Microstructure manipulation for enhancing the resistance of garnet

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Microstructure manipulation for enhancing the resistance of garnet-type solid electrolyte to “short circuit” by Li metal anode Yaoyu Ren, Yang Shen, Yuanhua Lin, and Cewen Nan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17954 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 23, 2019

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ACS Applied Materials & Interfaces

Microstructure Manipulation for Enhancing the Resistance of Garnet-type Solid Electrolyte to “Short Circuit” by Li Metal Anode Yaoyu Ren,*,†,⊥ Yang Shen,† Yuanhua Lin,† Ce-Wen Nan*,† †

School of Materials Science and Engineering, State Key Lab of New Ceramics and Fine

Processing, Tsinghua University, Beijing 100084, China Corresponding author * Yaoyu Ren, [email protected] * Ce-Wen Nan, [email protected] ⊥

Present address: Maryland Energy Innovation Institute & Department of Materials

Science and Engineering, University of Maryland, College Park, MD 20742, USA Keywords: garnet electrolyte, sintering, lithium dendrites, microstructure, grain boundary

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Abstract: Al-contained Li7-xLa3Zr2-xTaxO12 (xTa-LLZO) powder was synthesized via solid state reaction, where increasing the Ta doping level was found to reduce the average particle size and facilitate a higher relative density in the sintered pellet. 0.8Ta-LLZO pellets sintered at 1150 oC achieved a relative density of 96.2±0.2% and survived the Li striping/plating test under unidirectional current polarization of 0.5mA/cm2 for over 8 hrs without short-circuiting. In contrast, other xTa-LLZO sintered pellets with lower Ta doping levels were short-circuited by lithium dendrites after polarization for much shorter time periods. The microstructure of the sintered body played a more essential role for lithium dendrite prevention compared to relative density alone. By characterizing the microstructure of xTa-LLZO sintered pellets, a formation mechanism of the pathways for lithium dendrite growth was proposed.

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1. Introduction In developing safe lithium-based batteries, one key requirement is to prevent lithium dendrites from penetrating through the electrolyte. This is especially important when using metallic lithium as the anode to maximize the specific energy density of the battery. Solid electrolytes are regarded as the ultimate solution for such issue, yet building batteries with these electrolytes has proven problematic in bench-scale experiments. For instance, dendrite growth have been observed in situ through polymer electrolytes electrolytes 3. Monroe and Newman’s model

4

1-2

and sulfide

explained the mechanism for dendrites

growth based on the mechanical properties of the electrolyte and suggested that lithium dendrites can be inhibited by solid electrolytes with a shear modulus no less than 6 GPa. Solid oxide electrolytes largely satisfy this criterion 5. So far, the most promising solid oxide electrolyte material is the garnet-like Li7La3Zr2O12 (LLZO) and its doped derivatives due to their high conductivity, wide electrolytic window and good stability towards lithium metal. However, recently Yamamoto’s group reported the observation of a “short circuit” phenomenon in LLZO when applying high direct current (DC) density (0.5 mA·cm-2) through symmetric Li/LLZO-based electrolyte/Li cells 6-8

. Such phenomenon has later been correlated to lithium dendrite growth inside the LLZO

electrolyte by several groups via direct observation, including our group9-12. Based on these observation, we proposed that lithium dendrites grew through grain boundaries and interconnected pores9. Subsequent efforts to suppress lithium dendrites by modifying grain 3



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boundaries were largely successful, supporting this proposal 13. On the other hand, Porz et al.14 proposed a different mechanism that involved crack propagation driven by Li-plating on pre-existing flaws on the electrolyte surface, which they substantiated by in situ observations of the evolution of the electrolyte surface during Li-plating . However, the current density causing short-circuiting in their study was too high, e.g., 5 mA/cm2, as opposed to the typical value, 0.5 mA/cm2, observed previously. Therefore, their proposed mechanism is less likely to be directly correlated to the short circuit phenomenon widely observed in previous research. Following our proposed mechanism, in this study we developed a strategy to enhance the physical resistance of the garnet electrolyte to lithium dendrite growth. The initial idea was to increase the relative density of the sintered body by synthesizing powder with smaller particle sizes. To realize this, the Ta doping level in Ta-doped LLZO was increased to reduce the amount of lithium source required for synthesizing the Ta-doped LLZO powder. Our previous study suggested that when using LiOH as the lithium precursor powder, the formation of garnet phase could be facilitated by the melting LiOH 15. Such sintering aid effects of LiOH was also suggested in a recent study 13. It was then speculated that melting LiOH could also enhance particle growth during calcination and therefore reducing the amount of LiOH would reduce the particle size of the final calcined powder. Upon further study, it was found that the specific microstructure of the sinter pellet plays a more essential role on lithium dendrite prevention. 4


