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Li-ion conductive phosphosilicate glass ceramics synthesized by ion exchange
SOSI-13117; No of Pages 4 Solid State Ionics xxx (2013) xxx–xxx
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Li-ion conductive phosphosilicate glass ceramics synthesized by ion exchange Tomoyuki Tsujimura Research Center, Asahi Glass Co., Ltd., 1150 Hazawa-cho, Kanagawa-ku, Yokohama 221–8755, Japan
a r t i c l e
i n f o
Article history: Received 17 June 2013 Received in revised form 15 October 2013 Accepted 17 October 2013 Available online xxxx Keywords: Li-ion solid electrolyte Oxide glass ceramics Ion exchange NASICON
a b s t r a c t Ion exchange of LiNO3 molten salt into Na2O–ZrO2–P2O5–SiO2 glass ceramics was successfully performed in order to obtain Li-exchanged Na+ superionic conductor (NASICON)-type glass ceramics. This procedure was carried out to stabilize the Li-ion-exchanged NASICON-type crystals in a system composed of Li2O–ZrO2–P2O5–SiO2. The total conductivity of the prepared system increased as the X value in the 20Li2O–XZrO2–15P2O5–(65-X) SiO2 system increased, reaching a maximum conductivity value of 3.0 (±2.0) × 10−4 S/cm (X = 30). The conductivity was affected by the existence of the Li3PO4 phase, which was present in the Li-exchanged glass ceramics. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Li-ion solid electrolytes have recently emerged as alternatives to conventional liquid or gel electrolytes, and have drastically improved the safety features of next-generation secondary batteries, including all-solid-state and Li–air batteries, which have the potential to outperform conventional Li-ion batteries. However, the successful use of these batteries demands suitable materials with specific functionalities, such as high conductivities, high ionic transport numbers, and high stability against electrode materials. Inorganic oxide glassy electrolytes are promising candidates for solid electrolytes because of their stability, durability in air, and ease of fabrication, relative to conventional ceramics. In particular, Li2O–B2O3–SiO2 [1] and Li2O–B2O3–P2O5 glasses [2] exhibit high ionic conductivities among oxide materials. As a quantitative example, a maximum conductivity of 10− 6.7 S/cm can be obtained for 0.45Li2O–0.27B2O3–0.27P2O5 glass at room temperature [2]. Nevertheless, the ionic conductivities of these glasses are not sufficient for use as solid electrolytes in Li-ion secondary batteries. Additionally, a large amount of alkali-borate volatilization occurs during the glass melting process. Therefore, as an alternative, glass ceramics, which traditionally exist in the crystalline Li1 + x + yAlxTi2 − xSiyP3 − yO12 form and possess a Na+ superionic conductor (NASICON)-type structure, can also be used as solid oxide electrolytes [3]. The ionic conductivity of this system reaches ~ 10−4 S/cm at room temperature, which is the highest value obtained among all glassy oxide candidates. However, one important limiting issue is the narrow voltage window, which results from the Ti reduction because of the attachment with metal Li. Therefore, practical
E-mail address: [email protected].
applications require the development of a new glass or glass-type ceramic material that does not contain transition-metal cations. Li-ion conductive glass, or glass ceramics without transition metal cations, has not yet been reported, but there have been some literature reports on Na-ion conductive glasses or glass ceramics. For example, Susman et al. [4] have reported that Na-ion conducting phosphosilicate glasses, e.g., Na1 + xZr2 − x/3P3 − xSixO12 − x/3 (mol%), exhibit ionic conductivities as high as (1–2) × 10−3 S/cm at 300 °C. The conductivity of the system further increases by 1–2 orders of magnitude upon crystallization [5]. Herein, the NASICON skeleton facilitates fast diffusion pathways for other monovalent ions that possess ionic radii close to that of Na+. Furthermore, ion-exchange technology has the potential to produce new fast ion-conductive materials without changing the framework structure. For example, Yao and Kummer [6] have reported that the Na ions in a fast two-dimensional Na+-ion conductor, Na βalumina, have been exchanged in molten salts with K+, Ag+, Rb+, and Li+ ions. Additionally, Farrington and Dunn [7] have reported that the Na+ content of β″-alumina can be replaced by various divalent and trivalent cations in ion-exchange reactions. For Li-ion exchange in both βalumina [6] and the NASICON phase of Li3Zr2Si2PO12 [8], however, the ionic conductivities of the Li-ion-exchanged phases are much lower than those of the corresponding compounds. This result could be attributed to the smaller size of the Li+ ion. It has been suggested that the high charge-to-radius value for the small Li+ ions results in stronger bonding to the oxygen atoms of the framework than the original ions, while the Na+ ions are located near the center of the cavities and are thus more weakly bonded [8]. The present study has been motivated by the hypothesis that the positions of the Li sites in the Li crystals are more stable than those in the Li glass or glass ceramics with random network structures, suggesting that alkali-metal ions may be more
