Ionic Conductivity and Air Stability of Al-Doped Li7La3Zr2O12

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Ionic conductivity and air stability of Al-doped Li7La3Zr2O12 sintered in alumina and Pt crucibles Wenhao Xia, Biyi Xu, Huanan Duan, Yiping Guo, Hongmei Kang, Hua Li, and Hezhou Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12186 • Publication Date (Web): 09 Feb 2016 Downloaded from http://pubs.acs.org on February 16, 2016

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ACS Applied Materials & Interfaces

Ionic conductivity and air stability of Al-doped Li7La3Zr2O12 sintered in alumina and Pt crucibles Wenhao Xia, Biyi Xu, Huanan Duan*, Yiping Guo, Hongmei Kang, Hua Li, Hezhou Liu* State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P.R. China Corresponding authors: *E-mail: [email protected] *E-mail: [email protected]

Abstract Li7La3Zr2O12 (LLZO) is a promising electrolyte material for all-solid-state battery due to its high ionic conductivity and good stability with metallic lithium. In this article, we studied the effect of crucibles on the ionic conductivity and air stability by synthesizing 0.25Al doped LLZO pellets in Pt crucibles and alumina crucibles, respectively. The results show that the composition and microstructure of the pellets play important roles influencing the ionic conductivity, relative density, and air stability. Specifically, the 0.25Al-LLZO pellets sintered in Pt crucibles exhibit a high relative density (~96%) and high ionic conductivity (4.48 × 10-4 S cm-1). The ionic conductivity maintains at 3.6 × 10-4 S cm-1 after 3-month air exposure. In contrast, the ionic conductivity of the pellets from alumina crucibles is about 1.81 × 10-4 S cm-1 and drops to 2.39 × 10-5 S cm-1 three months later. The large grains and the reduced grain boundaries in the pellets sintered in Pt crucibles are favorable to obtain high ionic conductivity and good air stability. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy results suggest that the formation of Li2CO3 on the pellet surface is probably another main reason, which is also closely related to the relative density and the amount of grain boundary within the pellets. This work stresses the importance of synthesis parameters, crucibles included, to obtain the LLZO electrolyte with high ionic conductivity and good air stability. Keywords: solid electrolyte; lithium garnet; relative density; ionic conductivity; grain boundary; air stability 1. Introduction Lithium garnet is an alternative to traditional organic liquid electrolyte for Li ion battery (LIBs) due to the high conductivity (~10-4 S cm-1), good thermal and chemical stabilities.1-2 In 2007, Murugan et al. reported the synthesis of Li7La3Zr2O12 (LLZO), a fast lithium garnet, with excellent chemical stability against Li metal, large electrochemical stability window (6V) at room temperature.2 Later, it was revealed that the garnet type structure of LLZO exists in two polymorphs: tetragonal structure (I41/amd) stable at low temperature and cubic structure (Ia-3d) stable at high temperature; and the conductivity of the former is two magnitudes lower than that of the latter.3 Therefore, how to obtain garnet with the nominal composition of “Li7La3Zr2O12” and cubic structure has been attracting much attention. It has been suggested that supervalent cations can stabilize the cubic structure at room temperature by introducing Li vacancies and/or decreasing the Li content.4 In fact, stabilization of cubic LLZO through unintentionally incorporating Al by a reaction between the molten precursors (Li2CO3 or LiOH) and the alumina crucible wall during the heat treatment was first reported in 2011.5 Since then, a series of Al-doped LLZO was synthesized. Besides, many researches focused on