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2. Experimental Li7-xLa3Zr2-xTaxO12 (x=0.125, 0.25, 0.6, 0.8, xTa-LLZO) powders were synthesized by conventional solid-state reaction method with LiOH·H2O as lithium source as reported previously 15. Typically, stoichiometric amounts of LiOH·H2O (Aladdin, ≥99.0%), La2O3 (Sinopharm Chemical Reagent Co. Ltd, high purity, calcined at 900 oC for 12 h before using), ZrO2 (Sinopharm Chemical Reagent Co. Ltd, AR) and Ta2O5 (Aladdin, 99.99%) were thoroughly ball-milled with YSZ milling media in 2-propanol for 8 h. 10 wt% excess LiOH·H2O was added to compensate for the loss of lithium during the heating processes. Al2O3 (Sinopharm Chemical Reagent Co. Ltd, high purity, 0.15mol per mol Li7-xLa3Zr2xTaxO12)

was intentionally added to the raw materials as a sintering aid. The mixed slurries

were calcined at 900 oC for 6 h. The as-calcined powders were ball-milled in 2-propanol for 13 h, followed by drying process. The dried powders were pressed into pellets with diameter of 12 mm at 300 MPa, and then sintered at the temperature range from 1125 oC to 1200 oC for 4 h in air. The density of the sintered pellets was measured by the Archimedes method using ethanol as the medium. The particle size distribution of the powders was analyzed with a laser particle size analyzer (Mastersizer 2000). The phase composition of the powders was identified by Xray diffraction (XRD, Bruker D8 Advance A25) using Cu Kα radiation. The lattice parameters of Ta-doped LLZO were fitted and then used to calculate the theoretical density of each composition, from which the relative density of the sintered pellet was obtained. 5



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The morphology of the powders and the microstructure of the sintered pellets were observed by scanning electron microscopy (SEM, JSM-7001F JEOL Ltd.). Energy dispersive X-ray (EDX) analysis was performed to analyze the elemental distribution. Both parallel surfaces of the pellets were polished with sand paper, cleaned with 2propanol, and sputtered with Li-ion blocking Au electrodes for electrical measurement. Ionic conductivities of the sintered pellets were measured at temperatures between room temperature (25 ℃) and 80 °C with an Electrochemical Working Station (Zahner IM6) in the frequency range from 100 Hz to 8 MHz (the upper limit of this instrument). Agilent 4294A (with the measuring frequency up to 110 MHz) was also used to perform room temperature impedance spectroscopy on the pellets. The instrument was calibrated with built-in short and open circuit functions before performing impedance measurements. Li/xTa-LLZO/Li symmetric cells were prepared by sandwiching the xTa-LLZO pellet between thin lithium metal plates and the stack was sealed in stainless steel containers in an Ar-filled glove box16. The surface of the pellets was polished before assembling the cell and was heated to 180 oC (close to melting point of lithium metal) to ensure good contact between the lithium metal and the pellet 16. The electrochemical impedance spectrum (EIS) of the Li/LLZTO/Li cell was measured in the frequency range from 1 Hz to 8 MHz. DC polarization of the cells was conducted with a current density of 0.5 mA/cm2. Both measurements were performed on an Electrochemical Working Station (Zahner IM6) at room temperature. 6


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3. Results and Discussion 3.1 Materials characterization XRD verified that all the xTa-LLZO powder formed the cubic phase with garnet structure (Figure S1a). A minor LaAlO3 peak was present in all cases due to the addition of Al2O3. The diffraction peaks consistently shifted towards higher 2θ angles as the Ta doping level was increased (Figure S1b), indicating a decrease in cubic phase lattice parameter due to the smaller ionic radius of the Ta5+ ion (0.64 Å) as compared to the Zr4+ ion (0.72Å) 17.

Figure 1 (a)Particle size distribution and (b-e) SEM images for xTa-LLZO powders: (b)0.125Ta, (c)0.25Ta, (d)0.6Ta, (e)0.8Ta. As previously discussed, higher Ta doping levels could help to decrease the particle size of the Ta-doped LLZO because less LiOH would be necessary in the LLZO precursor powder and therefore the sintering aid effect of the LiOH would be reduced. Indeed, particle size distribution analysis verified that the average xTa-LLZO particle size 7



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decreased mildly with increasing Ta doping level, as expected (Figure 1a). This trend was also easily distinguished in SEM images (Figure 1b-e), where the particles were more sphere-like in shape for higher Ta doping levels. This quality, together with the narrow and uniform particle size distribution as shown in Figure 1a, was beneficial for subsequent ceramic forming, e.g., dry pressing and sintering. The relative densities of sintered xTa-LLZO pellets are compared in Figure 2a. Generally, for the same sintering temperature, the relative density increased with higher Ta doping level, as expected. The highest relative density achieved was 96.2±0.2% from 0.8Ta-LLZO sintered at 1150oC. Sintering this same composition at either higher or lower temperatures resulted in lower relative densities.