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weakly bonded to oxygen in glass or glass ceramics than they are in crystals. In our previous investigation [9], Li2O–ZrO2–P2O5–SiO2 glasses were synthesized by Li+/Na+ ion-exchange on Na-ion conducting Na2O– ZrO2–P2O5–SiO2 glasses in order to produce high Li-ion conductive glass materials. The maximum value of the ionic conductivity for the Na-type phosphosilicate glasses was found to be 6.2 × 10−7 S/cm, while the conductivity of the Li-ion-exchanged glass was observed to be 5.5 × 10− 5 S/cm at room temperature, suggesting that the ionexchange technique is suitable for producing new ionic conductive materials. The direct synthesis with batch melting method was also carried out for the Li2O–ZrO2–P2O5–SiO2 system, but it was difficult to vitrify the batch perfectly due to remaining of undissolved raw materials of SiO2 and ZrO2, and formation of Li3PO4 crystals. The conductivity of the obtained material was below 10−7S/cm because these crystals have low conductivity. This result indicates that the ion exchange is a useful technology to stabilize amorphous state in Na-system. Herein, the material obtained by Li+/Na+ ion-exchange on ionic conductivities of glass ceramics with Na-ion conductive NASICON, which are crystallized by heat treatment of Na-glass, are expected to have higher conductivity. The glass ceramics with NASICON crystals can be obtained with heat treatment of Na2O–ZrO2–P2O5–SiO2 glass, and the conductivity is increasing with X value in 20Na2O–XZrO2–15P2O5–(65-X)SiO2 system [5]. Using the same compositional glass batch, the effect of ion exchange on conductivity has been investigated in this study.
2. Experimental procedure The Li2O–ZrO2–P2O5–SiO2 material system was selected as a candidate for the development of materials with high ionic conductivities. NASICON-type glass ceramics were prepared through a conventional melting method. In the typical process, sodium phosphosilicate glass samples were prepared from 50-g batches containing appropriate amounts of precursors; particularly dried SiO2, ZrO2, (NH4)3PO4, and Na2CO3, accurately weighed and mixed in air, which were then used to provide the target compositional glass. These glass batches were placed in a platinum crucible and melted at 1650 °C for 1 h in air, and subsequently formed a 20Na2O–XZrO2–15P2O5–(65-X)SiO2 (12.5 ≤ X ≤ 35.0) system. The resulting melt was then annealed and slowly cooled to room temperature in order to remove the strain in the glass. The glass was heated at 800 °C and 1000 °C, which were temperatures referred to by Morimoto [5], in order to crystallize NASICON phase of the system. The glass ceramic was then polished to a thickness of 0.6 mm in order to obtain complete ion-exchange of the Na+ ions for Li+ ions in the Na2O–ZrO2–P2O5–SiO2 glass ceramic. The ion-exchange reaction was carried out as follows. First, the LiNO3 molten salt was maintained at 400 °C for at least 72 h. After cooling, the nitrate was dissolved in distilled water. The ion-exchanged materials were then powdered and dried at 105 °C to obtain precise measurements of the chemical compositions. After decomposition in acid, the Na and Li contents were determined using flame photometry. The P, Si, and Zr contents were subsequently determined using inductively coupled plasma after dissolution in alkaline solutions. X-ray diffraction (XRD) patterns of all the synthesized materials were determined before and after ion exchange. For the XRD measurements, 1 g of the samples was ground in ethanol, and the XRD measurements were then recorded using a RINT-TTRШ system (Rigaku) with Cu-Kα radiation at 50 kV and 300 mA, scanning in the 2θ range between 5° and 60°, with a step size of 0.02°. Furthermore, in order to determine useful structural information, such as the lattice parameters and unit cell volume of the NASICON-type crystal in the glass ceramic, precise XRD measurements were performed using synchrotron radiation with a wavelength of 0.07 nm at beamline 19B2 at SPring-8 (Japan).