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doping various cations (Fe3+, Ga3+, Nb4+, Ta5+, Te6+ etc) to stabilize cubic LLZO and obtained high ionic conductivities in the range of 1-10.2 × 10-4 S cm-1 at room temperature.6-10 High relative density is another crucial factor for LLZO to improve the ionic conductivity. For example, hot-pressing has been used to increase the relative density up to 96-98%, and high conductivity (8.7 × 10-4 S cm-1) was obtained.11-12 Guo et al. achieved high density of 96% and excellent conductivity of 7.4 × 10-4 S cm-1 by sintering lithium garnets in flowing oxygen.13 Other technologies such as field assisted sintering and spark plasma sintering have been adopted to prepare high-conductivity LLZO.14-15 All of these efforts improve the density of LLZO at the expense of high production cost, which is unsuitable for mass production. Besides relative density, microstructure including grain size and grain boundaries is vital to prepare desirable LLZO. Y. Jin et al. pointed out that grain boundaries were high interfacial energy regions for the reactions between LLZO and moisture, CO2 in the air; the reactions resulted in a drastic decrease of total conductivity of LLZO after one-week air exposure.16 Recently, L. Cheng et al., from the perspective of precursors, reported an elegant work to investigate the effects of grain size and surface composition on the air-stability of LLZO pellets by soft X-ray absorption and Raman spectroscopy, and suggested that LLZO with small grains and large amount of grain boundaries was more air stable.17-18 However, the relative density of the sintered pellets is merely ~92%, which may be responsible for the mediocre room-temperature ionic conductivity (2~2.5× 10-4 S cm-1) and air instability. Herein, we, from the perspective of synthesis, study the effect of crucible on the ionic conductivity and air stability of LLZO prepared through a conventional solid state reaction route. Various characterization techniques have been used to study the correlation between the chemical composition, the relative density, the amount of grain boundary, the ionic conductivity, and the surface reactions in the air atmosphere for the LLZO samples sintered in alumina crucibles and Pt crucibles. The results emphasize the benefits of Pt crucibles—preventing unintentional Al contamination from alumina crucibles and suppressing the excessive Li loss during high temperature sintering. By using Pt crucibles, we obtained well-densified LLZO with high relative density (96%), high conductivity (4.48 × 10-4 S cm-1), and good air stability. 2. Experimental 2.1 Sample preparation LLZO was synthesized by a conventional solid-state reaction route based on previous publications.19 All the chemicals—unless otherwise mentioned—were from Sinopharm Chemical Reagent Co. Ltd., China. Li2CO3 (Aladdin reagent, 99.7 %), La2O3 (99.95 %), ZrO2 (99.7 %), and fine-grained Al2O3 (≥ 99.7 %) powders were used as raw materials. The La2O3 and Al2O3 powders were heated in alumina crucibles at 900 °C for 12 h before use to remove residual moisture. The materials were mixed with the molar ratio of Li:La:Zr to be 7.7:3:2. A proper amount of Al2O3 was added to obtain a doping level of 0.25 mol of Al in one unit formula of LLZO (0.25Al-LLZO). The starting materials were ball milled at 200rpm in PTFE jars with zirconia balls in the media of 2-propanol (≥ 98%) for 12 h. The resulting mixture was heated at 900 oC for 12h, reground for another 12h, and then dried at 70 oC for 12h. Subsequently, the obtained powders were pressed into pellets with a diameter of 12 mm; the


