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Influence of lithium oxide additives on densification and ionic conductivity of garnet-type Li6.75La3Zr1.75Ta0.25O12 solid electrolytes
Solid State Ionics 253 (2013) 76–80
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Influence of lithium oxide additives on densification and ionic conductivity of garnet-type Li6.75La3Zr1.75Ta0.25O12 solid electrolytes Yiqiu Li, Yang Cao, Xiangxin Guo ⁎ State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding Xi Road, Shanghai 200050, China
a r t i c l e
i n f o
Article history: Received 8 March 2013 Received in revised form 2 September 2013 Accepted 2 September 2013 Available online 26 September 2013 Keywords: Lithium garnet electrolytes Densification Ionic conductivity Lithium oxide additives
a b s t r a c t The lithium garnet ceramic has been considered as one of promising solid-state electrolytes for secondary lithium batteries. However, improvement of its ionic conductivity is hindered by the high porosity related to the severe volatilization of lithium components during sintering. In order to find the solution for this problem, Li2O additives in concentration of 2–8 wt.% were introduced into the cubic Li6.75La3Zr1.75Ta0.25O12 (LLZTO) at the solid-state reaction processes. It is found that the Li2O additives lead to formation of glassy-like phases at the grain boundaries related to the liquid-phase sintering behavior, which eliminates the residual pores therein and increases the relative density from 91.5% to 97.3%. Measurement of conductivity indicates that 6 wt.% is the optimum concentration of Li2O which leads to an ionic conductivity of 6.4 × 10−4 S cm−1 at room temperature. This value is approximately 3 times larger than that of the ceramic without the additive. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Solid-state electrolytes are essential for all-solid-state lithium batteries, which are superior to the presently used organic liquid electrolytes in terms of safety issues such as dendrite formation, flammability and leakage problem [1,2]. So far, many types of solid-state electrolytes have been explored, including perovskite titanates [3–5], NASICON phosphates [6–8], LISICON sulfides [9,10] and garnets [11–14]. Among these materials, Li7La3Zr2O12 (LLZO) with a cubic garnet structure exhibited an ionic conductivity of approximately 2 × 10−4 S cm−1 at room temperature (RT), good thermal and chemical stability against prospective electrodes [13]. It was indicated that introduction of Al, Ga, Nb or Ta was necessary to stabilize such a cubic structure [15–21]. Otherwise, formation of a tetragonal structure might occur and cause a much lower ionic conductivity [22,23]. Besides the cubic structure, the relative density of the LLZO ceramic is also a critical issue. Because of the temperature limitation imposed by the volatilization of Li components during sintering, the dense LLZO ceramic is difficult to obtain via the conventional solid-state reaction. The solid electrolyte with a high porosity may cause a poor ionic conductivity [24] as well as a mechanical failure [25]. The hot-press sintering technique was applied for preparation of the LLZO ceramics, which achieved a relative density of 96–98% [16,26,27]. It is obvious that such technique needs dedicated furnaces. In contrast, the convention solid-state reactions with the liquid-phase sintering could also be a useful way to improve the ceramic density, since the liquid phases formed in between the grains could expel the residual pores [8,28–30]. However, using ⁎ Corresponding author. Tel.: +86 21 52411032; fax: +86 21 52411802. E-mail address: [email protected] (X. Guo). 0167-2738/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ssi.2013.09.005
this technique for preparing high-density LLZO ceramics has not ever been reported up to date. Li2O as the additive proved to be a good binder, which could lead to the liquid phase sintering behavior in preparation of NASICON phosphates [29,30]. It could also act as a flux to obtain a highly conductive layer at the grain boundaries [29]. In this work, we introduced Li2O as the additives into the Li6.75La3Zr1.75Ta0.25O12 (LLZTO) ceramics during the conventional solid-state reaction preparation. Through concentration variation of the Li2O additive, changes of crystallography, relative density and ionic conductivity of the ceramic electrolytes were investigated. The optimal concentration of the Li2O additive was analyzed and the relevant mechanism discussed. 2. Experimental Ceramics in composition of (1−x)LLZTO–xLi2O with x = 0 wt.%, 2 wt.%, 4 wt.%, 6 wt.% and 8 wt.%, were prepared via the conventional solid-state reaction. According to the stoichiometry of Li6.75La3Zr1.75Ta0.25O12, LiOH (Alfa Aesar, 99.995%), La(OH)3 (Alfa Aesar, 99.95%), ZrO2 (Aladdin Reagent, 99.99%) and Ta2O5 (Aladdin Reagent, 99.99%) were weighed and mixed. 15 wt.% excess LiOH was added to compensate volatile Li components during synthesis. The powders were ball-milled for 12 h, heated in air at 900 °C for 12 h and followed by another ball-milling for 12 h. The obtained LLZTO powders were in pure cubic phase, as confirmed by the XRD measurement. These cubic powders were mixed with Li2O (Alfa Aesar, 99.5%) in different weight ratio and ball-milled for 12 h. After the ballmilling, the powders of LLZTO–Li2O were pressed into pellets at 100 MPa, and sintered at 1170–1230 °C for 6 h in air when the pellets were covered with their respective mother powders. All the synthesis