Figure 2 Variation of (a) relative density and (b) room-temperature total ionic conductivity with Ta doping level and sintering temperature for xTa-LLZO pellets. The number in parenthesis beside each data point in (b) is the activation energy (in eV) of the corresponding xTa-LLZO pellet. 8


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Total ionic conductivities of xTa-LLZO were determined by measuring the impedance of the sintered pellet while sandwiched by Au current collectors. Room-temperature Nyquist plots for the xTa-LLZO pellets sintered at 1150 oC are shown as representatives in Figure S2. For all the samples, only one semicircle was observed at high frequency, and the high-frequency intercept with the real axis was zero. Total resistance (Rtotal) was then determined by simulating the impedance spectrum using the equivalent circuit (RtotalCPEtotal)CPEtail (inset of Figure S2). The capacitance (C) was calculated by C = 𝑅𝑡𝑜𝑡𝑎𝑙

(1−𝑃) 1 𝑃 𝑇𝑃

where P and T were parameters in the constant phase element, CPEtotal. The capacitance was calculated to be on the order of 10-11 F for all samples, indicating the bulk contribution. The ionic conductivities of the xTa-LLZO pellets sintered at different temperatures were calculated based on the dimensions of the pellet and summarized in Figure 2b. For the pellets sintered at 1150oC, the ionic conductivity increased from ~4.55×10-4 S/cm to ~8.53×10-4 S/cm with increasing Ta doping level from x=0.125 to x=0.25. Further increases in the Ta doping level to x=0.6 and 0.8 resulted in a decrease in ionic conductivity. Similar trends have also been reported by Wang et. al.18 in Ta-doped and by Ohta et al.19 in Nbdoped LLZO systems, which was due to the trade-off between the lithium ion concentration and the lithium ion vacancy concentration on the octahedral sites within the garnet structure 20-22

. Although the ionic conductivity of the sintered body was compromised by increasing

the Ta doping level, the measured values were still comparable to most cases for garnet type electrolytes5, 23. Also summarized in Figure 2b are the activation energies for Li ion 9



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transport in xTa-LLZO as determined in the temperature range of 27-80oC. The activation energy increased monotonically with increasing Ta doping level, i.e., from 0.33 eV for x=0.125 to 0.41 eV for x=0.8. The ab initio modelling by Xu et al.22 suggested two Li ion hopping routes with different energy barriers in the garnet structure. In addition, higher Li concentrations would promote more Li ion transport via the low energy barrier route as long as sufficient vacancies existed in the octahedral sites for ion migration. In our case, due to the charge compensation, higher Ta doping levels resulted in lower Li ion concentrations in the lattice, which then led to higher activation energies. For 0.8Ta-LLZO, sintering the pellets at temperatures either higher or lower than 1150oC decreased the ionic conductivity (Figure 2b). The similar activation energy of different samples indicated that the different ionic conductivity could be due to microstructural variation in the sintered pellet. This speculation was supported by the fact that the variation in ionic conductivity followed the trend of relative density (Figure 2a). 3.2 Polarization test of Li/xTa-LLZO/Li cell To test the ability of xTa-LLZO sintered pellets to block lithium dendrites, Li/xTaLLZO/Li symmetric cells were assembled and polarized under DC at 0.5 mA/cm2. The initial Nyquist plots of the cells are presented in the insets of Figure 3a and b. The incomplete semicircles at high frequency (>0.2MHz) were attributed to the total ohmic resistance contribution of the xTa-LLZO pellets by comparison with the impedance spectra of the Au/xTa-LLZO/Au cells in Figure S2. Accordingly, the semicircles at intermediate frequencies (40 Hz–0.2 MHz) were ascribed to the xTa-LLZO/Li interfacial resistance. The 10


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values of the interfacial resistance, i.e., ~50-150 Ω∙cm2, were comparable to those previously reported7-8, 24, suggesting a reasonable contact between xTa-LLZO and Li electrode.

