Transparent cubic garnet-type solid electrolyte of Al2O3-doped Li7La3Zr2O12

Solid State Ionics 278 (2015) 172–176

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Transparent cubic garnet-type solid electrolyte of Al2O3-doped Li7La3Zr2O12 Yosuke Suzuki a,c,⁎, K. Kami a, K. Watanabe b, A. Watanabe b, N. Saito b, T. Ohnishi b, K. Takada b, R. Sudo c, N. Imanishi c a b c

Denso Corporation, 500-1 Minamiyama, Komenoki-cho, Nisshin, Aichi 470-0111, Japan National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Graduate School of Engineering, Mie University, 1577 Kurimamachiya-cho, Tsu, Mie 514-8507, Japan

a r t i c l e

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Article history: Received 28 January 2015 Received in revised form 14 May 2015 Accepted 10 June 2015 Available online 24 June 2015 Keywords: Solid lithium ion conductor Garnet type Short-circuit Hot isostatic pressing

a b s t r a c t A transparent garnet-type lithium-ion conducting solid electrolyte of 1.0 wt% Al2O3-doped Li7La3Zr2O12 (A-LLZ) was prepared using hot isostatic pressing (HIP). The A-LLZ pellet sintered at 1180 °C for 36 h was followed by HIP treatment at 127 MPa and 1180 °C under an Ar atmosphere. The bulk conductivity of the HIP treated A-LLZ was 9.9 × 10−4 S cm−1 at 25 °C. The Li/HIP treated A-LLZ/Li cell showed no short-circuit due to lithium dendrite formation at 0.5 mA cm−2. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Developments in battery technology are increasingly required to address the problems associated with the expansion of energy demands in recent years [1]. Rechargeable batteries are an attractive energy storage system to achieve high energy conversion efficiency vehicles and to make efficient use of renewable energy from photovoltaic cells and wind power generation. In such applications, the capacity of batteries should be much larger than those for mobile phones and laptop computers. Safety, low cost and high energy density are also important issues for such large energy storage batteries. All-solid-state lithium batteries are expected to be safe for rigorous use because a nonflammable solid electrolyte is used instead of the flammable organic liquid electrolytes used in conventional lithium ion batteries. Various types of solid lithium ion-conducting electrolytes have been reported, such as a layered-type Li3N structure [2], Li2S-based glass [3], LISICON-type Li14Zn(GeO4)4 [4], Li10GeP2S12 [5], NASICON-type Li1+ xAxTi2−x(PO4)3 (A = Al, Fe, Sc, etc.) (LATP) [6], perovskite-type La2/3−xLi3xTiO3 (LLTO) [7], and garnet-type Li7La3Zr2O12 (LLZ) [8]. The highest room temperature lithium conductivity of 1.2 × 10−2 S cm− 1 was reported in Li10GeP2S12, which is higher than that of conventional organic liquid electrolytes. However, three types of high lithium ion conductivity solid electrolytes of LATP, LLTO and LLZ are not so sensitive to water in ⁎ Corresponding author at: Graduate School of Engineering, 1577 Kurimamachiya-cho, Mie University, Tsu, Mie 514-8507, Japan. Tel.: +81 561 75 1057. E-mail address: [email protected] (Y. Suzuki).

http://dx.doi.org/10.1016/j.ssi.2015.06.009 0167-2738/© 2015 Elsevier B.V. All rights reserved.

the atmosphere, while the other solid electrolytes are quite sensitive in atmosphere. LLZ is stable in contact with lithium metal and has an electrical conductivity that was reported to be 2.7 × 10− 4 S cm−1 at 25 °C [8]. Therefore, LLZ is an attractive electrolyte for all-solid-state lithium batteries. The tetragonal phase of genuine LLZ is stable at room temperature [9] and transforms to a cubic structure at 645 °C [10]. The electrical conductivity of tetragonal LLZ is as low as 1.63 × 10−6 S cm−1 at 25 °C. The high-temperature cubic phase was stabilized at room temperature by the substitution of Al3 + for Li+ [11], and of Ta5+ [12] and Nb5+ [13] for Zr4+. Geiger et al. [14] reported that the cubic phase was stabilized at room temperature by sintering LLZ at high temperature for a long period, due to contamination with aluminum from the alumina crucible. Recently, Imanishi et al. reported electrochemical performances of LLZ with lithium metal electrode and the formation of lithium dendrites in a Li/doped-LLZ/Li cell [15–17]. Li/Li6.75La3Zr1.75Nb0.25O12/Li [15], Li/Li6.25La3Ta0.25Zr1.75O12/Li [16] and Li/0.5 wt% Al2O3-doped Li7La3Zr2O12/Li [17] cells showed shortcircuiting after polarization for 280, 100, and 1000 s at 0.5 mA cm−2 and 25 °C, respectively. The relative densities of Li6.75La3Zr1.75Nb0.25O12, Li6.75La3Zr1.75Ta0.25O12, and 0.5 wt% Al2O3-doped Li7La3Zr2O12 are 92.8%, 96.7% and 93.7%, respectively. There is no clear dependence of the short-circuit period on the relative density of LLZ; alternatively, the lithium ion diffusion kinetics at the grain boundaries could be a key factor because Li6.75La3Zr1.75Ta0.25O12 with a high grain boundary conductivity of 2.5 × 10−3 S cm−1 at 25 °C and the highest relative density of 96.7% had a shorter short-circuit period than Li6.3La3Zr1.3Ta0.7O12 with a lower grain boundary conductivity of 1.37 × 10−3 S cm−1 at 25 °C and a