Y. Li et al. / Solid State Ionics 253 (2013) 76–80
processes were carried out in a box furnace, with alumina crucibles covered by alumina lids. Values of the relative density were obtained through the Archimedes method with alcohol for measurement. Phase analysis was carried out by X-ray diffraction (XRD, Bruker D2 Phaser) and rechecked at the beamline of BL14B1 in the Shanghai Synchrotron Radiation Facility. Characterization of microstructure was conducted by scanning electron microscopy (SEM, Hitachi S-3400 N) at a 5 kV accelerating voltage and transmission electron microscope (TEM, JEOL JEM-2100F) at a 200 kV accelerating voltage. For detection of cross-section morphology by the SEM, the cut samples were etched by the heat treatment of 1100 °C for 10 min in air. Elemental distributions were investigated by energy dispersive X-ray detector (EDX, Oxford Instruments) equipped on the SEM. The atomic ratios of La:Zr:Ta:Al were measured by inductively coupled plasma optical emission spectroscopy (ICP-OES, Agilent 725). For the TEM analysis, the samples were first polished down to a thickness of approximately 30 μm and then further thinned using argon ion milling with an incident energy of 4.5 keV. Phase distribution in the ceramic microstructure was detected by the selected-area electron diffraction (SAED) in combination with the TEM. Ionic conductivities of the sintered samples were measured by an impedance analyzer (Novocontrol Beta High Performance Impedance Analyzer) with an ac current of 10 mV from 0.1 to 20 M Hz in frequency. The ac impedance spectra were obtained with the lithium foils as electrodes which were pressed on both sides of the samples. Electronic conductivities were examined by the galvanostatic polarization measurement, using a battery test station (Arbin BT-2000) with an applied dc voltage of 5 V. For this measurement, the Au electrodes were coated on both sides of the samples by magnetron sputtering.
3. Results and discussion The relative densities of the (1 − x)LLZTO–xLi2O samples were first studied for determining optimal sintering temperatures. The value of the relative density is calculated through the formula ρexp⁄ρtheo, where ρexp is the density measured by the Archimedes method and ρtheo is the theoretical density. The latter is calculated by the formula 1=ρtheo ¼ ð1−xÞ=ρLLZTO þ x=ρLi2 O , where ρLLZTO and ρLi2 O are the theoretical densities of LLZTO and Li2O, respectively, x is the weight ratio of the Li2O additive. It is found that different concentrations of Li2O additives lead to different optimal sintering temperature in terms of the maximum relative density. The temperature of 1230 °C corresponds to the maximum relative density of 91.5% for the LLZTO without the additive. For both 98%LLZTO–2%Li2O and 96%LLZTO–4%Li2O, 1200 °C is the optimal temperature corresponding to the maximum relative densities of 92.3% and 95.4%, respectively. That of 1170 °C is for both 94%LLZTO–6%Li2O and 92%LLZTO–8%Li2O, corresponding to 97.2% and 97.3% as the maximum densities. These values are summarized in Table 1. It can be found that with increasing concentration of the additive the achievable maximum relative density increases while the optimum sintering temperature decreases. XRD patterns for the samples are shown in Fig. 1. It can be seen that each sample shows a structure similar to the cubic garnet Li5La2Nb2O12 (referring to JCPDS 01-084-1753). No peaks related to Li2O are detectable, indicating a negligible existence of the individual Li2O phase. With introduction of the Li2O, the diffraction peaks at the position greater than 25° are split. These splitting peaks indicate that the formation of the tetragonal phase most probably occurs owing to the incorporation of Li2O at the grain boundaries. This is in agreement with the results of TEM in combination with SAED, which reveals no tetragonal-phase LLZTO in the grains (as discussed in the following). Moreover, some peaks ascribed to Li0.5La2Al0.5O4 can be detected for the samples added with the Li2O additives. Such material mainly distributes at the grain boundaries, as indicated by the composition analysis discussed in the following.