The samples were placed in vacuum desiccators for over 24 h prior to conductivity measurements in order to avoid the moisture absorption, and the conductivity measurements were conducted under an Ar gas flow (10 mL/min). The total ionic conductivities of the glass ceramics before and after ion exchange were measured using a Solartron 1260 frequency response with a Solartron 1287 electrochemical interface in the frequency range of 10 MHz to 10 Hz, and an AC voltage of 100 mV was applied. Z plot software was employed for data analysis, and Al electrodes were attached to both surfaces of the glass ceramics through vacuum evaporation. Micro-Raman spectroscopic measurements at Institute for Study of the Earth's Interior, Okayama Univ. were applied in order to detect water concentration, because the phosphosilicate crystals and glasses can easily absorb water during the glass synthesis and ion exchange processes. 3. Results and discussion 3.1. Characterization of Na2O–ZrO2–P2O5–SiO2 glass ceramics The analyzed composition of the Na2O–ZrO2–P2O5–SiO2 glass ceramics is shown in Table 1. The amounts of Na2O and P2O5 components decreased by up to 2 mol.%, and those of SiO2 and ZrO2 components slightly increased when compared to the target composition. This result can be attributed to the volatilization of both Na2O and P2O5 components during glass melting process. The ionic conductivity values of the glasses were found to be ~10−9 S/cm, or below the detection limit. All samples became white upon further heating. The corresponding XRD patterns confirm that the prominent peaks could be attributed to the NASICON phase. For all samples, the Cole–Cole plot displays only one semicircle, since the capacitances of the bulk and the grain boundary are of the same order of magnitude for these materials. In this paper, the ionic conductivity values were calculated using the equation σ = 1/ ρ = d/A∙R, where σ is conductivity, ρ is resistivity, d is the sample thickness, A is the area of the electrodes, and R is the resistance of the sample. As shown in Fig. 1, the ionic conductivities of the glass ceramics are higher than that of the glass. This result could be attributed to the crystallization of NASICON. Additionally, the total conductivity was found as the X value increases in the 20Na2O–XZrO2–15P2O5–(65-X)SiO2 system. The lattice parameters a and c, and the unit cell volume increased with as the X value increased for this system, which likely increased the bottleneck size in the ionic pathway. For the ceramic materials, the bulk ionic conductivity also increased from 6 × 10−5 S/cm (Na3Zr1.583Si0.333 P2.667O12) to 6 × 10−4 S/cm (Na3Zr2Si2PO12) [10]. This result is due to the increase in the unit cell volume, which is consistent with the results observed in the present study. 3.2. Characterization of Li2O–ZrO2–P2O5–SiO2 glass ceramics The analyzed glass composition of the Li2O–ZrO2–P2O5–SiO2 sample is summarized in Table 1. Over 95% Na2O was exchanged into Li2O, and the major components of SiO2, ZrO2, and P2O5 did not change during ion exchange. The Raman spectroscopy shows there is no band near 3500 cm−1, which can be attributed to vibrations of the H2O molecules, for both Na2O–ZrO2–P2O5–SiO2 and Li2O–ZrO2–P2O5–SiO2 glass Table 1 Summary of analyzed compositions. Sample #
SiO2
P2O5
ZrO2
Na2O
Li2O
SPZN007 (X = 20) SPZN008 (X = 23) SPZN009 (X = 25) SPZN024 (X = 30) SPZL007 (X = 20) SPZL008 (X = 23) SPZL009 (X = 25) SPZL024 (X = 30)
46 42 40 36 45 41 41 35
14 14 15 14 14 14 14 15
20 24 25 31 20 25 25 30
20 20 20 19 1 1 1 0
– – – – 19 19 19 20
Please cite this article as: T. Tsujimura, Solid State Ionics (2013), http://dx.doi.org/10.1016/j.ssi.2013.10.034
T. Tsujimura / Solid State Ionics xxx (2013) xxx–xxx
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X value in 20Na2O–XZrO2–15P2O5–(65-X)SiO2
log σ (S/cm)
c b a
Fig. 1. Relationship between the ionic conductivity (log σ (S/cm)) and compositions of 20Na2O–XZrO2–15P2O5–(65-X)SiO2 glass (open circle) and glass ceramics (full circle).
