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A novel method for preparation of high dense tetragonal Li7La3Zr2O12
Journal of Power Sources 344 (2017) 56e61
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A novel method for preparation of high dense tetragonal Li7La3Zr2O12 Pengcheng Zhao, Yuehua Wen, Jie Cheng, Gaoping Cao*, Zhaoqing Jin, Hai Ming, Yan Xu, Xiayu Zhu Research Institute of Chemical Defense, Beijing Key Laboratory of Advanced Chemical Energy Storage Technology and Materials, Beijing 100191, China
h i g h l i g h t s High dense tetragonal LLZO has been firstly prepared by auto-consolidation method. No pressing operations are employed for the preparation of LLZO. The prepared tetragonal LLZO has the highest lithium ion conductivity in present research. An auto-consolidation mechanism for LLZO particles is proposed and discussed. This work provides a new and very promising method for ceramic solid electrolytes.
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Article history: Received 9 December 2016 Received in revised form 2 January 2017 Accepted 19 January 2017
For conventional preparation methods of Li7La3Zr2O12 (LLZO) solid state electrolytes, there is a stereotype that higher density always comes from higher pressure enforced upon the LLZO pellets. In this paper, a different way with an auto-consolidation mechanism is provided and discussed. No pressing operations are employed during the whole preparation process. Due to the surface tension of liquid melted Li2O at sintering temperature, LLZO particles could aggregate together freely and automatically. The preparation process for dense LLZO is greatly simplified. A dense tetragonal LLZO with high relative density about 93% has been prepared successfully by this auto-consolidation method. And there are no voids observed in the SEM images. At 30 C, the total conductivity is about 5.67 105 S cm1, which is the highest one for tetragonal LLZO in the reported issues, even two times higher than that prepared by hot-pressing method. The activation energy for total conductivity is ~0.35 eV atom1 at 30e120 C, slightly lower than the previous reported values. This work sheds light on the understanding of the consolidation mechanism for solid electrolytes and suggests a reliable route to syhthesize cemanic solid electrolytes. © 2017 Elsevier B.V. All rights reserved.
Keywords: Lithium ion battery Li7La3Zr2O12 solid state electrolyte Ionic conductivity Tetragonal structure
1. Introduction Lithium ion battery (LIB) is becoming the main storage device for commercial applications. However, its safety issues still haven't be resolved due to its liquid organic electrolytes which are volatile and flammable. All-solid-state LIB is viewed as a thorough solution for the safety issues [1]. Instead of liquid electrolytes, all-solid-state LIB employs solid electrolytes as both separators and ion conductors. Among current solid electrolyte materials, LLZO has attracted ever-increasing attention over the last few years due to its great advantages, such as high chemical stability with lithium metal
* Corresponding author. E-mail address: [email protected] (G. Cao). http://dx.doi.org/10.1016/j.jpowsour.2017.01.088 0378-7753/© 2017 Elsevier B.V. All rights reserved.
electrode, wide electrochemical window (0e5 V vs. Liþ/Li) and low bulk and grain boundary resistance [2e4]. LLZO was first reported by Murugan in 2007 [4]. Recent studies show that LLZO has two different structures, which are cubic and tetragonal, respectively [2e4]. For pure LLZO, cubic structure is unstable and tends to change into tetragonal structure at room temperature (RT). With doping of elements such as aluminum, niobium, wolfram or tantalum, cubic structure can be stabilized at RT with a high total conductivity between ~104 and ~103 S cm1 [5e11]. On the contrary, the total conductivity for tetragonal LLZO prepared by cold isostatic pressing (cold-pressing) method is about two orders lower, ~106 S cm1 with a relative density of 60e73% [10,12,13]. For solid electrolytes, higher density usually means higher ionic conductivity. In 2012, Jeff Wolfenstine prepared tetragonal LLZO by hot isostatic pressing (hot-pressing) method
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[14]. The relative density of tetragonal LLZO was promoted to ~98% with a total conductivity of 2.3 105 S cm1. It is the highest reported value for tetragonal LLZO presently. The preparation of LLZO electrolytes can be divided into two main steps, which are the preparation of powders from raw materials and the preparation of pellets from powders subsequently. In the first step, LLZO powders usually are prepared by solid reaction or solution technology [5,6,8,15e18]. Then, in the second step, LLZO powders are pressed for consolidation before or during sintering. There is a stereotype for the preparation of LLZO pellets that higher density always comes from higher pressure enforced upon the LLZO pellets. But for cold-pressing method, even when the pressure was promoted to 800 MPa, the highest reported density for tetragonal LLZO is still as low as 73% presently [12,13]. And for both cold and hot isostatic pressing methods, the demands for preparation equipment involving high pressure are so hard to be satisfied for practical production. In this paper, a different way with an auto-consolidation mechanism was provided and discussed. No pressing operations were employed during the whole preparation process. The schematic illustration for the preparation of LLZO by auto-consolidation method is shown in Fig. 1. The preparation process for dense LLZO is greatly simplified. A dense tetragonal LLZO with a relative density about 93% was prepared successfully. We believe that the results of this work could facilitate the development of LLZO membrane technology and the auto-consolidation method can be effective for other ceramic solid electrolytes, which could also be prepared by sintering technology.