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pellets were covered with the same mother powder (prepared at 900 °C for 12 h) and were sintered at 1200 °C for 24 hours in different crucibles. For comparison, pellets with same composition were prepared using the same heating schedule in alumina (Shanghai Shuocun Machine Electricity Hardware Limted Company, ≥99.99 %) and Pt (Tianjin Lucheng Technology Metal Manufacture, ≥99.95 %) crucibles. 2.2 Smaple characterization The microstructure of the samples was characterized by X-ray diffraction (XRD, D/MAX255ovl/84, Rigaku, Japan) with copper Kα radiation. For the air-aged samples, the top surface being x-rayed was exposed to air the entire time. The morphology of the sintered LLZO was observed with scanning electron microscopy (SEM, Sirion 200, FEI, USA). For the air-exposed samples, the pellets were fractured first and the cross section was aged in air for a period of time. The pellets densities were assessed from the ratio between their mass and geometric volume. Composition analysis of the samples was conducted by inductively coupled plasma atomic emission spectroscopy (ICP-OES, iCAP6000, Thermo, USA). The X-ray photoelectron spectroscopy (XPS, AXIS UltraDLD, Kratos, Japan) was carried out to identify the spectra of C 1s, O 1s, Li 1s, Zr 3d and La 3d at the top surface of LLZO after exposing in air for 3 months. The Raman spectroscopy mapping (RAM, Senterra R200-L, Bruker Optics, Germany) were performed to study the distribution of Li2CO3 on the surface of the pellets exposed in air for some time using 532nm laser. The ionic conductivity was determined using an impedance analyzer (Solartron, 1260) with frequency ranging from 1 Hz to 10 MHz at AC amplitude of 10 mV. The Ag paste was coated onto both sides of the finely polished pellets and dried at 175 °C for 2 h before test. For the air-exposed samples, the Ag paste was not removed during the air exposure. The total conductivity values σ were calculated by applying the equation σ = L/(A×R) where L is the thickness of the pellets, A is the area of the pellets and R is the total resistance of the pellets. The pellets were evaluated at temperature from 25 °C (room temperature, RT) to 90 °C with an interval of 10 °C. The activation energy was determined based on the Arrhenius equation. 3. Results and discussion 3.1 Microstructure and chemical composition analysis



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Figure 1. XRD patterns of the LLZO pellets sintered in Al2O3 and Pt crucibles before and after 3-month air exposure. Figure 1 shows the X-ray diffraction patterns of the pellets sintered in Al2O3 and Pt crucibles before and after 3-month air exposure. As shown in Figure 1, the XRD patterns of as-synthesized LLZO pellets sintered in Al2O3 and Pt crucibles match well with the standard pattern known as cubic garnet phase Li5La3Nb2O12 (PDF45-0109), which means that the pure cubic garnet phase was obtained in both crucibles. However, 3 months later, a minor impurity phase associated with Li2CO3 shows up for the pellets sintered in alumina crucibles. Figure 1 also shows that there is a relative peak intensity change for the LLZO pellets sintered in Al2O3 crucibles after 3-month air exposure, which can be attributed to the ionic exchange of Li+/H+ and/or the loss of Li during the air exposure.20,21 The interaction between the LLZO pellets and the moisture and CO2 in ambient atmosphere is discussed in more details in Section 3.4. Table 1. Composition of 0.25Al-LLZO sintered in Al2O3 and Pt crucibles by ICP-OES Elements

Al

La

Li

Zr

Pt

Wight ratio of pellet in Al2O3 crucibles Atomic ratio* Wight ratio of pellet in Pt crucibles Atomic ratio*

1.97 0.63 0.75 0.25

47.60 2.97 46.01 3

4.53 5.65 4.82 6.27

19.85 1.88 19.81 1.96

0.78 0.036

* Normalized to 12 O based on atomic ratios from ICP-OES results and charge balance.

The microstructure is closely related to the chemical composition of the LLZO pellets, which was analyzed by ICP-OES measurement and the results were listed in Table 1. It is interesting to note that the Al content for the LLZO samples sintered in Al2O3 crucibles is as


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high as 0.63 mol per unit formula and there is no evidence of Al impurity phases (e.g. LaAlO3) in Figure 1 with this high level of Al doping. The reasons for this absence of Al impurity phases are speculated to have two folds. This first one is related to the Al dopant content and the solubility limit of Al in cubic LLZO that may be related to the sintering temperature. Rangasamy et al.11 reported that LaAlO3 phase was observed in the XRD pattern for the LLZO samples sintered at 1000 °C when the content of Al was higher than 0.389 mol; the reason for the impurity formation is that the Al content exceeded the solubility limit of Al in cubic LLZO at 1000 °C. In the present work, the sintering temperature is 1200 °C, which may increase the solubility limit of Al in cubic LLZO and lead to the absence of impurity phases (e.g. LaAlO3) even when the content of Al is 0.63 mol per Li7La3Zr2O12. The second reason may be related to the Li concentration. Duvel’s work22 showed that the XRD pattern of Al-doping LLZO (Li7−3xAlxLa3Zr2O12) will have γ-LiAlO2 and LaAlO3 phases with x above 0.6, and these impurity phases were effectively suppressed when 1.25x Li2O was added. Table 2 shows that the Li contents are not less than 5.65 per unit formula, which helps suppress the Al impurity phases. Table 2. The influence of crucibles on the properties of 0.25 Al-LLZO