Figure 3 DC polarization test at 0.5 mA/cm2 and room temperature on Li/xTa-LLZO/Li cells with different garnet pellets: (a) xTa-LLZO sintered at 1150 ℃, (b) 0.8Ta-LLZO sintered at different temperatures, (c) 0.8Ta-LLZO sintered at 1150 ℃. The upper inset in (a) and the inset in (b) show the initial impedance spectra of the corresponding cells. The lower inset in (a) shows the initial parts of the polarization curves in (a). (d) Impedance spectra of the Li/0.8Ta-LLZO/Li cell measured at different time points corresponding to the dots marked as 1-5 in (c): 1 - before polarization test, 2 - immediately after the first 11



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polarization cut-off, 3 - after resting at open circuit for 44 hrs, 4 - after replacement of anodic lithium metal, 5 - immediately after the polarization test. The inset in (d) shows the enlarged view of the high frequency parts of the impedance spectra. Figure 3a-c show the changes in the cell voltage with polarization period. The onset voltages corresponded very well to the values calculated by multiplying the total impedance by the applied DC current. The cell voltages rose under applied DC polarization due to the formation of gaps at the lithium/garnet interface as lithium was locally depleted 26, 27

. The different rates of voltage increase for different cells could be due to different rates

of contact loss between the lithium and the electrolyte. This was expected for an unoptimized Li/garnet interface. After polarization for a variable period of time, the voltage dropped abruptly for samples of xTa-LLZO with x=0.125, 0.25, and 0.6 (Figure 3a) and 0.8Ta-LLZO sintered at temperatures other than 1150oC (Figure 3b), indicating the occurrence of short circuits. In contrast, no short circuit was observed for the sample of 0.8Ta-LLZO sintered at 1150oC during the entire test period, which lasted longer than any of the other samples (Figure 3a, c). The voltage fluctuation of the 0.6Ta-LLZO pellet after short-circuiting suggested that the short circuit was a reversible phenomenon, strongly indicating that it was due to the lithium dendrite growth rather than any irreversible processes, e.g., reduction of the garnet electrolyte 25. Similar phenomenon has also been observed in previous garnet samples6, 8. The mechanism of such fluctuations, or “fuse effect”26, is detailed as follows. Once lithium dendrites form, a major portion of the current applied to the cell will flow through the dendrites because of their extremely high 12


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electronically conductivity. Since the dendrites are very thin due to confinement within narrow paths through the garnet, i.e., interconnected pores and grain boundaries9, the current could generate large amounts of heat and melt the dendrites in less than one second (see detailed calculation based on Figure S3 in Supporting Information). Melting lithium may break at this time, thus the short circuit would appear to fluctuate. To further explore the potential of the 0.8Ta-LLZO composition to block lithium dendrites, DC polarization tests were continued on the symmetric cell under the same polarization direction. To minimize the possible irreversible damage to the electrolyte, e.g., the reduction by Li metal25, a voltage limit of 4.0V was set during polarization tests on the Li/0.8Ta-LLZO/Li cell. It was interesting to observe that the cell voltage at the resumption of testing was much lower than the previous cut-off voltage (as indicated by the dashed arrow in Figure 3c) when the current was re-applied after the cell was rested in open circuit conditions for some time. This phenomenon enabled us to continue with the galvanostatic polarization test for an extended period of time after a cut-off, although the restarting cell voltage was still higher than the original. Since the cell was under mechanical pressure controlled by a spring during polarization16, the gap between the lithium electrode and the electrolyte caused by local depletion of lithium could be narrowed and the cell voltage should then drop accordingly. However, if the current was too high for the relaxation process to follow up, the process would proceed after the polarization was removed, i.e., a delayed relaxation, which could cause the delayed cell voltage reduction. Moreover, some irreversible morphological changes, i.e., 13



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cracks and pulverization, could also occur inside the lithium electrode 27, making the full recovery of the interfacial contact almost impossible. Therefore, the restarting cell voltage was expected to be higher than the original cell voltage at the beginning of the polarization test. To prove this hypothesis, the interfacial structure was recovered by replacement of the anodic lithium electrode with a new piece of lithium metal, followed by restart of DC polarization in the same direction as before polarization cut-off. Indeed, the renewed cell exhibited essentially the same onset voltage as the original. It should be noted that the anodic lithium electrode was easily detached from the pellet when the cell was dissembled for the electrode replacement, which also indicated the loss of contact between the electrode and the electrolyte. To gain insight into the voltage variation, the impedances of the Li/0.8Ta-LLZO/Li cell immediately after the polarization cut-off and before the current was reapplied were also monitored (Figure 3d). All the Nyquist plots showed essentially the same ohmic resistance but a much different interfacial resistance, suggesting that the voltage variation was due to the interfacial resistance variation. The impedance spectrum obtained immediately after the polarization cut-off showed a much larger interfacial resistance than the initial (Figure 3d-2). After resting at open circuit, the interfacial resistance dropped dramatically (Figure 3d-3). After the replacement of the anodic lithium metal, the cell exhibited a similar initial impedance spectrum to the original one (Figure 3d-4), suggesting a full recovery of the interfacial structure. The final impedance spectrum obtained after the polarization test exhibited a large interfacial resistance, suggesting that the interfacial contact degraded 14