ceramics. The limitation of water is approximately 0.1 wt.% H2O with micro-Raman spectroscopy (Kanzaki, unpublished data), which is considerably lower than alkali component in the glass ceramics. These results confirm that the Li2O–ZrO2–P2O5–SiO2 glass ceramics was successfully obtained by the ion exchange. For all the Li-exchanged samples, the Cole–Cole plots exhibit only one semicircle, which can be attributed to the total of the bulk and the grain-boundary resistance. The conductivities of the Li-glass ceramics prepared by ion exchange (Fig. 2) are higher than those of Na-glass ceramics (Fig. 1) by approximately one order of magnitude. The XRD patterns revealed the presence of the NASICON phase in the Li-glass ceramics, indicating that Li-exchanged NASICON-type glass ceramics were successfully synthesized by ion exchange. Furthermore, the diffraction peak positions corresponding to NASICON crystals shifted to a higher angle, indicating a reduction in the unit cell volume during ion exchange (Fig. 3). Additionally, the diffraction peaks corresponding to the Li3PO4 phase were also observed for compositions in the region with lower X values for the 20Li2O–XZrO2–15P2O5–(65-X)SiO2 system (Fig. 4). These peaks could be observed only by XRD using synchrotron radiation, which suggests that the amount of the Li3PO4 phase may be below 10%. The relative peak intensity for Li3PO4, compared with that of NASICON, becomes lower in the systems with higher X values. The lattice parameters of the a and c axes and the unit cell volume decreased as a result of ion exchange, which can be attributed to the sites changing from Na-ion sites to Li-ion sites. Despite the positive correlation between the lattice parameters and composition for Na-glass ceramics, there is little correlation for Li-glass ceramics. This result indicates that the Li-ion conductive mechanism cannot be explained only by the crystalline structure of the NASICON phase. The decrease in ionic conductivity with a decrease in X value may be related to the formation of the Li3PO4 phase. The relative peak intensity of the Li3PO4 phase to that of the NASICON phase systematically decreased as the X value increased for the 20Li2O–XZrO2–15P2O5–(65-X)SiO2 system. When the
2θ (deg.) Fig. 3. Synchrotron X-ray diffraction patterns for (a) Na-glass (b) Na-glass ceramics and (c) Li-exchanged glass ceramics. The composition was 20R2O–20ZrO2–15P2O5–45SiO2 (R = Na, Li). The peaks (full circle) are attributed to the NASICON-type crystal.
X value is low, the high resistance of Li3PO4 causes low conductivity. The conductivity for the Li-exchanged glass ceramics is affected by the amount of the Li3PO4 phase, which forms during the ion-exchange process. 4. Conclusion Li-ion conductive materials without transition metals, which are potential electrolytes for secondary Li-ion batteries, such as all-solidstate and Li-air batteries, have been successfully developed. Based on systematic analyses, the following conclusions were drawn: (1) In order to stabilize the Li-NASICON phase in the system with a composition of Li2O–ZrO2–P2O5–SiO2, ion exchange in the LiNO3 molten salt of NASICON-type glass ceramics was performed. Li-exchanged glass ceramics can be successfully obtained using this technique. The conductivity becomes higher as the X value increases in the 20Li2O–XZrO2–15P2O5–(65-X)SiO2 system, with the maximum value of the ionic conductivity being 3.0 (±2.0) × 10−4 S/cm (X = 30). (2) The lattice parameters a and c, and the unit cell volume of the Li-exchanged NASICON phase in the Li-type NASICON glass ceramics were found to be smaller than those for the NASICON phase in the Na-type glass ceramics. This structural change can be attributed to the successful ion exchange from Na ions to Li ions. However, these parameters and the unit cell volume of the NASICON phase are independent of composition of the glass ceramics. The relationship between structure and conductivity for the NASICON phase in Li-exchanged glass ceramics is different from that of Na-type glass ceramics,
X value in 20Li2O–XZrO2–15P2O5–(65-X)SiO2
log σ (S/cm)
ZrO2 30.0 25.0 22.5 20.0 2θ (deg.)
Fig. 2. Relationship between the ionic conductivity (log σ (S/cm)) and compositions of glass ceramics for 20Li2O–XZrO2–15P2O5–(65-X)SiO2.
Fig. 4. Synchrotron X-ray diffraction patterns for Li-exchanged glass ceramics as a function of composition. The peak indicated by the full circle is attributed to NASICON-type crystal, and the full triangle represents the Li3PO4 phase.
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indicating that the alkali-ion conductive mechanism for the Li-exchanged glass ceramics is different from that of the Na-type glass ceramics. The Li-ion conductivity was affected by the existence of Li3PO4 phase, which was present in the Li-exchanged glass ceramics.
Acknowledgments The synchrotron radiation experiments were performed at BL19B2 with the approval of the SPring8 (proposal No. 2013A1202). The author is grateful to Dr. T. Itoh (AGC Seimi Chemical Co. Ltd.) for extending technical support for the synchrotron experiment and discussions, and
Prof. M. Kanzaki (Institute for Study of the Earth's Interior, Okayama Univ.) for permission to use the micro-Raman apparatus. References [1] C.E. Kim, H.C. Hwang, M.Y. Yoon, B.H. Choi, H.J. Hwang, J. Non-Cryst. Solids 357 (2011) 2863–2867. [2] S.H. Lee, K.I. Cho, J.B. Choi, D.W. Shin, J. Power Sources 162 (2006) 1341–1345. [3] J. Fu, Solid State Ionics 104 (1997) 191–194. [4] S. Susman, C.J. Delbecq, J.A. McMillan, M.F. Roche, Solid State Ionics 9–10 (1983) 667–674. [5] S. Morimoto, J. Ceram. Soc. Jpn. 97 (1989) 1097–1103. [6] Y.F. Yao, J.T. Kummer, J. Inorg. Nucl. Chem. 29 (1967) 2453–2475. [7] G.C. Farrington, B. Dunn, Solid State Ionics 7 (1982) 267–281. [8] H.Y.-P. Hong, Mater. Res. Bull. 13 (1978) 117–124. [9] T. Tsujimura, A. Kuroki, Y. Kuroki, ECS Trans. 45 (2012) 135–141. [10] O. Bohnke, S. Ronchetti, D. Mazza, Solid State Ionics 122 (1999) 127–136.