2. Experimental 2.1. Preparation for LLZO powder As shown in Fig. 1, LLZO powder was prepared by solid reaction and all the operations are conducted in air. Stoichiometric amounts of LiOH$H2O, La2O3, and ZrO2 precursor powders (All of the reagents are analytically pure from Sinopharm Chemical Reagent Co. Ltd) were ball-milled in a ZrO2 jar with ZrO2 balls filled with ethanol at 400 rpm min1 for 10 h. Commonly, an extra 10 wt% LiOH$H2O was added into the starting powders to compensate for the loss of Li during high-temperature sintering [4]. After ballmilling, the slurry was dried at 120 C for 10 h and calcined at 1150 C for 10 h in Al2O3 crucible. The heating rate from RT to 1150 C is 20 C/min. The primary LLZO powder was ball-milled again in a ZrO2 jar with ZrO2 balls at 500 rpm min1 for 10 h. After the second ball-milling, the primary powder was grinded by
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agate mortar and sieved by 100 meshes. Then tetragonal LLZO powder was prepared. 2.2. Preparation for LLZO pellet The LLZO powder was put in Al2O3 crucible and compacted by vibration. It was heated step by step. From RT to 900 C the heating rate is 10 C/min, followed by a heating rate of 1 C/min from 900 C to 1150 C. The powder was finally sintered to be bulk at 1150 C in air for 15 h. The dense LLZO bulk was sliced by a dimond saw and polished by 1000# diamond chip. A LLZO pellet with the dimension of 0.134 mm 1.740 cm 1.200 cm was prepared for characterization and test. 2.3. Characterization Malvern Mastersizer 2000 laser-diffraction particle size analyzer was employed to determine the particle size distribution of the LLZO powder before sintering. According to Archimedes principle, the density of LLZO bulk was determined from its weight and volume. The volume was determined by draining water. The relative density value was determined by dividing the bulk density by the theoretical density of tetragonal LLZO (~5.108 g cm3) [12]. X-ray diffraction (XRD, Bruker D8 advance X-ray diffractometer, Cu Ka radiation) was used to characterize the crystalline phases of the LLZO powders before and after sintering. The morphology of the sintered sample was characterized by scanning electron microscopy (Hitach S-4800 Field Emission SEM). Electrical conductivity measurements were performed on the LLZO pellet using two probes method. Au was sputter coated on the top and bottom surfaces of the sample (AujLLZOjAu). Impedance spectras were collected with an electrochemical interface (Solartron 1287, Solartron Analytical) and a frequency response analyzer (Solartron 1260, Solartron Analytical) under open-circuit voltage (OCV). The frequency range was from 1 MHz to 1 Hz and the ac amplitude was 30 mV. Temperature-dependent ionic conductivities were measured from the range of 30 Ce120 C. A constant dc voltage of 10 V was undertaken for 10 h between the two probes of AujLLZOjAu to determine electron conductivity. The electron conductivity was calculated from the constant voltage, end current and dimension of the sample. 3. Results and discussion the particle size distribution of LLZO powder is shown in Fig. 2. It indicates that the particle sizes of LLZO powder before sintering is
Fig. 1. Schematic illustration for the preparation of LLZO pellets by auto-consolidation method.
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Fig. 2. Particle size distribution of LLZO powder before sintering.
Fig. 3. Picture of the bulk (half) and slices of LLZO sample.
not uniform. The average particle size is about 10.668 mm with two main Gaussian distribution around ~0.5 mm and 9 mm. The LLZO powder can be viewed as two sizes particles with consecutive distribution and the amount of larger particles is several times to the amount of small ones. According to packing theories of particles [19]: 1) If the size of the particles is single, it will be hard to reach a high packing density. 2) For tow different sizes spheric particles, when the ratio between large particles and small particles is 7:3,
the packing density will be optimized. It means that consecutive particle sizes distribution is preferable to higher packing density, because small size particles can fill the interspace among large size particles. And the used particle size distribution of LLZO powder before sintering is in good agreement with the optimized packing theories above, so that the porosity of the packing powder will be minimized. The picture of LLZO bulk (half) and its slices is shown in Fig. 3. The volume of the LLZO bulk was determined to be 15.90 ml (75.5620 g). According to Archimedes principle, the density of LLZO bulk is ~4.7523 g cm3 and the relative density value is ~93%. The relative density value of LLZO is much higher than the previous reported value (60e73%) when prepared by cold-pressing method [12,13] and just a little lower than the reported value (98%) when prepared by hot-pressing method [14]. The SEM images of the surface of sintered sample are shown in Fig. 4. In Fig. 4a, it can be observed that the LLZO sample is built by different size particles gathering together. But in Fig. 4b there is no distinct boundaries between the particles. Different to hot-pressing samples, no voids are observed in the surface of the LLZO pellet. The result of SEM images is in good agreement with the high density measured above. Why so high dense LLZO could be gained from the sintering of powders directly without any other assist? At sintering temperature, Li2O is liquid and the liquid phase is accessible to the solidstate particles. So, the liquid would distribute among the surfaces of solid-state particles. The existing of surface tension from liquid phase would drive the solid-state particles to aggregate together instinctively. The consolidation mechanisms for different LLZO powders during sintering are shown in Fig. 5. Here, (a) cold-pressing refers to the samples pressed before sintering for conventional solid-state reaction; (b) hot-pressing refers to the samples pressed during sintering; (c) auto-consolidation refers to the samples neither pressed before or during sintering (provided by this paper). On the left of Fig. 5, the main forces around the particles are shown. Gravity of the particles is neglected in this case. On the right of Fig. 5, relative migration of the particles is shown. For simple, only three neighbor particles are chose to show the consolidation mechanisms for LLZO powders in line-dimension scope. For cold-pressing samples (pressed pellet), as shown in Fig. 5ae1, besides of surface tension from the liquid phase there are great inner stresses among the particles due to the distortion of the particles resulting from pressing process. In another word, the solid state particles are trapped. Only when the force of surface tension is
Fig. 4. SEM images of dense tetragonal LLZO pellet.