Sintered in Al2O3 crucibles (Pristine) Sintered in Al2O3 crucibles (3 months later) Sintered in Pt crucibles (Pristine) Sintered in Pt crucibles (3 months later)

Ion conductivity (S cm-1)

Relative density

Lattice parameter (Å)

Activation energy (eV)

1.81×10-4

89%

12.945

0.37

2.04×10-5

89%

12.985

0.37

4.48×10-4

96%

12.967

0.32

3.06×10-4

96%

12.969

0.32

Table 2 summarizes the lattice parameters of the pellets. The lattice parameter of the LLZO pellets sintered in alumina crucibles is 12.945 Å, which is smaller than those sintered in Pt crucibles (12.976 Å). The decrease can be attributed to Al doping from the alumina crucible to the garnet lattice; replacing Li+ by Al3+ will shrink the LLZO framework since the ionic radius of Al3+ (0.53 Å) was smaller than that of Li+ (0.76 Å).23 After 3 months exposure, the cell parameters of pellets sintered in alumina crucibles increased from 12.945Å to 12.985 Å. The cell expansion may imply the Li+/H+ exchange in LLZO, which is a replacement of strong Li-O bonds by weak hydrogen O-H…O bonds.24-25 Interestingly by contrast, the cell parameters of pellets sintered in Pt crucibles retained essentially unchanged after exposing, indicating that the samples were more chemically stable in air and the Li+/H+ exchange did not occur to the same extent as for the pellets sintered in alumina crucibles. 3.2 Morphology analysis



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Figure 2. SEM images of (a) as-synthesized 0.25Al-LLZO, (b, c) 0.25Al-LLZO after exposed to air for 3 months sintered in Al2O3 crucibles; (d) as-synthesized 0.25Al-LLZO, and (e, f) 0.25Al-LLZO after exposed to air for 3-month sintered in Pt crucibles. Figure 2 shows typical SEM images of the 0.25Al-LLZO samples synthesized in different crucibles before and after 3-month air exposure. Clearly in Figure 2d, pellets sintered in Pt crucibles are compact with high relative density (~96 %); little grain-boundary and porosity are observed. Similar morphology was observed by other groups by applying hot-pressing, field assisted sintering technology, and so on.11-12, 26 In contrast, samples sintered in alumina crucibles exhibit nonuniform grains with clear grain boundaries and pores, consistent with the lower relative density (~89 %) (Figure 2a). The lower density in Al2O3 crucibles can be attributed to the fact that excessive Al coming from crucibles lead to loss of Li during high temperature sintering (as shown in Table 1), which will leave pores during grain growth. After 3 months of air exposure, a rough reaction layer was observed on the cross section of the samples sintered in alumina crucibles (Figure 2b and Figure 2c). For the samples sintered in Pt crucibles, on the other hand, the morphology change is much less significant. This difference implies that the morphology and relative density play important roles in the formation of impurity phases with the effect of air. It has been reported that the reaction layer


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preferred to grow along the grain boundaries, due to the fact that the grain boundaries have higher interfacial energy than the grains.16 3.4 Electric property



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Figure 3. (a) Impedance spectra measured at room temperature for 0.25Al-LLZO samples sintered in Al2O3 crucibles and Pt crucibles. (b) Arrhenius plots of total conductivities for 0.25Al-LLZO samples sintered in alumina crucibles and Pt crucibles.