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Table 1 Influence of Li2O concentration on optimal sintering temperature, maximum relative density, total conductivity, activation energy and electronic conductivity of the ceramic electrolytes investigated here. x (wt.%)a
Optimal sintering T (°C)
Maximum density (%)b
σtotal (S cm−1)c
0 2 4 6 8
1230 1200 1200 1170 1170
91.5 92.3 95.4 97.2 97.3
2.2 3.3 4.4 6.4 2.7
a b c d e
× × × × ×
10−4 10−4 10−4 10−4 10−4
Ea (eV)d
σe (S cm−1)e
0.41 0.38 0.33 0.30 0.39
3.9 4.0 4.3 5.3 5.7
× × × × ×
10−7 10−7 10−7 10−7 10−7
x: concentration of the Li2O additive. Maximum relative density after optimization of sintering temperature. σtotal: total conductivity. Ea: activation energy. σe: electronic conductivity.
Fig. 2 shows SEM images for cross sections of the samples with and without the Li2O additives. As for the LLZTO without the additive (Fig. 2(a)), small pores can be observed in between the grains. The pore size ranges from 0.5 to 2.0 μm, while the grain size varies from 2 to 20 μm. By introduction of the Li2O additive, a significant change in microstructure can be seen in Fig. 2(b)–(e). The grains become more connected and the amount of pores at the grain boundaries becomes much smaller. EDX spectra for the grains and grain boundaries as shown in Fig. 3 clearly indicate that the Al element is solely detectable at the grain boundaries. Its existence in the grains is negligible. In order to clarify crystallography for the materials of the grains as well as the grain boundaries, TEM in combination with SAED were carried out for the samples of 94%LLZTO–6%Li2O. Fig. 4(a) shows the image for the ionbeam-cut cross section of the sample. SAED reveals that the materials at the grain boundaries consist of two sets of diffused diffraction rings, which can be attributed to weakly crystallized LLZTO and Li0.5La2Al0.5O4, as shown in Fig. 4(b). In the grains, only one set of ordered sharp diffraction spots can be observed, clearly indicating the high-quality crystallized LLZTO, as shown in Fig. 4(c). Therefore, the above results demonstrate that the grains are composed of crystalline LLZTO while the grain boundaries consist of glassy-like materials of the LLZTO mixed with the Li0.5La2Al0.5O4. The atomic ratios of La:Zr:Ta:Al for the 94%LLZTO–6%Li2O samples were investigated by the ICP technique, which are 3.00:1.76:0.24:0.23. This means that the concentration of Al is comparable to that of Ta in
Fig. 1. XRD curves for the LLZTO with the Li2O additive concentration ranging from 0 wt.% to 8 wt.%. The diffraction patterns of cubic garnet Li5La3Nb 2O12 referring to JCPDS 01-084-1753 are given for comparison. “♦” denotes the Li0.5La2Al0.5O4 formed at the grain boundaries, as revealed by the composition analysis.
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Fig. 2. SEM images for cross sections of the ceramic electrolytes: (a) 100%LLZTO, (b) 98%LLZTO–2%Li2O, (c) 96%LLZTO–4%Li2O, (d) 94%LLZTO–6%Li2O, (e) 92%LLZTO–8%Li2O. The cut samples were etched by the heat treatment of 1100 °C for 10 min in air before the SEM measurement.