Please cite this article as: T. Tsujimura, Solid State Ionics (2013), http://dx.doi.org/10.1016/j.ssi.2013.10.034
Contents lists available at ScienceDirect
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Li-ion conductive phosphosilicate glass ceramics synthesized by ion exchange Tomoyuki Tsujimura Research Center, Asahi Glass Co., Ltd., 1150 Hazawa-cho, Kanagawa-ku, Yokohama 221–8755, Japan
a r t i c l e
i n f o
Article history: Received 17 June 2013 Received in revised form 15 October 2013 Accepted 17 October 2013 Available online xxxx Keywords: Li-ion solid electrolyte Oxide glass ceramics Ion exchange NASICON
a b s t r a c t Ion exchange of LiNO3 molten salt into Na2O–ZrO2–P2O5–SiO2 glass ceramics was successfully performed in order to obtain Li-exchanged Na+ superionic conductor (NASICON)-type glass ceramics. This procedure was carried out to stabilize the Li-ion-exchanged NASICON-type crystals in a system composed of Li2O–ZrO2–P2O5–SiO2. The total conductivity of the prepared system increased as the X value in the 20Li2O–XZrO2–15P2O5–(65-X) SiO2 system increased, reaching a maximum conductivity value of 3.0 (±2.0) × 10−4 S/cm (X = 30). The conductivity was affected by the existence of the Li3PO4 phase, which was present in the Li-exchanged glass ceramics. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Li-ion solid electrolytes have recently emerged as alternatives to conventional liquid or gel electrolytes, and have drastically improved the safety features of next-generation secondary batteries, including all-solid-state and Li–air batteries, which have the potential to outperform conventional Li-ion batteries. However, the successful use of these batteries demands suitable materials with specific functionalities, such as high conductivities, high ionic transport numbers, and high stability against electrode materials. Inorganic oxide glassy electrolytes are promising candidates for solid electrolytes because of their stability, durability in air, and ease of fabrication, relative to conventional ceramics. In particular, Li2O–B2O3–SiO2 [1] and Li2O–B2O3–P2O5 glasses [2] exhibit high ionic conductivities among oxide materials. As a quantitative example, a maximum conductivity of 10− 6.7 S/cm can be obtained for 0.45Li2O–0.27B2O3–0.27P2O5 glass at room temperature [2]. Nevertheless, the ionic conductivities of these glasses are not sufficient for use as solid electrolytes in Li-ion secondary batteries. Additionally, a large amount of alkali-borate volatilization occurs during the glass melting process. Therefore, as an alternative, glass ceramics, which traditionally exist in the crystalline Li1 + x + yAlxTi2 − xSiyP3 − yO12 form and possess a Na+ superionic conductor (NASICON)-type structure, can also be used as solid oxide electrolytes [3]. The ionic conductivity of this system reaches ~ 10−4 S/cm at room temperature, which is the highest value obtained among all glassy oxide candidates. However, one important limiting issue is the narrow voltage window, which results from the Ti reduction because of the attachment with metal Li. Therefore, practical
E-mail address: [email protected].