To evaluate the impact of crucibles on the ion conduction property, AC impedance spectroscopy was employed. Figure 3 compares typical Nyquist profiles for 0.25Al-LLZO sintered in alumina and Pt crucibles. The solid line represents the fit to the experimental data based on the equivalent circuit consisting of (RbCPEb) (RgbCPEgb) (CPE) elements using the ZVIEW program, where R is the resistance, CPE is the constant phase element, and the subscript b and gb refer to the bulk and grain-boundary contribution, respectively. As shown in Figure 3a, one suppressed semicircle is observed. The suppressed semicircle at high frequency is attributed to the total resistance (grain boundary and bulk) of the sample. Compared with the impedance profile of LLZO sintered in alumina crucibles, the semicircle in the high frequency has similar shape but a smaller diameter for LLZO sintered in Pt crucibles. This suppressed shape is consistent with previous results of high density LLZO with low grain boundary impedance.12, 27-28 The extracted conductivity data are listed in Table 2. Clearly, the Li+ conductivity of pellets from Pt crucibles is 4.48 × 10-4 S cm-1, which is nearly 2.5 times than that (1.81 × 10-4 S cm-1) from alumina crucibles. In addition, the activation energies of the pellets was determined from 25 oC to 90 oC based on the Arrhenius equation to be 0.32 eV and 0.37 eV for Pt crucibles and alumina crucibles, respectively, which are in the range of reported values.2, 13, 29 The high ionic conductivity and low activation energy of the pellets from Pt crucibles may be attributed to three inter-connected factors: chemical composition, improved density, and doping effect. Firstly, as shown in Table 1, the content of Li ions in the pellets sintered in Pt crucibles (4.82 wt%) is higher than that from alumina crucibles (4.53 wt%). Generally speaking, increasing lithium content in the lithium garnet results in an improvement in the occupational and positional disorder along with the enhancement in Li+ conductivity. From an analysis of trajectory and topology, Ceder et al. predicated the increased concentration of Li will destabilize the lithium ions and push them into higher energy sites allowing for more lithium diffusion to occur.30 Based on neutron diffraction observation, Xie et al. further suggested that optimum Li+ concentration required to achieved the maximum Li+ conductivity in LixA3B2O12 garner is around x = 6.4 ± 0.1 and x = 7.5 is the upper limit for x that lithium can be tolerated in LixA3B2O12.31 As shown in Table 1, the Li content in the derived formula (Li6.27La3Al0.25Zr1.96Pt0.036O12) for 0.25Al-LLZO sintered in Pt crucibles is more suitable for high conductivity LLZO comparing to Li5.65La2.97Al0.63Zr1.88O12 in alumina crucibles. The excess loss of Li is closely associated to the excessive substitution by Al that comes from alumina crucibles. Indeed as shown in Table 1, the Al contents in the pellets synthesized in alumina crucibles and Pt crucibles were 1.97 and 0.75 wt%, respectively. Obviously, much more Al3+ entered into the garnet pellet synthesized in alumina crucibles compared with Pt crucibles. The excess replacement of Al3+ with 3 Li+ will decrease the Li content in garnet and the excess Al3+ ion at Li+ sites will partially block the Li+ diffusion routes at the same time. It has been shown that Al dopants originating from the alumina crucibles can be included in the garnet structure by residing tetrahedral 24d sites, sharing that sites with lithium.32 The


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immobile Al3+ ions on 24d sites of the transport network block the highly accessible Li+ ion diffusion pathways (24d-96h-48g-96h-24d), resulting in low Li+ ion conductivity. Secondly, increasing density and reducing grain boundary are beneficial to increase the ionic conductivity of LLZO. As shown in Figure 2, the pellets were well sintered in Pt crucibles with a relative density of 96% and the amount of grain boundary was much less than that in alumina crucibles. The reducing grain boundary resulted in a low grain-boundary resistance, which in turn contributed to a high total ionic conductivity.28 While a great amount of grain boundary in the LLZO sintered in alumina crucibles contributed to a low ionic conductivity. Thirdly, the doping effect of Pt may be another reason. As shown in Table 1, the content of Pt in the pellets sintered in Pt crucibles is 0.78 wt%. The result suggests that a small amount of Pt entered into the pellets sintered in Pt crucibles, which is in accord with the previous results.33 G. Ceder et.al. suggested that Pt4+ may act as an isovalent dopant for the Zr sites, and the Pt4+ (Zr) defect energy was only 0.27 eV, smaller than the defect energy of Ta5+ (Zr) (0.94 eV) and Nb5+ (Zr) (0.92 eV).34 Li-ion conductivity of LLZO enhances with the Zr sites doping, such as Ta5+ doping and Nb5+ doping.8, 29 Hence, we conclude that the Pt crucibles is helpful to control the unintentional incorporation of Al into the structure, suppress Li loss, gain high relative density, and have Pt doping effect. 3.4 Air stability