P. Zhao et al. / Journal of Power Sources 344 (2017) 56e61
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Fig. 5. Consolidation mechanisms for LLZO powders during sintering: (a) cold-pressing, (b) hot-pressing, (c) auto-consolidation.
larger than inner stresses, can the particles gather together. As the common case shown in Fig. 5ae2, particle II in the center moves closer to particle I but broader to particle III relatively. Usually, the bulk volume of the sample shrinks little. When the pressure for cold-pressing is increased, for one side the distances between particles will become closer. But for the other side, the particles would be harder to move together due to the higher inner stress. As a result, the relative density will be as low as 60e73% [12,13] and very hard to be promoted further. For hot-pressing samples (pressing pellet), there is a large pressure enforcing the particles to move from one side to the other side, as shown in Fig. 5b. Both the inner stress and the surface tension can be neglected. As shown in Fig. 5be2, both particle II and particle III are pressed to particle I against the “wall”. The relative density is supposed to be much higher. But the particles can't moving freely so that the voids can't be let out freely either. It is hard to eliminate the voids thoroughly. As a result, even the relative density reaches to ~98% but there are still numerous visible voids in the SEM images [5,14,20]. In Fig. 5ce1, for auto-consolidation samples (packing powders), the main force around each particle is the surface tension from liquid phase. During sintering, the force of surface tension from liquid phase drives particles to aggregate together freely and automatically. As shown in Fig. 5ce2, both particles I and III move closer to particle II in the center. With the consolidation of solidstate particles, the bulk volume of the sample shrinks and the voids are minimized and eliminated finally. As shown in Fig. 4, no voids are observed in the SEM images. Additionally, there are three key points for the preparation of dense LLZO electrolytes. The first one is the calcination temperature for the preparation of LLZO powder which was promoted to 1150 C. So when the gas components (such as H2O, CO2 and other impurities) in the raw materials are being let out, LLZO particles are formed at the same time. The second one is the packing density of LLZO powder before sintering, which is related to the particle size distribution. The last and most important one is the sintering rules.
During the temperature range of sintering, the heating rate should be slower enough. Based on the three key points above, the autoconsolidation mechanism began to do its work and dense LLZO bulk was gained. The XRD patterns for LLZO samples are shown in Fig. 6. No second phases is observed in the XRD patterns. The peak splitting shown in Fig. 6c is indicative of LLZO with a tetragonal structure (space group I41/acd (No.142)) [12]. In Fig. 6b, the X-ray pattern of the calcined powder is similar to the curve of Fig. 6a, suggesting that no structural change occurred during sintering. The results of Fig. 6 are in agreement with the previous reported results that, with no impurities, only LLZO with tetragonal structure is stable at room temperature [2,10,12e14,21e25]. Rietveld refinement of the
Fig. 6. X-ray diffraction patterns for LLZO samples: (a) powder before sintering, (b) bulk after sintering, (c) tetragonal phase from Ref. [12].
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Fig. 7. (a) Nyquist plots for the total (bulkþgrain boundary) conductivity measured from 30 C to 120 C, and (b) Arrhenius plot for total conductivity of dense tetragonal LLZO sample (AujLLZOjAu) at heating and cooling process subsequently.
diffraction data for tetragonal LLZO using atom positions from Awaka et al. [12] yielded lattice parameters: a ¼ 13.122 Å, c ¼ 12.690 Å, with a unit cell volume of 2184.860 Å. These values are in good agreement with the values of a ¼ 13.134 Å and c ¼ 12.663 Å [12], a ¼ 13.129 Å and c ¼ 12.672 Å [26], and a ¼ 13.130 Å and c ¼ 12.688(3) Å [27] for tetragonal LLZO. The conductivity of LLZO sample was examined by AC impedance spectroscopy using Li-ion blocking Au electrode. Fig. 7a shows the impedance plots measured from 30 C to 120 C for AujLLZOjAu. At 30 C, the data separates into a high frequency region which contains two semicircles and a low frequency region which contains a spike. The semicircles are attributed to bulk resistance and grain boundary resistance separately and the intercept of the semicircle of higher frequency on the real Z axis gives the total ionic resistance (latticeþgrain boundary) [14]. This inclined curve is referred to the frequency dispersion phenomenon and can be described using capacitance or constant phase element (CPE) [28,29]. Each experimental data was fitted with an equivalent circuit consisting of Rtotal (R1CPE1) (R2CPE2) (R3CPE3), as shown in Fig. 7a. Rtotal refers to the total resistance of the sample. In Fig. 7a, the fitting lines are shown in solid type. Using the value of Rtotal and the sample dimensions, the total conductivity of the tetragonal LLZO sample was calculated. The total conductivity of tetragonal LLZO is examined to be ~5.67 105 S cm1. Although it is still much lower than the maximum value for cubic LLZO (~1 103 S cm1), as shown in Table 1, it is the highest reported value for tetragonal LLZO, about 10e500 times higher than previously reported values of conventional solid-state sintering and