applications require the development of a new glass or glass-type ceramic material that does not contain transition-metal cations. Li-ion conductive glass, or glass ceramics without transition metal cations, has not yet been reported, but there have been some literature reports on Na-ion conductive glasses or glass ceramics. For example, Susman et al. [4] have reported that Na-ion conducting phosphosilicate glasses, e.g., Na1 + xZr2 − x/3P3 − xSixO12 − x/3 (mol%), exhibit ionic conductivities as high as (1–2) × 10−3 S/cm at 300 °C. The conductivity of the system further increases by 1–2 orders of magnitude upon crystallization [5]. Herein, the NASICON skeleton facilitates fast diffusion pathways for other monovalent ions that possess ionic radii close to that of Na+. Furthermore, ion-exchange technology has the potential to produce new fast ion-conductive materials without changing the framework structure. For example, Yao and Kummer [6] have reported that the Na ions in a fast two-dimensional Na+-ion conductor, Na βalumina, have been exchanged in molten salts with K+, Ag+, Rb+, and Li+ ions. Additionally, Farrington and Dunn [7] have reported that the Na+ content of β″-alumina can be replaced by various divalent and trivalent cations in ion-exchange reactions. For Li-ion exchange in both βalumina [6] and the NASICON phase of Li3Zr2Si2PO12 [8], however, the ionic conductivities of the Li-ion-exchanged phases are much lower than those of the corresponding compounds. This result could be attributed to the smaller size of the Li+ ion. It has been suggested that the high charge-to-radius value for the small Li+ ions results in stronger bonding to the oxygen atoms of the framework than the original ions, while the Na+ ions are located near the center of the cavities and are thus more weakly bonded [8]. The present study has been motivated by the hypothesis that the positions of the Li sites in the Li crystals are more stable than those in the Li glass or glass ceramics with random network structures, suggesting that alkali-metal ions may be more
0167-2738/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ssi.2013.10.034
Please cite this article as: T. Tsujimura, Solid State Ionics (2013), http://dx.doi.org/10.1016/j.ssi.2013.10.034
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weakly bonded to oxygen in glass or glass ceramics than they are in crystals. In our previous investigation [9], Li2O–ZrO2–P2O5–SiO2 glasses were synthesized by Li+/Na+ ion-exchange on Na-ion conducting Na2O– ZrO2–P2O5–SiO2 glasses in order to produce high Li-ion conductive glass materials. The maximum value of the ionic conductivity for the Na-type phosphosilicate glasses was found to be 6.2 × 10−7 S/cm, while the conductivity of the Li-ion-exchanged glass was observed to be 5.5 × 10− 5 S/cm at room temperature, suggesting that the ionexchange technique is suitable for producing new ionic conductive materials. The direct synthesis with batch melting method was also carried out for the Li2O–ZrO2–P2O5–SiO2 system, but it was difficult to vitrify the batch perfectly due to remaining of undissolved raw materials of SiO2 and ZrO2, and formation of Li3PO4 crystals. The conductivity of the obtained material was below 10−7S/cm because these crystals have low conductivity. This result indicates that the ion exchange is a useful technology to stabilize amorphous state in Na-system. Herein, the material obtained by Li+/Na+ ion-exchange on ionic conductivities of glass ceramics with Na-ion conductive NASICON, which are crystallized by heat treatment of Na-glass, are expected to have higher conductivity. The glass ceramics with NASICON crystals can be obtained with heat treatment of Na2O–ZrO2–P2O5–SiO2 glass, and the conductivity is increasing with X value in 20Na2O–XZrO2–15P2O5–(65-X)SiO2 system [5]. Using the same compositional glass batch, the effect of ion exchange on conductivity has been investigated in this study.
2. Experimental procedure The Li2O–ZrO2–P2O5–SiO2 material system was selected as a candidate for the development of materials with high ionic conductivities. NASICON-type glass ceramics were prepared through a conventional melting method. In the typical process, sodium phosphosilicate glass samples were prepared from 50-g batches containing appropriate amounts of precursors; particularly dried SiO2, ZrO2, (NH4)3PO4, and Na2CO3, accurately weighed and mixed in air, which were then used to provide the target compositional glass. These glass batches were placed in a platinum crucible and melted at 1650 °C for 1 h in air, and subsequently formed a 20Na2O–XZrO2–15P2O5–(65-X)SiO2 (12.5 ≤ X ≤ 35.0) system. The resulting melt was then annealed and slowly cooled to room temperature in order to remove the strain in the glass. The glass was heated at 800 °C and 1000 °C, which were temperatures referred to by Morimoto [5], in order to crystallize NASICON phase of the system. The glass ceramic was then polished to a thickness of 0.6 mm in order to obtain complete ion-exchange of the Na+ ions for Li+ ions in the Na2O–ZrO2–P2O5–SiO2 glass ceramic. The ion-exchange reaction was carried out as follows. First, the LiNO3 molten salt was maintained at 400 °C for at least 72 h. After cooling, the nitrate was dissolved in distilled water. The ion-exchanged materials were then powdered and dried at 105 °C to obtain precise measurements of the chemical compositions. After decomposition in acid, the Na and Li contents were determined using flame photometry. The P, Si, and Zr contents were subsequently determined using inductively coupled plasma after dissolution in alkaline solutions. X-ray diffraction (XRD) patterns of all the synthesized materials were determined before and after ion exchange. For the XRD measurements, 1 g of the samples was ground in ethanol, and the XRD measurements were then recorded using a RINT-TTRШ system (Rigaku) with Cu-Kα radiation at 50 kV and 300 mA, scanning in the 2θ range between 5° and 60°, with a step size of 0.02°. Furthermore, in order to determine useful structural information, such as the lattice parameters and unit cell volume of the NASICON-type crystal in the glass ceramic, precise XRD measurements were performed using synchrotron radiation with a wavelength of 0.07 nm at beamline 19B2 at SPring-8 (Japan).