the investigated samples. Note that in the present work, the powders of cubic Li6.75La3Zr1.75Ta0.25O12 (LLZTO) were first synthesized by the heating at 900 °C followed by a ball-milling. The ICP measurement reveals negligible Al in these cubic powders. This indicates that incorporation of the Al occurs in the following sintering of the LLZTO–Li2O powders at the elevated temperatures of 1170–1230 °C, owing to the diffusion of Al from the used alumina crucibles into the samples. Recalling that the Al is mainly distributed at the grain boundaries, the Al compound should work as a sintering aid and play a role in densification of the ceramic samples. Impedance spectra for all the samples in Nyquist plot are drawn in Fig. 5(a). Though the grain and grain boundary contributions cannot be separated, the overall resistance of the sample can be obtained from the intersection of the semicircle with the real axis. According to σtotal = L/SRtotal where L is the sample thickness, S is the area of the electrodes and Rtotal is the overall resistance, the total conductivities are calculated, as listed in Table 1. Note that these spectra were obtained with the Li electrodes, which were pressed on the sides of pellet
samples. They were compared with those measured with the Au electrodes. As can be seen in Fig. 5(b) for the 94%LLZTO–6%Li2O sample, the overall resistance (i.e. Rtotal) measured with the Li electrodes is nearly the same as that with the Au electrodes. However, as shown in Fig. 5(c), for the Li electrodes the semicircle attributed to the charge transfer resistance of Li/LLZTO interface impedance can be seen though it is as large as ~ 4 × 104 Ω. For the Au electrodes such semicircle cannot be obtained, which is consistent with the ionblocking behavior of the Au electrodes. Curves of conductivity as a function of inverse temperature are plotted in Fig. 6. According to σtotalT = Aexp(− Ea/kBT) where A is the pre-exponential parameter, Ea the activation energy, kB the Boltzmann constant and T the absolute temperature, the values for Ea are calculated and summarized in Table 1. From Fig. 6 and Table 1, it can be seen that σtotal increases from 2.2 × 10−4 to 6.4 × 10−4 S cm−1 as the Li2O concentration increases from 0 wt.% to 6 wt.%. Meanwhile Ea decreases from 0.41 eV to 0.30 eV. This means that the increased density of the sample improves the total conductivity while reducing
Y. Li et al. / Solid State Ionics 253 (2013) 76–80
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that the Li2O additives lead to formation of glassy-like materials at the grain boundaries, which is helpful for expelling the residual pores and improving the ceramic density. The 6 wt.% is the optimal additive
Fig. 3. EDX results of elemental distributions in the grains and at the grain boundaries of the 94%LLZTO–6%Li2O sample. The Al element can solely be observed at the grain boundaries. In combination with the XRD and the TEM results, the fact that the Li0.5 La 2 Al 0.5 O 4 is formed at the grain boundary is demonstrated.
the activation energy when the Li2O concentration is below 6 wt.%. As the Li2O concentration increases to 8 wt.%, σtotal decreases and Ea increases though the relative density slightly increases. Concerning that the activation energy of the dense Li6.75La3Zr1.75Ta0.25O12 ceramic electrolytes is 0.22 eV [16], the above results indicate that the appropriate Li2O additive most probably leads to the grains fused together, thus improving the connection between the grains and enhancing the migration of lithium ions in the overall ceramic. The glassy-like materials formed at the grain boundaries have a lower conductivity than LLZTO, thus the excess additives causing deterioration of the lithium ion transport. Galvanostatic polarization behaviors of the above samples were investigated to check the electronic contribution. Each sample shows the typical exponential relationship between the current and the measuring time. The electronic contributions can be estimated by the steady state currents [31], which are found to be approximately three orders of magnitude lower than the ionic counter parts. This demonstrates that the investigated ceramics here are ionically conducting materials. 4. Conclusions Cubic garnet LLZTO ceramic electrolytes with the Li2O additives have been synthesized via the conventional solid-state reaction. It is found
Fig. 4. (a) A typical image for the ion-beam-cut cross section of the 94%LLZTO–6%Li2O sample. (b) SAED pattern for the materials at the grain boundaries. Two sets of diffused diffraction rings are identified: (220), (420) and (431) for LLZTO and (110) for Li0.5La2Al0.5O4. (c) SAED pattern for the materials at the grains. One set of ordered sharp diffraction spots clearly indicates the high-quality crystallized LLZTO at the grains.
Fig. 5. (a) Impedance spectra measured at 28 °C for the LLZTO ceramics with the Li2O additive concentration ranging from 0 wt.% to 8 wt.%. The Li electrodes were used for these samples. The total conductivities are calculated from these spectra. (b) and (c) Comparison of the impedance spectra measured with the Li electrodes and those measured with the Au electrodes. The measured samples are the LLZTO ceramics with the Li2O additive concentration of 6 wt.%.
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References
Fig. 6. Arrhenius plots based on the impedance spectra measured at 28 °C for the LLZTO ceramics with the Li2O additive concentration ranging from 0 wt.% to 8 wt.%.
concentration, which leads to the optimum total ionic conductivity of 6.4 × 10−4 S cm−1 of the samples investigated here. Further increase of the additive concentration does not help improve the ionic conduction, mostly probably attributed to the worse conduction of the glassy-like materials at the grain boundaries compared to that of the LLZTO at the grains. Overall, the results here demonstrate that introduction of the additive in an appropriate concentration is a useful way to improve the ionic conductivities of the solid-state LLZTO electrolytes.