even 2 times higher than that of hot-pressing. This vast improvement in total conductivity is a result of the high density and nonvoids LLZO samples. Fig. 7b shows the Arrhenius plots for tetragonal LLZO sample. In order to check the reversibility of the sample, the data was measured at heating and cooling process subsequently. From left to right, the spots refer to 120, 110, 100, 90, 80, 70, 60, 50, 40 and 30 C separately. The activation energy for total conduction was obtained by fitting the conductivity measurement to the Arrhenius equation [12]. The activation energy is determined from the slope of the ln(sT) versus 1/T plot for the total conductivity. According to Fig. 7b, the Ea-heating and Ea-cooling are nearly equal, ~0.35 eV atom1 for 30e120 C. However, the fitting line isn't a very fine straight at 30e70 C. The ion conductivities for both 60 C and 70 C at cooling process are slightly higher than those at heating process (this phenomenon has been checked by heating back after the cooling process). It indicates that there might be different lithium ion conductivity mechanisms for the sample in different temperature ranges. For tetragonal LLZO, three different Li sites have been identified and the distances between Li sites are 2.5 Å, 2.6 Å and 2.7 Å respectively [26]. The Li sites distribution would be distorted with the increasing of temperature and a tetragonal-cubic transition of LLZO has been observed at 100e150 C [30]. It could be implied that there might be a structure distortion and relaxation phenomenon at 60e70 C for the sample corresponding to different conduction pathway. The I-t curve for AujLLZOjAu at 10 V lasting 10 h is measured. The electron conductivity was determined by the constant voltage, end
Table 1 Relative density, conductivity and activation energy data for tetragonal LLZO electrolytes around RT. Preparation method Cold-pressing at 200 Cold-pressing Cold-pressing Cold-pressing at 100 Cold-pressing at 200 Cold-pressing Cold-pressing at 800 Cold-pressing at 800 Cold-pressing Cold-pressing at 300 Hot-pressing Auto-consolidation
MPa
MPa MPa MPa MPa MPa
Major sintering rules
Relative density (%)
Total conductivity (S cm1)
1050 C for 10 h, 1230 C for 30 h. 900 C for 5 h 900 C for 5 h 1140 C for 48 h 800 C for 20 h. 1100 C for 1 h 1100 C for 1 h 1130 C for 12 h 1200 C for 24 h 1050 C at 40 MPa pressure for 1 h 1150 C for 15 h
e e e e 60 e 73 66 e 91 98 93
1.0 2.3 3.1 3.2 4.0 4.4 1.3 5.2 2.0 1.0 2.3 5.7
107 107 107 107 107 107 106 106 106 105 105 105
Activation energy (eV atom1)
Reference
e 0.49 0.67 e 0.54 0.54 0.43 0.45 0.49 0.43 0.41 0.35
[30] [31] [32] [33] [12] [22] [13] [13] [10] [34] [14] This study
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current and the dimension of LLZO pellet. The value of the end current is ~1.17 106 A. The electron conductivity was calculated to be 7.51 109 S cm1. It indicates that the tetragonal LLZO is electron insulated. That is in good agreement with the AC impedance plots in Fig. 7a. The appearance of the tail at low frequencies in the case of ion blocking electrodes is an indication that the investigated material is ionically conducting in nature [4,14]. 4. Conclusion For LLZO solid electrolytes, higher density usually means higher ionic conductivity. In order to get high density samples, researchers preferred to choose high pressure technology such as cold or hot isostatic pressing methods. Instead, this work provides a novel method from a different scope. The LLZO powder was just compacted by vibration before sintering. No pressing operations were employed during the whole preparation process. An autoconsolidation mechanism was proposed for the consolidation of LLZO particles. Due to the surface tension of liquid Li2O at sintering temperature, LLZO particles could aggregate together freely and automatically. The preparation process for dense LLZO is greatly simplified. A dense tetragonal LLZO with high relative density about 93% was prepared successfully by this auto-consolidation method. And no voids were observed from the SEM images. At 30 C, the total conductivity is about 5.67 105 S cm1, which is the highest one for tetragonal LLZO in the reported issues. The activation energy for total conductivity is about 0.35 eV atom1 at 30e120 C, slightly lower than the previous reported values. By the way, autoconsolidation method has been applied to prepare doped cubic LLZO successfully in our further study. This work sheds light on the understanding of the consolidation mechanism for solid electrolytes and suggests a reliable route to syhthesize cemanic electrolytes, which could also be prepared by sintering technology. Acknowledgments This work is financially supported by the National Key Research and Development Plan of China (No. 2016YFB0901503). References [1] Z.F. Yow, Y.L. Oh, W. Gu, R.P. Rao, S. Adams, Effect of Liþ/Hþ exchange in water treated Ta-doped Li7La3Zr2O12, Solid State Ion. 292 (2016) 122e129. [2] S. Kumazaki, Y. Iriyama, K.-H. Kim, R. Murugan, K. Tanabe, K. Yamamoto, T. Hirayama, Z. Ogumi, High lithium ion conductive Li7La3Zr2O12 by inclusion of both Al and Si, Electrochem. Commun. 13 (2011) 509e512. [3] R. Murugan, W. Weppner, P. Schmid-Beurmann, V. Thangadurai, Structure and lithium ion conductivity of bismuth containing lithium garnets Li5La3Bi2O12 and Li6SrLa2Bi2O12, Mater. Sci. Eng. B 143 (2007) 14e20. [4] R. Murugan, V. Thangadurai, W. Weppner, Fast lithium ion conduction in garnet-type Li(7)La(3)Zr(2)O(12), Angew. Chem. 46 (2007) 7778e7781. [5] Y. Kim, H. Jo, J.L. Allen, H. Choe, J. Wolfenstine, J. Sakamoto, G. Pharr, The effect of relative density on the mechanical properties of hot-pressed cubic Li7La3Zr2O12, J. Am. Ceram. Soc. 99 (2016) 1367e1374. [6] M. Bitzer, T. Van Gestel, S. Uhlenbruck, B. Hans Peter, Sol-gel synthesis of thin solid Li7La3Zr2O12 electrolyte films for Li-ion batteries, Thin Solid Films 615 (2016) 128e134. [7] Y. Li, Z. Wang, Y. Cao, F. Du, C. Chen, Z. Cui, X. Guo, W-doped Li7La3Zr2O12 ceramic electrolytes for solid state Li-ion batteries, Electrochim. Acta 180 (2015) 37e42. [8] Y. Zhang, F. Chen, R. Tu, Q. Shen, L. Zhang, Field assisted sintering of dense Alsubstituted cubic phase Li7La3Zr2O12 solid electrolytes, J. Power Sources 268 (2014) 960e964. [9] H. Katsui, T. Goto, Preparation of cubic and tetragonal Li7La3Zr2O12 film by metal organic chemical vapor deposition, Thin Solid Films 584 (2015)