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again (Figure 3d-5). Benefiting from both the relaxation process and the replacement of the lithium electrode, the polarization testing on the Li/0.8Ta-LLZO/Li cell in one current direction was able to be performed for over 8 h. No short circuit was detected during this test. The total areal charge density passed through the electrolyte was 4 mAh/cm2, higher than the typical areal capacity of today’s commercial lithium-ion batteries (3 mAh/cm2)28-30. Previous studies suggested that lithium dendrite growth could be induced by high local current densities at the lithium/electrolyte interface10. Interfacial modification by coating a thin layer of lithiable metal or oxide on the electrolyte surface can help create uniform current distribution across the interface, thus dramatically increasing the current density that triggers lithium dendrites10, 31. In this study, no surface modification was made to the garnet electrolyte. Consequently, a local current density much higher than 0.5mA/cm2 would be expected to be applied at random locations across the garnet surface. Moreover, one direction current can quickly create many defects at the electrolyte/Li interface (uniform Li stripping rate at 0.5mA/cm2 is approx. 40nm Li per minute; nonuniform Li stripping rate will be higher locally), leading to even higher local current densities. Therefore, the currently reported results indicated that the garnet electrolyte can sustain lithium plating/striping at current densities much higher than 0.5mA/cm2. The results from polarization tests generally followed our expectation that the densest pellet exhibited the highest resistance to lithium dendrites. However, it was reported that a Ta-doped LLZO pellet with a relative density of 99% was shorted within 160s under 15



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0.5mA/cm2 DC polarization10. This result contrasts to our observation that the 0.8Ta-LLZO pellet with a relative density of 96% can survive the same polarization condition for 8h. This contradiction suggests that there are fundamental factors in addition to relative ceramic density that affect the short circuit phenomenon. It has been established in the study of solid sodium ion conducting electrolytes that the maximum tolerable current density, or so called “critical current density”, of the electrolyte was a function of ionic conductivity32. In this work, short circuit occurred for pellets with both higher and lower ionic conductivities as compared to the 0.8Ta-LLZO pellet sintered at 1150oC (Figure 2b), indicating that the ionic conductivity was not a factor causing the short circuit phenomenon. Doeff et al.33 demonstrated that the surface microstructure of garnet sintered pellets can affect the uniformity of Li plating/striping at the Li/garnet interface and the consequent current density for shorting the pellet. Their results suggested that smaller grain size (i.e., more grain boundaries) was able to more uniformly distribute the current across the Li/garnet interface and hence improve the ability of the pellet to prevent lithium dendrite formation. On the other hand, Sakamoto et al.34 found that the “critical current density” increased with increasing grain size in the garnet sintered pellet. They ascribed this trend to the reduced grain boundary area where possible failure points were located. While these studies contradicted each other, both underlined the importance of the microstructure on lithium dendrite prevention. 3.3 Microstructural effect on Li dendrite prevention To further explore the fundamental factors influencing the lithium dendrite prevention 16


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ability of the sintered pellet, the microstructure of the xTa-LLZO pellets before and after short-circuiting was carefully characterized.

Figure 4 (a)(c)(e)(g) Surface and (b)(d)(f)(h) cross-sectional SEM images of xTa-LLZO pellets sintered at 1150 ℃: (a)(b) 0.125Ta, (c)(d) 0.25Ta, (e)(f) 0.6Ta, (g)(h) 0.8Ta. Inset in (h) shows the enlarged view. Arrows in (a)(c)(e) indicate holes. Figure 4 shows the microstructure of surface and cross section of the xTa-LLZO pellets sintered at 1150oC before polarization test. Lower magnification surface SEM images of these samples are also provided in Figure S5. On the finely polished surface of the xTaLLZO pellets, some holes were observed in 0.125Ta-LLZO, 0.25Ta-LLZO, and 0.6TaLLZO (indicated by arrows in Figure 4a, c, e; Figure S5a-c), while no such holes were observed on the surface of 0.8Ta-LLZO (Figure 4g, Figure S5d). From the cross section of the pellets, two microstructural features were identified. First, pores with diameters of 25μm were largely observed (Figure 4b, d, f, h). The pores were fewer and smaller as the Ta doping level increased from x=0.125 to x=0.8, a trend in agreement with the increase in relative density (Figure 2a). The pores mainly originated from the space between particles 17



