6(Gd0.4Nd0.6)2O3–0.16LiNO3–0.24KNO3 was developed
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