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A novel method for preparation of high dense tetragonal Li7La3Zr2O12 Pengcheng Zhao, Yuehua Wen, Jie Cheng, Gaoping Cao*, Zhaoqing Jin, Hai Ming, Yan Xu, Xiayu Zhu Research Institute of Chemical Defense, Beijing Key Laboratory of Advanced Chemical Energy Storage Technology and Materials, Beijing 100191, China
h i g h l i g h t s High dense tetragonal LLZO has been firstly prepared by auto-consolidation method. No pressing operations are employed for the preparation of LLZO. The prepared tetragonal LLZO has the highest lithium ion conductivity in present research. An auto-consolidation mechanism for LLZO particles is proposed and discussed. This work provides a new and very promising method for ceramic solid electrolytes.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 December 2016 Received in revised form 2 January 2017 Accepted 19 January 2017
For conventional preparation methods of Li7La3Zr2O12 (LLZO) solid state electrolytes, there is a stereotype that higher density always comes from higher pressure enforced upon the LLZO pellets. In this paper, a different way with an auto-consolidation mechanism is provided and discussed. No pressing operations are employed during the whole preparation process. Due to the surface tension of liquid melted Li2O at sintering temperature, LLZO particles could aggregate together freely and automatically. The preparation process for dense LLZO is greatly simplified. A dense tetragonal LLZO with high relative density about 93% has been prepared successfully by this auto-consolidation method. And there are no voids observed in the SEM images. At 30 C, the total conductivity is about 5.67 105 S cm1, which is the highest one for tetragonal LLZO in the reported issues, even two times higher than that prepared by hot-pressing method. The activation energy for total conductivity is ~0.35 eV atom1 at 30e120 C, slightly lower than the previous reported values. This work sheds light on the understanding of the consolidation mechanism for solid electrolytes and suggests a reliable route to syhthesize cemanic solid electrolytes. © 2017 Elsevier B.V. All rights reserved.
Keywords: Lithium ion battery Li7La3Zr2O12 solid state electrolyte Ionic conductivity Tetragonal structure
1. Introduction Lithium ion battery (LIB) is becoming the main storage device for commercial applications. However, its safety issues still haven't be resolved due to its liquid organic electrolytes which are volatile and flammable. All-solid-state LIB is viewed as a thorough solution for the safety issues [1]. Instead of liquid electrolytes, all-solid-state LIB employs solid electrolytes as both separators and ion conductors. Among current solid electrolyte materials, LLZO has attracted ever-increasing attention over the last few years due to its great advantages, such as high chemical stability with lithium metal
* Corresponding author. E-mail address: [email protected] (G. Cao). http://dx.doi.org/10.1016/j.jpowsour.2017.01.088 0378-7753/© 2017 Elsevier B.V. All rights reserved.
electrode, wide electrochemical window (0e5 V vs. Liþ/Li) and low bulk and grain boundary resistance [2e4]. LLZO was first reported by Murugan in 2007 [4]. Recent studies show that LLZO has two different structures, which are cubic and tetragonal, respectively [2e4]. For pure LLZO, cubic structure is unstable and tends to change into tetragonal structure at room temperature (RT). With doping of elements such as aluminum, niobium, wolfram or tantalum, cubic structure can be stabilized at RT with a high total conductivity between ~104 and ~103 S cm1 [5e11]. On the contrary, the total conductivity for tetragonal LLZO prepared by cold isostatic pressing (cold-pressing) method is about two orders lower, ~106 S cm1 with a relative density of 60e73% [10,12,13]. For solid electrolytes, higher density usually means higher ionic conductivity. In 2012, Jeff Wolfenstine prepared tetragonal LLZO by hot isostatic pressing (hot-pressing) method
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[14]. The relative density of tetragonal LLZO was promoted to ~98% with a total conductivity of 2.3 105 S cm1. It is the highest reported value for tetragonal LLZO presently. The preparation of LLZO electrolytes can be divided into two main steps, which are the preparation of powders from raw materials and the preparation of pellets from powders subsequently. In the first step, LLZO powders usually are prepared by solid reaction or solution technology [5,6,8,15e18]. Then, in the second step, LLZO powders are pressed for consolidation before or during sintering. There is a stereotype for the preparation of LLZO pellets that higher density always comes from higher pressure enforced upon the LLZO pellets. But for cold-pressing method, even when the pressure was promoted to 800 MPa, the highest reported density for tetragonal LLZO is still as low as 73% presently [12,13]. And for both cold and hot isostatic pressing methods, the demands for preparation equipment involving high pressure are so hard to be satisfied for practical production. In this paper, a different way with an auto-consolidation mechanism was provided and discussed. No pressing operations were employed during the whole preparation process. The schematic illustration for the preparation of LLZO by auto-consolidation method is shown in Fig. 1. The preparation process for dense LLZO is greatly simplified. A dense tetragonal LLZO with a relative density about 93% was prepared successfully. We believe that the results of this work could facilitate the development of LLZO membrane technology and the auto-consolidation method can be effective for other ceramic solid electrolytes, which could also be prepared by sintering technology.