Y. Suzuki et al. / Solid State Ionics 278 (2015) 172–176

lower relative density of 96.0% [16]. Therefore, the grain boundary of LLZ with high lithium ion conductivity should be reduced to suppress the formation of lithium dendrites during lithium deposition on lithium at high current density. In this study, we have attempted to prepare grain boundary-free LLZ to avoid ionic diffusion and growth of the dendrite along the grain boundary. High sintering temperature and a long sintering period are required to prepare grain boundary-free high-density LLZ using conventional solid state reactions. However, at high sintering temperatures, it is difficult to obtain a dense pellet of LLZ due to lithium evaporation. Hot isostatic pressing (HIP) is a useful technique to obtain a dense pellet [18–20]. Closed pores typically disappear following HIP treatment because the pellet is isostatically pressed under high gas pressure that is beneficial to suppress gasification of chemical species in the substance. In this study, an attempt was made to prepare dense cubic LLZ pellets by HIP treatment of Al2O3-doped LLZ sintered at 1180 °C for 36 h and examined its electrochemical performance. 2. Experimental The LLZ precursor was prepared by solid state reaction using Li2CO3, La2O3 and ZrO2 (Nacalai Tesque, 99.9%). La2O3 was dried at 1000 °C for 1 h. The molar ratio of Li:La:Zr was controlled to 7.7:3:2. Excess Li was added to compensate for the expected Li loss during high-temperature heat treatment. The starting materials were ball-milled with a zirconia vessel and balls using a high-energy mechanical mill (Fritsch Planetary Micro Mill) and the sample powder was isostatically pressed into pellets at 150 MPa and then calcined at 900 °C for 5 h. The reaction products were ball-milled again and the sample powder was isostatically pressed into pellets and calcined at 950 °C for 12 h. The calcined pellets were then ball-milled with γ-Al2O3 (Alfa) and pressed into pellets that were then placed in alumina crucibles, covered with the mother powder (prepared at 950 °C for 12 h), and sintered at 1180 °C for 36 h under an air atmosphere. The obtained Al2O3-doped Li7La3Zr2O12 (A-LLZ) pellets were subjected to HIP treatment of 127 MPa at 1160 °C in Ar atmosphere for 2 h using zirconia crucibles (Kobelco O2-Dr.HIP). In HIP treatment, a crucible with a sample is put into a chamber, and then the chamber is subjected to high temperature and high pressure. To prevent contamination from the zirconia crucible, HIP treatment was performed for two LLZ pellets in the following way; one pellet was in contact with the crucible and the other was placed on the pellet. The upper pellet was used for characterization and further study. X-ray diffraction (XRD) patterns were obtained using a Rigaku RINT 2500 diffractometer with a rotating copper cathode. The electrical conductivities of the sintered samples (10 mm diameter and 0.8–0.5 mm thick) with sputtered copper electrodes were measured using an impedance gain-phase analyzer (Solartron 1260) in the frequency range of 0.1 Hz to 1 MHz with a voltage amplitude at 10 mV. ZView software was employed for data analysis and presentation of the impedance results. Direct current measurements of the Li/LLZ/Li cells were performed using a multichannel potentio-galvanostat (Bio-Logic Science Instruments VMP3). Laser beam microscopy (Keyence VR-3000) and scanning electron microscopy (SEM; Jeol JSM-7000 F) were used to analyze the surface morphology of the pellets. Light transmittance measurements were conducted using a Jasco V-550 spectrophotometer.