The samples were placed in vacuum desiccators for over 24 h prior to conductivity measurements in order to avoid the moisture absorption, and the conductivity measurements were conducted under an Ar gas flow (10 mL/min). The total ionic conductivities of the glass ceramics before and after ion exchange were measured using a Solartron 1260 frequency response with a Solartron 1287 electrochemical interface in the frequency range of 10 MHz to 10 Hz, and an AC voltage of 100 mV was applied. Z plot software was employed for data analysis, and Al electrodes were attached to both surfaces of the glass ceramics through vacuum evaporation. Micro-Raman spectroscopic measurements at Institute for Study of the Earth's Interior, Okayama Univ. were applied in order to detect water concentration, because the phosphosilicate crystals and glasses can easily absorb water during the glass synthesis and ion exchange processes. 3. Results and discussion 3.1. Characterization of Na2O–ZrO2–P2O5–SiO2 glass ceramics The analyzed composition of the Na2O–ZrO2–P2O5–SiO2 glass ceramics is shown in Table 1. The amounts of Na2O and P2O5 components decreased by up to 2 mol.%, and those of SiO2 and ZrO2 components slightly increased when compared to the target composition. This result can be attributed to the volatilization of both Na2O and P2O5 components during glass melting process. The ionic conductivity values of the glasses were found to be ~10−9 S/cm, or below the detection limit. All samples became white upon further heating. The corresponding XRD patterns confirm that the prominent peaks could be attributed to the NASICON phase. For all samples, the Cole–Cole plot displays only one semicircle, since the capacitances of the bulk and the grain boundary are of the same order of magnitude for these materials. In this paper, the ionic conductivity values were calculated using the equation σ = 1/ ρ = d/A∙R, where σ is conductivity, ρ is resistivity, d is the sample thickness, A is the area of the electrodes, and R is the resistance of the sample. As shown in Fig. 1, the ionic conductivities of the glass ceramics are higher than that of the glass. This result could be attributed to the crystallization of NASICON. Additionally, the total conductivity was found as the X value increases in the 20Na2O–XZrO2–15P2O5–(65-X)SiO2 system. The lattice parameters a and c, and the unit cell volume increased with as the X value increased for this system, which likely increased the bottleneck size in the ionic pathway. For the ceramic materials, the bulk ionic conductivity also increased from 6 × 10−5 S/cm (Na3Zr1.583Si0.333 P2.667O12) to 6 × 10−4 S/cm (Na3Zr2Si2PO12) [10]. This result is due to the increase in the unit cell volume, which is consistent with the results observed in the present study. 3.2. Characterization of Li2O–ZrO2–P2O5–SiO2 glass ceramics The analyzed glass composition of the Li2O–ZrO2–P2O5–SiO2 sample is summarized in Table 1. Over 95% Na2O was exchanged into Li2O, and the major components of SiO2, ZrO2, and P2O5 did not change during ion exchange. The Raman spectroscopy shows there is no band near 3500 cm−1, which can be attributed to vibrations of the H2O molecules, for both Na2O–ZrO2–P2O5–SiO2 and Li2O–ZrO2–P2O5–SiO2 glass Table 1 Summary of analyzed compositions. Sample #
SiO2
P2O5
ZrO2
Na2O
Li2O
SPZN007 (X = 20) SPZN008 (X = 23) SPZN009 (X = 25) SPZN024 (X = 30) SPZL007 (X = 20) SPZL008 (X = 23) SPZL009 (X = 25) SPZL024 (X = 30)
46 42 40 36 45 41 41 35
14 14 15 14 14 14 14 15
20 24 25 31 20 25 25 30
20 20 20 19 1 1 1 0
– – – – 19 19 19 20
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X value in 20Na2O–XZrO2–15P2O5–(65-X)SiO2
log σ (S/cm)
c b a
Fig. 1. Relationship between the ionic conductivity (log σ (S/cm)) and compositions of 20Na2O–XZrO2–15P2O5–(65-X)SiO2 glass (open circle) and glass ceramics (full circle).