2. Experimental 2.1. Preparation for LLZO powder As shown in Fig. 1, LLZO powder was prepared by solid reaction and all the operations are conducted in air. Stoichiometric amounts of LiOH$H2O, La2O3, and ZrO2 precursor powders (All of the reagents are analytically pure from Sinopharm Chemical Reagent Co. Ltd) were ball-milled in a ZrO2 jar with ZrO2 balls filled with ethanol at 400 rpm min1 for 10 h. Commonly, an extra 10 wt% LiOH$H2O was added into the starting powders to compensate for the loss of Li during high-temperature sintering [4]. After ballmilling, the slurry was dried at 120 C for 10 h and calcined at 1150 C for 10 h in Al2O3 crucible. The heating rate from RT to 1150 C is 20 C/min. The primary LLZO powder was ball-milled again in a ZrO2 jar with ZrO2 balls at 500 rpm min1 for 10 h. After the second ball-milling, the primary powder was grinded by
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agate mortar and sieved by 100 meshes. Then tetragonal LLZO powder was prepared. 2.2. Preparation for LLZO pellet The LLZO powder was put in Al2O3 crucible and compacted by vibration. It was heated step by step. From RT to 900 C the heating rate is 10 C/min, followed by a heating rate of 1 C/min from 900 C to 1150 C. The powder was finally sintered to be bulk at 1150 C in air for 15 h. The dense LLZO bulk was sliced by a dimond saw and polished by 1000# diamond chip. A LLZO pellet with the dimension of 0.134 mm 1.740 cm 1.200 cm was prepared for characterization and test. 2.3. Characterization Malvern Mastersizer 2000 laser-diffraction particle size analyzer was employed to determine the particle size distribution of the LLZO powder before sintering. According to Archimedes principle, the density of LLZO bulk was determined from its weight and volume. The volume was determined by draining water. The relative density value was determined by dividing the bulk density by the theoretical density of tetragonal LLZO (~5.108 g cm3) [12]. X-ray diffraction (XRD, Bruker D8 advance X-ray diffractometer, Cu Ka radiation) was used to characterize the crystalline phases of the LLZO powders before and after sintering. The morphology of the sintered sample was characterized by scanning electron microscopy (Hitach S-4800 Field Emission SEM). Electrical conductivity measurements were performed on the LLZO pellet using two probes method. Au was sputter coated on the top and bottom surfaces of the sample (AujLLZOjAu). Impedance spectras were collected with an electrochemical interface (Solartron 1287, Solartron Analytical) and a frequency response analyzer (Solartron 1260, Solartron Analytical) under open-circuit voltage (OCV). The frequency range was from 1 MHz to 1 Hz and the ac amplitude was 30 mV. Temperature-dependent ionic conductivities were measured from the range of 30 Ce120 C. A constant dc voltage of 10 V was undertaken for 10 h between the two probes of AujLLZOjAu to determine electron conductivity. The electron conductivity was calculated from the constant voltage, end current and dimension of the sample. 3. Results and discussion the particle size distribution of LLZO powder is shown in Fig. 2. It indicates that the particle sizes of LLZO powder before sintering is
Fig. 1. Schematic illustration for the preparation of LLZO pellets by auto-consolidation method.
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Fig. 2. Particle size distribution of LLZO powder before sintering.
Fig. 3. Picture of the bulk (half) and slices of LLZO sample.
not uniform. The average particle size is about 10.668 mm with two main Gaussian distribution around ~0.5 mm and 9 mm. The LLZO powder can be viewed as two sizes particles with consecutive distribution and the amount of larger particles is several times to the amount of small ones. According to packing theories of particles [19]: 1) If the size of the particles is single, it will be hard to reach a high packing density. 2) For tow different sizes spheric particles, when the ratio between large particles and small particles is 7:3,
the packing density will be optimized. It means that consecutive particle sizes distribution is preferable to higher packing density, because small size particles can fill the interspace among large size particles. And the used particle size distribution of LLZO powder before sintering is in good agreement with the optimized packing theories above, so that the porosity of the packing powder will be minimized. The picture of LLZO bulk (half) and its slices is shown in Fig. 3. The volume of the LLZO bulk was determined to be 15.90 ml (75.5620 g). According to Archimedes principle, the density of LLZO bulk is ~4.7523 g cm3 and the relative density value is ~93%. The relative density value of LLZO is much higher than the previous reported value (60e73%) when prepared by cold-pressing method [12,13] and just a little lower than the reported value (98%) when prepared by hot-pressing method [14]. The SEM images of the surface of sintered sample are shown in Fig. 4. In Fig. 4a, it can be observed that the LLZO sample is built by different size particles gathering together. But in Fig. 4b there is no distinct boundaries between the particles. Different to hot-pressing samples, no voids are observed in the surface of the LLZO pellet. The result of SEM images is in good agreement with the high density measured above. Why so high dense LLZO could be gained from the sintering of powders directly without any other assist? At sintering temperature, Li2O is liquid and the liquid phase is accessible to the solidstate particles. So, the liquid would distribute among the surfaces of solid-state particles. The existing of surface tension from liquid phase would drive the solid-state particles to aggregate together instinctively. The consolidation mechanisms for different LLZO powders during sintering are shown in Fig. 5. Here, (a) cold-pressing refers to the samples pressed before sintering for conventional solid-state reaction; (b) hot-pressing refers to the samples pressed during sintering; (c) auto-consolidation refers to the samples neither pressed before or during sintering (provided by this paper). On the left of Fig. 5, the main forces around the particles are shown. Gravity of the particles is neglected in this case. On the right of Fig. 5, relative migration of the particles is shown. For simple, only three neighbor particles are chose to show the consolidation mechanisms for LLZO powders in line-dimension scope. For cold-pressing samples (pressed pellet), as shown in Fig. 5ae1, besides of surface tension from the liquid phase there are great inner stresses among the particles due to the distortion of the particles resulting from pressing process. In another word, the solid state particles are trapped. Only when the force of surface tension is
Fig. 4. SEM images of dense tetragonal LLZO pellet.