Acknowledgments The authors would like to thank the project supported by the National Natural Science Foundation of China under Grant No. U1232111. The beamline of BL14B1 in the Shanghai Synchrotron Radiation Facility is acknowledged for the X-ray measurement and data analysis.
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Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Influence of lithium oxide additives on densification and ionic conductivity of garnet-type Li6.75La3Zr1.75Ta0.25O12 solid electrolytes Yiqiu Li, Yang Cao, Xiangxin Guo ⁎ State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding Xi Road, Shanghai 200050, China
a r t i c l e
i n f o
Article history: Received 8 March 2013 Received in revised form 2 September 2013 Accepted 2 September 2013 Available online 26 September 2013 Keywords: Lithium garnet electrolytes Densification Ionic conductivity Lithium oxide additives
a b s t r a c t The lithium garnet ceramic has been considered as one of promising solid-state electrolytes for secondary lithium batteries. However, improvement of its ionic conductivity is hindered by the high porosity related to the severe volatilization of lithium components during sintering. In order to find the solution for this problem, Li2O additives in concentration of 2–8 wt.% were introduced into the cubic Li6.75La3Zr1.75Ta0.25O12 (LLZTO) at the solid-state reaction processes. It is found that the Li2O additives lead to formation of glassy-like phases at the grain boundaries related to the liquid-phase sintering behavior, which eliminates the residual pores therein and increases the relative density from 91.5% to 97.3%. Measurement of conductivity indicates that 6 wt.% is the optimum concentration of Li2O which leads to an ionic conductivity of 6.4 × 10−4 S cm−1 at room temperature. This value is approximately 3 times larger than that of the ceramic without the additive. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Solid-state electrolytes are essential for all-solid-state lithium batteries, which are superior to the presently used organic liquid electrolytes in terms of safety issues such as dendrite formation, flammability and leakage problem [1,2]. So far, many types of solid-state electrolytes have been explored, including perovskite titanates [3–5], NASICON phosphates [6–8], LISICON sulfides [9,10] and garnets [11–14]. Among these materials, Li7La3Zr2O12 (LLZO) with a cubic garnet structure exhibited an ionic conductivity of approximately 2 × 10−4 S cm−1 at room temperature (RT), good thermal and chemical stability against prospective electrodes [13]. It was indicated that introduction of Al, Ga, Nb or Ta was necessary to stabilize such a cubic structure [15–21]. Otherwise, formation of a tetragonal structure might occur and cause a much lower ionic conductivity [22,23]. Besides the cubic structure, the relative density of the LLZO ceramic is also a critical issue. Because of the temperature limitation imposed by the volatilization of Li components during sintering, the dense LLZO ceramic is difficult to obtain via the conventional solid-state reaction. The solid electrolyte with a high porosity may cause a poor ionic conductivity [24] as well as a mechanical failure [25]. The hot-press sintering technique was applied for preparation of the LLZO ceramics, which achieved a relative density of 96–98% [16,26,27]. It is obvious that such technique needs dedicated furnaces. In contrast, the convention solid-state reactions with the liquid-phase sintering could also be a useful way to improve the ceramic density, since the liquid phases formed in between the grains could expel the residual pores [8,28–30]. However, using ⁎ Corresponding author. Tel.: +86 21 52411032; fax: +86 21 52411802. E-mail address: [email protected] (X. Guo). 0167-2738/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ssi.2013.09.005
this technique for preparing high-density LLZO ceramics has not ever been reported up to date. Li2O as the additive proved to be a good binder, which could lead to the liquid phase sintering behavior in preparation of NASICON phosphates [29,30]. It could also act as a flux to obtain a highly conductive layer at the grain boundaries [29]. In this work, we introduced Li2O as the additives into the Li6.75La3Zr1.75Ta0.25O12 (LLZTO) ceramics during the conventional solid-state reaction preparation. Through concentration variation of the Li2O additive, changes of crystallography, relative density and ionic conductivity of the ceramic electrolytes were investigated. The optimal concentration of the Li2O additive was analyzed and the relevant mechanism discussed. 2. Experimental Ceramics in composition of (1−x)LLZTO–xLi2O with x = 0 wt.%, 2 wt.%, 4 wt.%, 6 wt.% and 8 wt.%, were prepared via the conventional solid-state reaction. According to the stoichiometry of Li6.75La3Zr1.75Ta0.25O12, LiOH (Alfa Aesar, 99.995%), La(OH)3 (Alfa Aesar, 99.95%), ZrO2 (Aladdin Reagent, 99.99%) and Ta2O5 (Aladdin Reagent, 99.99%) were weighed and mixed. 