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Fig. 1. XRD patterns of undoped LLZ and 1.0 wt% Al2O3-doped LLZ.

crucible, where the Al2O3 content was 1.16 wt%. The cubic phase of the undoped LLZ in the present work could be explained by the inclusion of aluminum from the alumina crucible. The relative densities of the undoped LLZ and 1.0 wt% A-LLZ samples were 89.3% and 91.5%, respectively. Lithium metal is the best anode candidate for high-energy density batteries because it has a high theoretical capacity and low negative potential. However, the formation of lithium dendrites during lithium deposition in a conventional liquid electrolyte limits the use of lithium metal as the anode for lithium batteries [22]. Monroe and Newman [23] predicted that if a homogenous solid electrolyte with a bending modulus of 6 GPa or more was obtained, then the lithium dendrite problem would be solved. The bending modulus of ceramic materials is generally higher than 6 GPa; therefore, no lithium dendrite formation would be expected between LLZ and lithium metal. The galvanostatic test was performed for the symmetric Li/A-LLZ/Li cell with a, 0.8-mm-thick garnet electrolyte. Fig. 2 shows a typical cell voltage change with the polarization period at 0.5 mA cm− 2 and 25 °C. The cell voltage gradually increased at the initial stage and then dropped suddenly to 0 V after 250 s polarization. Similar results were reported for another Al2O3-doped LLZ electrolyte [17]. The cell was disassembled and the A-LLZ pellet was immersed in water to remove the lithium metal from the surface. The A-LLZ surface on the cathode side was examined using SEM. Black spots that were not observed in the pristine A-LLZ (Fig.3a) were evident on the surface, as shown in

3. Result and Discussion Fig. 1 shows XRD patterns of the undoped (LLZ) and 1.0 wt% Al2O3-doped LLZ (A-LLZ) samples synthesized by solid state reaction at 1180 °C for 32 h. All XRD patterns of undoped LLZ and 1.0 wt% A-LLZ were assigned to the garnet-type cubic structure reported by Awaka et al. [21]. The nominal Li7La3Zr2O12 composition is in the tetragonal phase at room temperature [9]; however, Geiger et al. reported that a sample of the nominal Li7La3Zr2O12 composition sintered at 1100 °C overnight was in cubic phase due to aluminum contamination from the

Fig. 2. Cell voltage vs. polarization period for Li/1.0 wt% Al2O3-doped LLZ/Li at 0.5 mA cm−2 and 25 °C.

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Fig. 4. (a) Cross-sectional SEM image of 1.0 wt% Al2O3-doped LLZ and (b) cross-sectional laser beam microscope images of the HIP-treated 1.0 wt% Al2O3-doped LLZ. Fig. 3. SEM image of the cathode side surface of Li/1.0 wt% Al2O3-doped LLZ/Li (a) polished after sintering (b) after 250 s polarization at 0.5 mA cm−2.

Fig. 3b. Energy-dispersive X-ray spectroscopy (EDX) analysis of the black spot area showed that the compositional ratio of La, Zr, O, and Al was almost the same as that of the bulk. Chemical elements other than those previously mentioned could not be detected using the EDX spectrometer at the black spot area. The black spots may be the grain boundaries or voids that lithium dendrite grows through and become hollow after washing with water. The HIP treatment is effective for densification of the pellets with many closed pores because of the compression with a high-pressure gas at high temperature. Fig. 4 shows a cross-sectional SEM image of A-LLZ before and after the HIP treatment. The sample before the HIP treatment has many pores, the size of which is up to approximately 10 μm, as shown in Fig. 4a. Almost all of these pores are considered to be closed pores. Cross-sectional laser beam microscope images of the HIP-treated A-LLZ shows no pores (Fig. 4b). The relative density of A-LLZ was 91.5%, which was increased to 99.1% by HIP treatment. The HIP treatment was thus effective in removing the closed pores in A-LLZ. White areas were observed near the grain boundary in Fig. 4b, which accounted for ca. 1% of the entire sample surface. These white areas may be impurity phases that were concentrated at the grain boundaries by HIP treatment. The sintered A-LLZ before HIP treatment was white-colored and opaque, whereas after HIP treatment, the A-LLZ was transparent and had few white areas. As a result of HIPtreated A-LLZ XRD measurements, almost the same structure as A-LLZ before HIP was obtained (Fig. 5). The light transmittance of the transparent

parts and of the entire A-LLZ sample were 30% and 20%, respectively, as shown in Fig. 6. Raman scattering measurements confirmed that the white parts of the sample contained Li2CO3 and LiOH (Fig. 7). Galven et al. reported that in ambient air, spontaneous Li+/H+ exchange reaction occurs in garnet-type Li7La3Sn2O12 [24]. LiOH reacts with CO2 in ambient air to produce Li2CO3. The Li2CO3 and LiOH observed in A-LLZ may be due to contamination by water and CO2 in the air. Fig. 8 shows an impedance profile of the HIP treated A-LLZ with sputtered Cu electrodes measured at 25 °C. The impedance profile

Fig. 5. XRD patterns of (a) A-LLZ and (b) HIP-treated A-LLZ.