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Fig. 5. Consolidation mechanisms for LLZO powders during sintering: (a) cold-pressing, (b) hot-pressing, (c) auto-consolidation.
larger than inner stresses, can the particles gather together. As the common case shown in Fig. 5ae2, particle II in the center moves closer to particle I but broader to particle III relatively. Usually, the bulk volume of the sample shrinks little. When the pressure for cold-pressing is increased, for one side the distances between particles will become closer. But for the other side, the particles would be harder to move together due to the higher inner stress. As a result, the relative density will be as low as 60e73% [12,13] and very hard to be promoted further. For hot-pressing samples (pressing pellet), there is a large pressure enforcing the particles to move from one side to the other side, as shown in Fig. 5b. Both the inner stress and the surface tension can be neglected. As shown in Fig. 5be2, both particle II and particle III are pressed to particle I against the “wall”. The relative density is supposed to be much higher. But the particles can't moving freely so that the voids can't be let out freely either. It is hard to eliminate the voids thoroughly. As a result, even the relative density reaches to ~98% but there are still numerous visible voids in the SEM images [5,14,20]. In Fig. 5ce1, for auto-consolidation samples (packing powders), the main force around each particle is the surface tension from liquid phase. During sintering, the force of surface tension from liquid phase drives particles to aggregate together freely and automatically. As shown in Fig. 5ce2, both particles I and III move closer to particle II in the center. With the consolidation of solidstate particles, the bulk volume of the sample shrinks and the voids are minimized and eliminated finally. As shown in Fig. 4, no voids are observed in the SEM images. Additionally, there are three key points for the preparation of dense LLZO electrolytes. The first one is the calcination temperature for the preparation of LLZO powder which was promoted to 1150 C. So when the gas components (such as H2O, CO2 and other impurities) in the raw materials are being let out, LLZO particles are formed at the same time. The second one is the packing density of LLZO powder before sintering, which is related to the particle size distribution. The last and most important one is the sintering rules.
During the temperature range of sintering, the heating rate should be slower enough. Based on the three key points above, the autoconsolidation mechanism began to do its work and dense LLZO bulk was gained. The XRD patterns for LLZO samples are shown in Fig. 6. No second phases is observed in the XRD patterns. The peak splitting shown in Fig. 6c is indicative of LLZO with a tetragonal structure (space group I41/acd (No.142)) [12]. In Fig. 6b, the X-ray pattern of the calcined powder is similar to the curve of Fig. 6a, suggesting that no structural change occurred during sintering. The results of Fig. 6 are in agreement with the previous reported results that, with no impurities, only LLZO with tetragonal structure is stable at room temperature [2,10,12e14,21e25]. Rietveld refinement of the
Fig. 6. X-ray diffraction patterns for LLZO samples: (a) powder before sintering, (b) bulk after sintering, (c) tetragonal phase from Ref. [12].
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Fig. 7. (a) Nyquist plots for the total (bulkþgrain boundary) conductivity measured from 30 C to 120 C, and (b) Arrhenius plot for total conductivity of dense tetragonal LLZO sample (AujLLZOjAu) at heating and cooling process subsequently.
diffraction data for tetragonal LLZO using atom positions from Awaka et al. [12] yielded lattice parameters: a ¼ 13.122 Å, c ¼ 12.690 Å, with a unit cell volume of 2184.860 Å. These values are in good agreement with the values of a ¼ 13.134 Å and c ¼ 12.663 Å [12], a ¼ 13.129 Å and c ¼ 12.672 Å [26], and a ¼ 13.130 Å and c ¼ 12.688(3) Å [27] for tetragonal LLZO. The conductivity of LLZO sample was examined by AC impedance spectroscopy using Li-ion blocking Au electrode. Fig. 7a shows the impedance plots measured from 30 C to 120 C for AujLLZOjAu. At 30 C, the data separates into a high frequency region which contains two semicircles and a low frequency region which contains a spike. The semicircles are attributed to bulk resistance and grain boundary resistance separately and the intercept of the semicircle of higher frequency on the real Z axis gives the total ionic resistance (latticeþgrain boundary) [14]. This inclined curve is referred to the frequency dispersion phenomenon and can be described using capacitance or constant phase element (CPE) [28,29]. Each experimental data was fitted with an equivalent circuit consisting of Rtotal (R1CPE1) (R2CPE2) (R3CPE3), as shown in Fig. 7a. Rtotal refers to the total resistance of the sample. In Fig. 7a, the fitting lines are shown in solid type. Using the value of Rtotal and the sample dimensions, the total conductivity of the tetragonal LLZO sample was calculated. The total conductivity of tetragonal LLZO is examined to be ~5.67 105 S cm1. Although it is still much lower than the maximum value for cubic LLZO (~1 103 S cm1), as shown in Table 1, it is the highest reported value for tetragonal LLZO, about 10e500 times higher than previously reported values of conventional solid-state sintering and