15 wt.% excess LiOH was added to compensate volatile Li components during synthesis. The powders were ball-milled for 12 h, heated in air at 900 °C for 12 h and followed by another ball-milling for 12 h. The obtained LLZTO powders were in pure cubic phase, as confirmed by the XRD measurement. These cubic powders were mixed with Li2O (Alfa Aesar, 99.5%) in different weight ratio and ball-milled for 12 h. After the ballmilling, the powders of LLZTO–Li2O were pressed into pellets at 100 MPa, and sintered at 1170–1230 °C for 6 h in air when the pellets were covered with their respective mother powders. All the synthesis
Y. Li et al. / Solid State Ionics 253 (2013) 76–80
processes were carried out in a box furnace, with alumina crucibles covered by alumina lids. Values of the relative density were obtained through the Archimedes method with alcohol for measurement. Phase analysis was carried out by X-ray diffraction (XRD, Bruker D2 Phaser) and rechecked at the beamline of BL14B1 in the Shanghai Synchrotron Radiation Facility. Characterization of microstructure was conducted by scanning electron microscopy (SEM, Hitachi S-3400 N) at a 5 kV accelerating voltage and transmission electron microscope (TEM, JEOL JEM-2100F) at a 200 kV accelerating voltage. For detection of cross-section morphology by the SEM, the cut samples were etched by the heat treatment of 1100 °C for 10 min in air. Elemental distributions were investigated by energy dispersive X-ray detector (EDX, Oxford Instruments) equipped on the SEM. The atomic ratios of La:Zr:Ta:Al were measured by inductively coupled plasma optical emission spectroscopy (ICP-OES, Agilent 725). For the TEM analysis, the samples were first polished down to a thickness of approximately 30 μm and then further thinned using argon ion milling with an incident energy of 4.5 keV. Phase distribution in the ceramic microstructure was detected by the selected-area electron diffraction (SAED) in combination with the TEM. Ionic conductivities of the sintered samples were measured by an impedance analyzer (Novocontrol Beta High Performance Impedance Analyzer) with an ac current of 10 mV from 0.1 to 20 M Hz in frequency. The ac impedance spectra were obtained with the lithium foils as electrodes which were pressed on both sides of the samples. Electronic conductivities were examined by the galvanostatic polarization measurement, using a battery test station (Arbin BT-2000) with an applied dc voltage of 5 V. For this measurement, the Au electrodes were coated on both sides of the samples by magnetron sputtering.
3. Results and discussion The relative densities of the (1 − x)LLZTO–xLi2O samples were first studied for determining optimal sintering temperatures. The value of the relative density is calculated through the formula ρexp⁄ρtheo, where ρexp is the density measured by the Archimedes method and ρtheo is the theoretical density. The latter is calculated by the formula 1=ρtheo ¼ ð1−xÞ=ρLLZTO þ x=ρLi2 O , where ρLLZTO and ρLi2 O are the theoretical densities of LLZTO and Li2O, respectively, x is the weight ratio of the Li2O additive. It is found that different concentrations of Li2O additives lead to different optimal sintering temperature in terms of the maximum relative density. The temperature of 1230 °C corresponds to the maximum relative density of 91.5% for the LLZTO without the additive. For both 98%LLZTO–2%Li2O and 96%LLZTO–4%Li2O, 1200 °C is the optimal temperature corresponding to the maximum relative densities of 92.3% and 95.4%, respectively. That of 1170 °C is for both 94%LLZTO–6%Li2O and 92%LLZTO–8%Li2O, corresponding to 97.2% and 97.3% as the maximum densities. These values are summarized in Table 1. It can be found that with increasing concentration of the additive the achievable maximum relative density increases while the optimum sintering temperature decreases. XRD patterns for the samples are shown in Fig. 1. It can be seen that each sample shows a structure similar to the cubic garnet Li5La2Nb2O12 (referring to JCPDS 01-084-1753). No peaks related to Li2O are detectable, indicating a negligible existence of the individual Li2O phase. With introduction of the Li2O, the diffraction peaks at the position greater than 25° are split. These splitting peaks indicate that the formation of the tetragonal phase most probably occurs owing to the incorporation of Li2O at the grain boundaries. This is in agreement with the results of TEM in combination with SAED, which reveals no tetragonal-phase LLZTO in the grains (as discussed in the following). Moreover, some peaks ascribed to Li0.5La2Al0.5O4 can be detected for the samples added with the Li2O additives. Such material mainly distributes at the grain boundaries, as indicated by the composition analysis discussed in the following.