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Figure 4. (a) Impedance spectra of the as-sintered pellets and the pellets exposed to air for a week; (b) the change of ionic conductivity of LLZO sintered in Pt and alumina crucibles over time. A comparative impedance study of the pellets synthesized in alumina crucibles and Pt crucibles after exposing in ambient air was performed. As shown in Figure 4a, the AC impedance plots of the pellets sintered in Pt crucibles are almost overlapping before and after 1-week exposure. In contrast, for the pellets sintered in Alumina crucibles, impedance was found to be increasing after 1-week exposure. The extracted conductivity data are compiled in Figure 4b. Obviously, 3 months later, the conductivity of the pellets from Pt crucibles declined from 4.48 × 10-4 S cm-1 to 3.06 × 10-4 S cm-1; nevertheless, a sharp decrease from 1.81 × 10-4 S cm-1 to 2.04 × 10-5 S cm-1 was observed for the pellets sintered in alumina crucibles. It is noted that the pellets for AC impedance spectroscopy were coated with silver contacts the whole time during air exposure, so we can expect that the conductivity drop will be more significant without the silver paste. This conductivity difference may be attributed to the difference of microstructure and phase composition of these pellets. Compared with the pellets sintered in alumina crucibles, the pellets sintered in Pt crucibles were well densified and contained less grain boundaries (Figure 2). The reactions between LLZO and H2O, CO2 prefer to occur at grain boundaries after air exposure.16 So, larger content of grain boundaries means more reactions taking place during air exposure. Considering that Li2CO3 is a high resistance phase that will hinder the Li+ transport and decrease the ion conductivity, more Li2CO3 leads to bigger decline of the ionic conductivity for the LLZO sintered in alumina crucibles. What’s more, the relative density will also affect the air-stability of LLZO. A compact structure could reduce the rate of Li+/H+ exchange, which will render the formation of LiOH and Li2CO3.35 It had been proved that Li+/H+ exchange occurred on the octahedral


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sites, implying less Li ions in garnet are mobile for ion conduction.36

Figure 5. (a) C 1s, O 1s, La 3d, Zr 3d, and (b) Li 1s XPS spectra collected from LLZO pellet surfaces sintered in Pt and alumina crucibles after 3-month air exposure. To further study the surface chemistry of the air-aged LLZO, XPS was carried out and the results are depicted in Figure 5. The spectra of C 1s, O 1s, Li 1s, Zr 3d and La 3d for the



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LLZO sintered in Pt and alumina crucibles confirm the formation of Li2CO3 on the surface of LLZO pellets after 3 months air exposure. Specifically, two C 1s peaks around 285 eV and 290 eV show up for the LLZO sintered in both alumina and Pt crucibles, which can be assigned to adventitious carbon37 and carbonate38, respectively. Compared with the pellets sintered in alumina crucibles, the C 1s peak of the carbonate for the pellets sintered in Pt crucibles is less strong. The peak of O 1s about 531 eV is attributed to the Li-O bond in LLZO. The peaks of La 3d and Zr 3d are not observed for both pellets, suggesting that the formation of Li2CO3 layer blocks the La and Zr photoelectron signals. As shown in Figure 5b, the peak of Li 1s for the LLZO sintered in alumina crucibles consist of two peaks at around 55.5 eV and 54.9 eV, which are associated with Li2CO3 and Li-O, respectively.37-38 In contrast, for the LLZO sintered in Pt crucibles, these two peaks are present too; but the area ratio of Li2CO3 to Li-O is much less, suggesting less Li2CO3 formation in the case of Pt crucibles.