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inside the green body after pressing15, as evidenced by observations that the pores inside the 0.8Ta-LLZO sintered pellet were located only between grains (inset in Figure 4h). The second microstructural feature was that the grain size decreased with increasing Ta doping level. No grain boundary was observed within the whole area of Figure 4b for 0.125TaLLZO, while some grain boundaries were observed in images for 0.25Ta-LLZO and 0.6TaLLZO with a same area (Figure 4d, f), indicating that 0.125Ta-LLZO sample exhibited larger grain size. The grains in xTa-LLZO (x=0.125, 0.25, 0.6) were extremely large (>100μm) (Figure 4a-c), which formed through rapid grain growth15. These grains contrasted with those observed in the 0.8Ta-LLZO pellet (inset in Figure 4d), which were of comparable size to the original particles of the powder, indicative of bare grain growth. From the microstructural analysis, a preliminary hypothesis on the route for the formation and growth of lithium dendrites through the sintered pellet was conceived. In brief, lithium dendrites formed at the holes on the surface of the sintered pellet and grew through the pellet via the pathways provided by the residual interconnected pores inside the pellet. The formation of lithium dendrites at the surface hole was supported by observations that parts of lithium dendrites visible in the cross section of the shorted pellets were found close to the Li/pellet interface (Figure S6). As further evidence for void spaces as pathways for lithium dendrite growth, interconnected pores have been recently imaged by 3D synchrotron X‑ray tomography inside sintered garnet electrolytes that were fabricated in a similar process to our samples, and these pores were correlated to the pathway of lithium dendrite growth35. Lithium dendrites in interconnected pores have also 18


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been directly observed in our previous work on 0.25Ta-LLZO sintered at 1100oC 9 and in the current work on 0.25Ta-LLZO sintered at 1150oC (Figure 5). The lithium dendrite, as identified by backscattered electron images (Figure 5a) and EDS (Figure 5b), appeared continuously distributed across the cross-sectional area of the pellet, which was different from our previous observation that the lithium dendrites were more discretely distributed 9

. The different morphology of lithium dendrites suggested a higher concentration of

interconnected pores inside the pellet that was sintered at higher temperature, which was consistent with the observations by synchrotron X‑ray tomography35. Our observations clearly showed that lithium dendrites grew along grain boundaries (Ref. 9 and Figure 5), further pointing out that the grain boundaries were the location for the interconnected pores that facilitated lithium dendrite growth. The growth of lithium dendrites along grain boundaries was also widely reported by other researchers 12-13, 33. The combination of this fact and the observation that the holes on the surface of the sintered pellet were more concentrated along grain boundaries for 0.25Ta-LLZO and 0.6Ta-LLZO (Figure 4c, e; Figure S5b, c) shows consistency with our speculation that lithium dendrites form at the holes in the electrolyte surface and grow through the pellet via interconnected pores.

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Figure 5 (a) cross-sectional SEM images in secondary electron (SE) and backscattered electron (BSE) modes of the short-circuited 0.25Ta-LLZO pellet sintered at 1150 ℃; (b) EDX line scan along the yellow line in a local region covered by lithium dendrite. It should be noted that the grain boundaries mentioned above were between extremely large grains (>100μm) as observed in xTa-LLZO (x=0.125, 0.25, 0.6) (Figure 4a-f). The formation of these grain boundaries was different from that in 0.8Ta-LLZO that had much 20


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smaller grains, which could cause the different ability of the sintered pellet to suppress lithium dendrite growth. To gain insight into the formation mechanism of the grain boundaries between large grains, the microstructural evolution of the pellets with sintering temperature was investigated and discussed below. For Al-containing lithium garnets, the rapid grain growth was controlled by two dimensional nucleation on a singular grain with atomically smooth interface structures and facilitated by the presence of liquid Al-containing phases, such as a composition between Li5AlO4 and LiAlO2 15, 36. As evidence for the existence of the liquid Al-containing phase, segregations of Al to the grain boundaries were observed in 0.25Ta-LLZO (Figure 6a, Figure S7) and 0.8Ta-LLZO pellets (Figure S8). The amount of the liquid Al-containing phases formed could be influenced by different amounts of Li source added for different Ta doping level, which further influenced the microstructure of the sintered body. Specifically, a composition around Li:Al=3.3:1 would liquify at a temperature as low as 1055oC 37, which was identified as the cause of the rapid grain growth within 0.25Ta-LLZO samples at 1100oC15. Reducing the ratio, i.e., less Li source added for higher Ta doping level, will increase the temperature required for liquifying the equal mole of Al-containing phase 37. Therefore, the onset temperature for rapid grain growth would be higher for higher Ta doping levels due to the corresponding lower amounts of Li source added. Indeed, for 0.25Ta-LLZO, the rapid grain growth caused a microstructural variation from fine grain dominating to mixed large and fine grains and then to large grain dominating as the sintering temperature increased from 1050 to 1100 and then to 1150oC (ref. 15 and Figure 21