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Fig. 6. Transmittance vs. wavelength curves for (a) the transparent area and (b) the entire area of 1.0 wt% Al2O3-doped LLZ after HIP treatment, and (c) 1.0 wt% Al2O3-doped LLZ before HIP treatment. Fig. 8. Impedance profile for the Cu/HIP-treated 1.0 wt% Al2O3-doped LLZ/Cu cell at 25 °C.

shows a semicircle, which is attributed to the grain boundary resistance of the sample. The intercept of the semicircle on the real axis at high frequency represents the bulk resistance [16]. The semicircle due to the bulk was not observed because this was out of the frequency range of the frequency analyzer. The estimated bulk and grain boundary conductivities at 25 °C are thus 9.9 × 10−4 and 3.2 × 10−4 S cm−1, respectively. The bulk conductivity is higher than that of 4.4 × 10−4 S cm−1 for A-LLZ and comparable to that of 1.0 × 10−3 S cm−1 for Li1.4La3Zr1.4Ta0.6O12 reported by Li et al. [25], which is the highest bulk conductivity reported previously in the garnet-type lithium conductors. The grain boundary conductivity is lower than that of 9.4 × 10−4 S cm− 4 for A-LLZ and those reported previously [16,17,25]. The low grain boundary conductivity may be caused by the decrease in the amounts of grain boundaries and the impurities concentrated at the grain boundaries. The activation energy for bulk conductivity was estimated to be 0.37 eV from the temperature dependence of the bulk conductivity in the temperature range of 25 °C to −100 °C, which is slightly higher than that of LLZ sintered at 1180 °C for 32 h [26]. Fig. 9 shows the change in the cell voltage with the polarization period at 0.5 mA cm−2 and 25 °C for the Li/HIP-treated A-LLZ/Li cells, where HIP-treated A-LLZ with (Fig. 7a) and without (Fig. 7b) white areas were examined. The cell with HIP-treated A-LLZ containing white areas showed a sudden voltage drop after 100 s polarization, as observed for the Li/A-LLZ/Li cell shown in Fig. 2, where the cell voltage suddenly dropped to zero after 100 s polarization due to lithium dendrite formation. However, cells with HIP-treated transparent A-LLZ

Fig. 7. Raman spectra for the white parts of the sample.

containing no white areas showed no sudden cell voltage drop, but instead the cell voltage increased gradually with the polarization period. The rapid cell voltage increase after 850 s of polarization may be due to a loss of lithium metal at the anode side because the lithium electrodes were prepared by vapor deposition and the thickness of

Fig. 9. Cell voltage vs. polarization period curves for Li/HIP-treated 1.0 wt% Al2O3-doped LLZ/Li (a) with and (b) without white areas.

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the deposited lithium was ca. 2 μm. No short-circuit for the transparent A-LLZ suggests that the grain boundary and impurity phase-free transparent LLZ suppresses lithium dendrite growth through the electrolyte. 4. Conclusions LLZ is an attractive candidate as the solid electrolyte for all-solidstate lithium batteries due to its stability with lithium metal and high lithium ion conductivity at room temperature. However, the suppression of lithium dendrite formation during the lithium deposition process at high current density is required for the application of LLZ as an electrolyte with a lithium metal electrode. In this study, transparent 1.0 wt% Al2O3-doped LLZ was prepared by HIP treatment of a sintered LLZ pellet. The relative density of the LLZ pellet was increased from 91.5% to 99.1% by HIP treatment. The HIP-treated samples contained both white and transparent areas. The bulk conductivity of the HIPtreated LLZ was as high as 9.9 × 10−4 S cm−1 at 25 °C. The LLZ electrolyte that included white areas was short-circuited after 250 s polarization at 0.5 mA cm−2 due to lithium dendrite formation, whereas the transparent LLZ without white areas was not short-circuited. This result suggests that the grain boundary in LLZ may play an important role for lithium dendrite formation during the lithium deposition process. References [1] [2] [3] [4]

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