ceramics. The limitation of water is approximately 0.1 wt.% H2O with micro-Raman spectroscopy (Kanzaki, unpublished data), which is considerably lower than alkali component in the glass ceramics. These results confirm that the Li2O–ZrO2–P2O5–SiO2 glass ceramics was successfully obtained by the ion exchange. For all the Li-exchanged samples, the Cole–Cole plots exhibit only one semicircle, which can be attributed to the total of the bulk and the grain-boundary resistance. The conductivities of the Li-glass ceramics prepared by ion exchange (Fig. 2) are higher than those of Na-glass ceramics (Fig. 1) by approximately one order of magnitude. The XRD patterns revealed the presence of the NASICON phase in the Li-glass ceramics, indicating that Li-exchanged NASICON-type glass ceramics were successfully synthesized by ion exchange. Furthermore, the diffraction peak positions corresponding to NASICON crystals shifted to a higher angle, indicating a reduction in the unit cell volume during ion exchange (Fig. 3). Additionally, the diffraction peaks corresponding to the Li3PO4 phase were also observed for compositions in the region with lower X values for the 20Li2O–XZrO2–15P2O5–(65-X)SiO2 system (Fig. 4). These peaks could be observed only by XRD using synchrotron radiation, which suggests that the amount of the Li3PO4 phase may be below 10%. The relative peak intensity for Li3PO4, compared with that of NASICON, becomes lower in the systems with higher X values. The lattice parameters of the a and c axes and the unit cell volume decreased as a result of ion exchange, which can be attributed to the sites changing from Na-ion sites to Li-ion sites. Despite the positive correlation between the lattice parameters and composition for Na-glass ceramics, there is little correlation for Li-glass ceramics. This result indicates that the Li-ion conductive mechanism cannot be explained only by the crystalline structure of the NASICON phase. The decrease in ionic conductivity with a decrease in X value may be related to the formation of the Li3PO4 phase. The relative peak intensity of the Li3PO4 phase to that of the NASICON phase systematically decreased as the X value increased for the 20Li2O–XZrO2–15P2O5–(65-X)SiO2 system. When the
2θ (deg.) Fig. 3. Synchrotron X-ray diffraction patterns for (a) Na-glass (b) Na-glass ceramics and (c) Li-exchanged glass ceramics. The composition was 20R2O–20ZrO2–15P2O5–45SiO2 (R = Na, Li). The peaks (full circle) are attributed to the NASICON-type crystal.
X value is low, the high resistance of Li3PO4 causes low conductivity. The conductivity for the Li-exchanged glass ceramics is affected by the amount of the Li3PO4 phase, which forms during the ion-exchange process. 4. Conclusion Li-ion conductive materials without transition metals, which are potential electrolytes for secondary Li-ion batteries, such as all-solidstate and Li-air batteries, have been successfully developed. Based on systematic analyses, the following conclusions were drawn: (1) In order to stabilize the Li-NASICON phase in the system with a composition of Li2O–ZrO2–P2O5–SiO2, ion exchange in the LiNO3 molten salt of NASICON-type glass ceramics was performed. Li-exchanged glass ceramics can be successfully obtained using this technique. The conductivity becomes higher as the X value increases in the 20Li2O–XZrO2–15P2O5–(65-X)SiO2 system, with the maximum value of the ionic conductivity being 3.0 (±2.0) × 10−4 S/cm (X = 30). (2) The lattice parameters a and c, and the unit cell volume of the Li-exchanged NASICON phase in the Li-type NASICON glass ceramics were found to be smaller than those for the NASICON phase in the Na-type glass ceramics. This structural change can be attributed to the successful ion exchange from Na ions to Li ions. However, these parameters and the unit cell volume of the NASICON phase are independent of composition of the glass ceramics. The relationship between structure and conductivity for the NASICON phase in Li-exchanged glass ceramics is different from that of Na-type glass ceramics,
X value in 20Li2O–XZrO2–15P2O5–(65-X)SiO2
log σ (S/cm)
ZrO2 30.0 25.0 22.5 20.0 2θ (deg.)
Fig. 2. Relationship between the ionic conductivity (log σ (S/cm)) and compositions of glass ceramics for 20Li2O–XZrO2–15P2O5–(65-X)SiO2.
Fig. 4. Synchrotron X-ray diffraction patterns for Li-exchanged glass ceramics as a function of composition. The peak indicated by the full circle is attributed to NASICON-type crystal, and the full triangle represents the Li3PO4 phase.
Please cite this article as: T. Tsujimura, Solid State Ionics (2013), http://dx.doi.org/10.1016/j.ssi.2013.10.034
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indicating that the alkali-ion conductive mechanism for the Li-exchanged glass ceramics is different from that of the Na-type glass ceramics. The Li-ion conductivity was affected by the existence of Li3PO4 phase, which was present in the Li-exchanged glass ceramics.
Acknowledgments The synchrotron radiation experiments were performed at BL19B2 with the approval of the SPring8 (proposal No. 2013A1202). The author is grateful to Dr. T. Itoh (AGC Seimi Chemical Co. Ltd.) for extending technical support for the synchrotron experiment and discussions, and
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Please cite this article as: T. Tsujimura, Solid State Ionics (2013), http://dx.doi.org/10.1016/j.ssi.2013.10.034