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4d). For 0.6Ta-LLZO, reducing the sintering temperature from 1150oC to 1125oC can retain the feature of clusters of fine grains between large grains (Figure S9a, b); whereas, for 0.8Ta-LLZO, the sintering temperature needed to be increased from 1150oC to 1175oC to promote the formation of large grains (Figure S9c, d). For 0.6Ta-LLZO sintered at 1125oC and 0.8Ta-LLZO sintered at 1175oC, the smooth surface of the large grain as indicated by the straight lines (guided by dotted lines in Figure S9b, d) formed in the interface between the large grain and the fine grain matrix was a clear sign of the grain growth mechanism with atomically smooth interface structures 15, 36. The same feature has also been observed in 0.25Ta-LLZO sintered at 1100oC15.

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Figure 6 (a) SEM image and EDX mapping of a grain surface on the cross section of 0.25Ta-LLZO pellet sintered at 1150 ℃. Cross-sectional SEM images of (b) 0.6Ta-LLZO pellet sintered at 1175 ℃ and (c) 0.8Ta-LLZO pellet sintered at 1200 ℃. Dotted lines circle out intergranular fracture areas. Figure 6a shows a grain boundary region revealed by intergranular fracture in the cross section of 0.25Ta-LLZO pellets sintered at 1150oC. As identified by EDX mapping, a LiAl-O layer partially covers the grain surface, leaving voids around. These voids connected with the pores on the grain surface to form an interconnected tunnel, which could provide the pathway for lithium dendrite growth. The porous nature of the grain boundaries between large grains can be more largely revealed in the pellets sintered at higher temperature where more intergranular fracture occurs (circled out in Figure 6b, c). Based on the above observations and discussion, the mechanism for the microstructural evolution of xTa-LLZO pellet upon sintering is proposed below. Upon sintering, liquified Li-Al-O phase initially forms on the particle surface to facilitate particle rearrangement. As sintering proceeds, some of the liquid phase is isolated to the grain boundaries, carrying with it some pores originated from green body; the liquid phase thereby provides a pathway for the fast escaping of these pores. In addition, the liquid phase could also facilitate lithium evaporation at high temperature. Accordingly, the presence of liquid phase at the grain boundaries will cause preferential lithium evaporation from the grain boundaries. During grain growth, additional pores will move to grain boundaries along with the remainder of the Li-Al-O phase and the pores will subsequently become interconnected to each other. 23



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On the other hand, lithium evaporation will reduce the lithium concentration, which could increase the melting point of the Li-Al-O phase, thus enabling its re-solidification. Once cooled, the porous nature of the grain boundary between large grains was retained and provided the pathway for lithium dendrite growth (ref. 9 and Figure 5). It was obvious that the holes observed on the polished surfaces of 0.25Ta-LLZO and 0.6Ta-LLZO were more concentrated along grain boundaries (Figure 4c, e; Figure S5b, c). These holes, later acting as the entrance for lithium dendrites, were initially the exit of the escaping pores and evaporated lithium species. Both the formation of interconnected pores and lithium evaporation weakened the grain boundary and made it more porous than inner grains. The effect is more severe at higher temperatures, which caused the fracture of the sintered body to propagate from transgranularly to intergranularly (Figure 6b, c). The fact that more pores were observed in the intergranular fracture area than in the transgranular fracture area suggested that pores were prone to moving to grain boundaries.