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Table 1 Influence of Li2O concentration on optimal sintering temperature, maximum relative density, total conductivity, activation energy and electronic conductivity of the ceramic electrolytes investigated here. x (wt.%)a
Optimal sintering T (°C)
Maximum density (%)b
σtotal (S cm−1)c
0 2 4 6 8
1230 1200 1200 1170 1170
91.5 92.3 95.4 97.2 97.3
2.2 3.3 4.4 6.4 2.7
a b c d e
× × × × ×
10−4 10−4 10−4 10−4 10−4
Ea (eV)d
σe (S cm−1)e
0.41 0.38 0.33 0.30 0.39
3.9 4.0 4.3 5.3 5.7
× × × × ×
10−7 10−7 10−7 10−7 10−7
x: concentration of the Li2O additive. Maximum relative density after optimization of sintering temperature. σtotal: total conductivity. Ea: activation energy. σe: electronic conductivity.
Fig. 2 shows SEM images for cross sections of the samples with and without the Li2O additives. As for the LLZTO without the additive (Fig. 2(a)), small pores can be observed in between the grains. The pore size ranges from 0.5 to 2.0 μm, while the grain size varies from 2 to 20 μm. By introduction of the Li2O additive, a significant change in microstructure can be seen in Fig. 2(b)–(e). The grains become more connected and the amount of pores at the grain boundaries becomes much smaller. EDX spectra for the grains and grain boundaries as shown in Fig. 3 clearly indicate that the Al element is solely detectable at the grain boundaries. Its existence in the grains is negligible. In order to clarify crystallography for the materials of the grains as well as the grain boundaries, TEM in combination with SAED were carried out for the samples of 94%LLZTO–6%Li2O. Fig. 4(a) shows the image for the ionbeam-cut cross section of the sample. SAED reveals that the materials at the grain boundaries consist of two sets of diffused diffraction rings, which can be attributed to weakly crystallized LLZTO and Li0.5La2Al0.5O4, as shown in Fig. 4(b). In the grains, only one set of ordered sharp diffraction spots can be observed, clearly indicating the high-quality crystallized LLZTO, as shown in Fig. 4(c). Therefore, the above results demonstrate that the grains are composed of crystalline LLZTO while the grain boundaries consist of glassy-like materials of the LLZTO mixed with the Li0.5La2Al0.5O4. The atomic ratios of La:Zr:Ta:Al for the 94%LLZTO–6%Li2O samples were investigated by the ICP technique, which are 3.00:1.76:0.24:0.23. This means that the concentration of Al is comparable to that of Ta in
Fig. 1. XRD curves for the LLZTO with the Li2O additive concentration ranging from 0 wt.% to 8 wt.%. The diffraction patterns of cubic garnet Li5La3Nb 2O12 referring to JCPDS 01-084-1753 are given for comparison. “♦” denotes the Li0.5La2Al0.5O4 formed at the grain boundaries, as revealed by the composition analysis.
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Fig. 2. SEM images for cross sections of the ceramic electrolytes: (a) 100%LLZTO, (b) 98%LLZTO–2%Li2O, (c) 96%LLZTO–4%Li2O, (d) 94%LLZTO–6%Li2O, (e) 92%LLZTO–8%Li2O. The cut samples were etched by the heat treatment of 1100 °C for 10 min in air before the SEM measurement.