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Figure 6. (a) Typical Raman spectra of the air-exposed 0.25Al-LLZO pellets. Raman mapping images of the air-exposed 0.25Al-LLZO sintered in (b) Pt crucibles and in (c) Al2O3 crucibles. The formation of Li2CO3 on the surface of the air-exposed pellets was analyzed by Raman spectra as shown in Figure 6a. The peaks at 156 cm-1, 192 cm-1, 711 cm-1 and 1100 cm-1 can be assigned to Li2CO339, suggesting the presence of Li2CO3 impurity. The distribution of the Li2CO3 was studied by Raman spectroscopy mapping of 10×10 µm2 areas on the surface of pellets. Figures 6b-6c show the integrated Raman signal between 1050 cm-1 and 1150 cm-1 covering the most intense peak of Li2CO3 at 1100 cm-1 (shaded area in Fig 6a). As shown in Figure 6b-6c, the distribution of Li2CO3 on the surface of pellets was not uniform, but on average much more Li2CO3 formed on the samples sintered in Al2O3 crucibles than in Pt crucibles, which is consistent with the XPS data. The formation of Li2CO3 on the surface of LLZO after long time air exposure was observed by other groups.36-37 It is generally believed that the formation of Li2CO3 may occur through two routs –the extraction of Li from the garnet structure and the Li+/H+ exchange–as expressed by the following reactions.36 Li7 La3Zr2O12 + xCO2 → Li7-xLa3Zr2O12-x + x Li2CO3 (1) Li7 La3Zr2O12 + x HO2 →Li7-xLa3Zr2O12-x + x LiOH (2) Both reactions are thermodynamically favorable. In equation (1), the LLZO interact with CO2 in air directly; in equation (2), the water vapor from the atmosphere might enter into the lattice and replace the lithium ions by protons to form O-H bonds, leading to the formation of LiOH. It is noted that the Li2CO3 maybe also came from the reaction between the LiOH formed on ion-exchange and CO2 in the air. In the present work, even though the impurity phase of Li2CO3 is detected by XPS and Raman, it is absent in the XRD data for the samples synthesized in Pt crucibles, which is probably because that the quantity is below the detection limit. 4. Conclusions The effects of crucibles on the ionic conductivity and air stability of Al-doped LLZO have been studied. The 0.25Al-LLZO pellets sintered in Pt crucibles exhibit high relative density (~96%), high ionic conductivity (4.48 × 10-4 S cm-1), and good stability in ambient air. XRD



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and ICP results suggest that the increased ionic conductivity is associated with the controlled amount of Al dopant and the desirable amount of Li content in the LLZO samples. Whereas for the samples sintered in alumina crucibles, the low ionic conductivity can be attributed primarily to the excessive Al dopant came from alumina crucibles, which leads to Li loss during high temperature sintering. Moreover, the LLZO pellets sintered in Pt crucibles maintain much higher ion conductivity than those in alumina crucibles after 3-month exposure in ambient air. The air stability is believed to be directly correlated with the formation of Li2CO3, as shown by XPS and Raman Spectroscopy. The differences in air stability are attributed to variations in the morphology: the 0.25Al-LLZO pellets sintered in Pt crucibles have higher relative density, larger grains and less grain boundaries than in alumina crucibles, which provide less high interfacial energy regions for the reactions between LLZO and water, CO2 in the air. These results indicate that the synthesis parameters, crucibles included, have profound influence on the ionic conductivity and air stability of LLZO pellets. Acknowledgements This work was supported by the Natural Science Foundation of China (no. 11304198) and SMC-Chen Xing Young Scholar Award of SJTU. Instrumental Analysis Center of Shanghai Jiao Tong University and National Engineering Research Center for Nanotechnology are gratefully acknowledged for assisting with relevant analyses.

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