even 2 times higher than that of hot-pressing. This vast improvement in total conductivity is a result of the high density and nonvoids LLZO samples. Fig. 7b shows the Arrhenius plots for tetragonal LLZO sample. In order to check the reversibility of the sample, the data was measured at heating and cooling process subsequently. From left to right, the spots refer to 120, 110, 100, 90, 80, 70, 60, 50, 40 and 30 C separately. The activation energy for total conduction was obtained by fitting the conductivity measurement to the Arrhenius equation [12]. The activation energy is determined from the slope of the ln(sT) versus 1/T plot for the total conductivity. According to Fig. 7b, the Ea-heating and Ea-cooling are nearly equal, ~0.35 eV atom1 for 30e120 C. However, the fitting line isn't a very fine straight at 30e70 C. The ion conductivities for both 60 C and 70 C at cooling process are slightly higher than those at heating process (this phenomenon has been checked by heating back after the cooling process). It indicates that there might be different lithium ion conductivity mechanisms for the sample in different temperature ranges. For tetragonal LLZO, three different Li sites have been identified and the distances between Li sites are 2.5 Å, 2.6 Å and 2.7 Å respectively [26]. The Li sites distribution would be distorted with the increasing of temperature and a tetragonal-cubic transition of LLZO has been observed at 100e150 C [30]. It could be implied that there might be a structure distortion and relaxation phenomenon at 60e70 C for the sample corresponding to different conduction pathway. The I-t curve for AujLLZOjAu at 10 V lasting 10 h is measured. The electron conductivity was determined by the constant voltage, end
Table 1 Relative density, conductivity and activation energy data for tetragonal LLZO electrolytes around RT. Preparation method Cold-pressing at 200 Cold-pressing Cold-pressing Cold-pressing at 100 Cold-pressing at 200 Cold-pressing Cold-pressing at 800 Cold-pressing at 800 Cold-pressing Cold-pressing at 300 Hot-pressing Auto-consolidation
MPa
MPa MPa MPa MPa MPa
Major sintering rules
Relative density (%)
Total conductivity (S cm1)
1050 C for 10 h, 1230 C for 30 h. 900 C for 5 h 900 C for 5 h 1140 C for 48 h 800 C for 20 h. 1100 C for 1 h 1100 C for 1 h 1130 C for 12 h 1200 C for 24 h 1050 C at 40 MPa pressure for 1 h 1150 C for 15 h
e e e e 60 e 73 66 e 91 98 93
1.0 2.3 3.1 3.2 4.0 4.4 1.3 5.2 2.0 1.0 2.3 5.7
107 107 107 107 107 107 106 106 106 105 105 105
Activation energy (eV atom1)
Reference
e 0.49 0.67 e 0.54 0.54 0.43 0.45 0.49 0.43 0.41 0.35
[30] [31] [32] [33] [12] [22] [13] [13] [10] [34] [14] This study
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current and the dimension of LLZO pellet. The value of the end current is ~1.17 106 A. The electron conductivity was calculated to be 7.51 109 S cm1. It indicates that the tetragonal LLZO is electron insulated. That is in good agreement with the AC impedance plots in Fig. 7a. The appearance of the tail at low frequencies in the case of ion blocking electrodes is an indication that the investigated material is ionically conducting in nature [4,14]. 4. Conclusion For LLZO solid electrolytes, higher density usually means higher ionic conductivity. In order to get high density samples, researchers preferred to choose high pressure technology such as cold or hot isostatic pressing methods. Instead, this work provides a novel method from a different scope. The LLZO powder was just compacted by vibration before sintering. No pressing operations were employed during the whole preparation process. An autoconsolidation mechanism was proposed for the consolidation of LLZO particles. Due to the surface tension of liquid Li2O at sintering temperature, LLZO particles could aggregate together freely and automatically. The preparation process for dense LLZO is greatly simplified. A dense tetragonal LLZO with high relative density about 93% was prepared successfully by this auto-consolidation method. And no voids were observed from the SEM images. At 30 C, the total conductivity is about 5.67 105 S cm1, which is the highest one for tetragonal LLZO in the reported issues. The activation energy for total conductivity is about 0.35 eV atom1 at 30e120 C, slightly lower than the previous reported values. By the way, autoconsolidation method has been applied to prepare doped cubic LLZO successfully in our further study. This work sheds light on the understanding of the consolidation mechanism for solid electrolytes and suggests a reliable route to syhthesize cemanic electrolytes, which could also be prepared by sintering technology. Acknowledgments This work is financially supported by the National Key Research and Development Plan of China (No. 2016YFB0901503). References [1] Z.F. Yow, Y.L. Oh, W. Gu, R.P. Rao, S. Adams, Effect of Liþ/Hþ exchange in water treated Ta-doped Li7La3Zr2O12, Solid State Ion. 292 (2016) 122e129. [2] S. Kumazaki, Y. Iriyama, K.-H. Kim, R. Murugan, K. Tanabe, K. Yamamoto, T. Hirayama, Z. Ogumi, High lithium ion conductive Li7La3Zr2O12 by inclusion of both Al and Si, Electrochem. Commun. 13 (2011) 509e512. [3] R. Murugan, W. Weppner, P. Schmid-Beurmann, V. Thangadurai, Structure and lithium ion conductivity of bismuth containing lithium garnets Li5La3Bi2O12 and Li6SrLa2Bi2O12, Mater. Sci. Eng. B 143 (2007) 14e20. [4] R. Murugan, V. Thangadurai, W. Weppner, Fast lithium ion conduction in garnet-type Li(7)La(3)Zr(2)O(12), Angew. Chem. 46 (2007) 7778e7781. [5] Y. Kim, H. Jo, J.L. Allen, H. Choe, J. Wolfenstine, J. Sakamoto, G. Pharr, The effect of relative density on the mechanical properties of hot-pressed cubic Li7La3Zr2O12, J. Am. Ceram. Soc. 99 (2016) 1367e1374. [6] M. Bitzer, T. Van Gestel, S. Uhlenbruck, B. Hans Peter, Sol-gel synthesis of thin solid Li7La3Zr2O12 electrolyte films for Li-ion batteries, Thin Solid Films 615 (2016) 128e134. [7] Y. Li, Z. Wang, Y. Cao, F. Du, C. Chen, Z. Cui, X. Guo, W-doped Li7La3Zr2O12 ceramic electrolytes for solid state Li-ion batteries, Electrochim. Acta 180 (2015) 37e42. [8] Y. Zhang, F. Chen, R. Tu, Q. Shen, L. Zhang, Field assisted sintering of dense Alsubstituted cubic phase Li7La3Zr2O12 solid electrolytes, J. Power Sources 268 (2014) 960e964. [9] H. Katsui, T. Goto, Preparation of cubic and tetragonal Li7La3Zr2O12 film by metal organic chemical vapor deposition, Thin Solid Films 584 (2015)
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