the investigated samples. Note that in the present work, the powders of cubic Li6.75La3Zr1.75Ta0.25O12 (LLZTO) were first synthesized by the heating at 900 °C followed by a ball-milling. The ICP measurement reveals negligible Al in these cubic powders. This indicates that incorporation of the Al occurs in the following sintering of the LLZTO–Li2O powders at the elevated temperatures of 1170–1230 °C, owing to the diffusion of Al from the used alumina crucibles into the samples. Recalling that the Al is mainly distributed at the grain boundaries, the Al compound should work as a sintering aid and play a role in densification of the ceramic samples. Impedance spectra for all the samples in Nyquist plot are drawn in Fig. 5(a). Though the grain and grain boundary contributions cannot be separated, the overall resistance of the sample can be obtained from the intersection of the semicircle with the real axis. According to σtotal = L/SRtotal where L is the sample thickness, S is the area of the electrodes and Rtotal is the overall resistance, the total conductivities are calculated, as listed in Table 1. Note that these spectra were obtained with the Li electrodes, which were pressed on the sides of pellet
samples. They were compared with those measured with the Au electrodes. As can be seen in Fig. 5(b) for the 94%LLZTO–6%Li2O sample, the overall resistance (i.e. Rtotal) measured with the Li electrodes is nearly the same as that with the Au electrodes. However, as shown in Fig. 5(c), for the Li electrodes the semicircle attributed to the charge transfer resistance of Li/LLZTO interface impedance can be seen though it is as large as ~ 4 × 104 Ω. For the Au electrodes such semicircle cannot be obtained, which is consistent with the ionblocking behavior of the Au electrodes. Curves of conductivity as a function of inverse temperature are plotted in Fig. 6. According to σtotalT = Aexp(− Ea/kBT) where A is the pre-exponential parameter, Ea the activation energy, kB the Boltzmann constant and T the absolute temperature, the values for Ea are calculated and summarized in Table 1. From Fig. 6 and Table 1, it can be seen that σtotal increases from 2.2 × 10−4 to 6.4 × 10−4 S cm−1 as the Li2O concentration increases from 0 wt.% to 6 wt.%. Meanwhile Ea decreases from 0.41 eV to 0.30 eV. This means that the increased density of the sample improves the total conductivity while reducing
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that the Li2O additives lead to formation of glassy-like materials at the grain boundaries, which is helpful for expelling the residual pores and improving the ceramic density. The 6 wt.% is the optimal additive
Fig. 3. EDX results of elemental distributions in the grains and at the grain boundaries of the 94%LLZTO–6%Li2O sample. The Al element can solely be observed at the grain boundaries. In combination with the XRD and the TEM results, the fact that the Li0.5 La 2 Al 0.5 O 4 is formed at the grain boundary is demonstrated.
the activation energy when the Li2O concentration is below 6 wt.%. As the Li2O concentration increases to 8 wt.%, σtotal decreases and Ea increases though the relative density slightly increases. Concerning that the activation energy of the dense Li6.75La3Zr1.75Ta0.25O12 ceramic electrolytes is 0.22 eV [16], the above results indicate that the appropriate Li2O additive most probably leads to the grains fused together, thus improving the connection between the grains and enhancing the migration of lithium ions in the overall ceramic. The glassy-like materials formed at the grain boundaries have a lower conductivity than LLZTO, thus the excess additives causing deterioration of the lithium ion transport. Galvanostatic polarization behaviors of the above samples were investigated to check the electronic contribution. Each sample shows the typical exponential relationship between the current and the measuring time. The electronic contributions can be estimated by the steady state currents [31], which are found to be approximately three orders of magnitude lower than the ionic counter parts. This demonstrates that the investigated ceramics here are ionically conducting materials. 4. Conclusions Cubic garnet LLZTO ceramic electrolytes with the Li2O additives have been synthesized via the conventional solid-state reaction. It is found
Fig. 4. (a) A typical image for the ion-beam-cut cross section of the 94%LLZTO–6%Li2O sample. (b) SAED pattern for the materials at the grain boundaries. Two sets of diffused diffraction rings are identified: (220), (420) and (431) for LLZTO and (110) for Li0.5La2Al0.5O4. (c) SAED pattern for the materials at the grains. One set of ordered sharp diffraction spots clearly indicates the high-quality crystallized LLZTO at the grains.
Fig. 5. (a) Impedance spectra measured at 28 °C for the LLZTO ceramics with the Li2O additive concentration ranging from 0 wt.% to 8 wt.%. The Li electrodes were used for these samples. The total conductivities are calculated from these spectra. (b) and (c) Comparison of the impedance spectra measured with the Li electrodes and those measured with the Au electrodes. The measured samples are the LLZTO ceramics with the Li2O additive concentration of 6 wt.%.
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References
Fig. 6. Arrhenius plots based on the impedance spectra measured at 28 °C for the LLZTO ceramics with the Li2O additive concentration ranging from 0 wt.% to 8 wt.%.
concentration, which leads to the optimum total ionic conductivity of 6.4 × 10−4 S cm−1 of the samples investigated here. Further increase of the additive concentration does not help improve the ionic conduction, mostly probably attributed to the worse conduction of the glassy-like materials at the grain boundaries compared to that of the LLZTO at the grains. Overall, the results here demonstrate that introduction of the additive in an appropriate concentration is a useful way to improve the ionic conductivities of the solid-state LLZTO electrolytes.
Acknowledgments The authors would like to thank the project supported by the National Natural Science Foundation of China under Grant No. U1232111. The beamline of BL14B1 in the Shanghai Synchrotron Radiation Facility is acknowledged for the X-ray measurement and data analysis.
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