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Figure 7 Schematic diagram of the microstructure of (a) lithium-dendrite-free xTa-LLZO pellet and (b) lithium-dendrite-penetrated xTa-LLZO pellet. (blue: garnet; red: Li-Al-O phase) Yamada et al. have suggested that well-sintered grain boundaries without impurities would decrease interconnected pores and would be mechanically strong with hardly any openings for lithium dendrite growth13. The above proposed mechanism indicated that such well-sintered grain boundaries would only be possible if the pellet was sintered at the stage before rapid grain growth. For 0.8Ta-LLZO, this stage should correspond to sintering at 1150oC. As seen in Figure 4h, the uniformly strong grain boundary was indicated by the flat feature of the cross section with transgranular fracture; this feature suggested that the grain boundary was as strong as the inner grain. When sintered at this stage, particles flattened their contact to each other with the aid of liquified Li-Al-O phase, getting themselves ready for grain growth through Ostwald ripening. The particles (grains) have well-sintered to each other, leaving no interconnected pores at grain boundaries. Therefore, the pellet sintered at this stage was hardly penetrated by lithium dendrites (Figure 7a). Sintering the pellet at higher temperatures resulted in porous grain boundaries between large grains that may provide pathways for lithium dendrites growth (Figure S9c, d, Figure 6c, Figure 7b). Sintering at lower temperatures would not be enough to fully sinter the particles, as indicated by the rough feature of the cross section (Figure S10). Consequently, all the pellets sintered at temperatures other than 1150 oC were short-circuited (Figure 3b). These results suggest that a suitable temperature for well-sintering an xTa-LLZO pellet in 25



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order to prevent lithium dendrites would be located between the melting point of Li-Al-O phase and the onset temperature for rapid grain growth. A consistent result can be observed in our previous work, which demonstrated that the longest polarization period among all the 0.25Ta-LLZO pellets sintered at different temperatures was achieved from the pellet sintered at 1075oC9. This temperature was well above the melting point (1055oC) of Li-AlO phase15. However, to fully suppress lithium dendrites, the particle size of the initial powder also needs to be carefully controlled, as demonstrated in this work. While the above proposed mechanism is straightforward for explaining the situation occurred in xTa-LLZO (x=0.25, 0.6, 0.8), it needs elaboration to be applied to the case of 0.125Ta-LLZO. It was noticed that the grain boundaries were barely observed even in the whole cross section of the 0.125Ta-LLZO pellets sintered at 1150oC and below (Figure S11a, b), and only appeared in the pellets sintered at higher temperatures (Figure S11c, d). On the surface of 0.125Ta-LLZO sintered at 1150oC, the holes are scattered over the entire area with no grain boundaries identified (Figure 4a), consistent with the cross-sectional observation (Figure 4b and Figure S11b). Since 0.125Ta-LLZO powder had the largest particle size and hence least sintering driving force compared to the other powders, the sintering body could always have pores that were large and plenty enough to keep connection for evaporation of trapped gas and lithium species during the whole sintering process. Only at higher temperatures when sintering was accelerated would residual interconnected pores begin to agglomerate to form porous grain boundaries. In 0.125TaLLZO pellet sintered at 1150oC, the interconnected pores provided the pathway for lithium 26


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dendrite growth (Figure S6). Based on its origination, it was reasonable to expect a higher tortuosity, i.e., longer distance, of such pathways compared to the pathways provided by the porous grain boundaries inside 0.25Ta-LLZO pellet sintered at 1150oC. This can explain why 0.125Ta-LLZO had distinctly lower relative density than 0.25Ta-LLZO but its short-circuiting period was longer (Figure 3a). 4. Conclusions Al-contained xTa-LLZO (x=0.125, 0.25, 0.6, and 0.8) powder was synthesized via solid state reaction. The particle size of the powder decreased with increasing Ta doping level, resulting in a higher relative density of the sintered pellet. 0.8Ta-LLZO pellet sintered at 1150oC had a relative density of 96.2±0.2% and survived the Li striping/plating test under direct current polarization of 0.5mA/cm2 for over 8h without short-circuiting. Whereas, other xTa-LLZO sintered pellets investigated were shorted by lithium dendrites after polarization for much shorter time periods. The microstructure of the sintered body played a more essential role on lithium dendrite prevention. The dense sintered body with wellsintered grain boundaries was essential for lithium dendrite growth prevention. The formation of extremely large grains was accompanied by the formation of porous grain boundaries, which served as pathways for lithium dendrite growth. This work suggests that garnet type electrolytes that are dense enough to prevent lithium dendrites could be obtained using conventional sintering process by tuning the microstructure.

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Acknowledgment This work was supported by China Postdoctoral Science Foundation funded project (grant No. 043202211) received by Dr. Yaoyu Ren and the National Natural Science Foundation of China (grant No. 51221291) received by Prof. Ce-Wen Nan. Supporting Information. Additional details including XRD patterns, EIS results, SEM images, and EDX analysis results References (1) Orsini, F.; du Pasquier, A.; Beaudouin, B.; Tarascon, J. M.; Trentin, M.; Langenhuizen, N.; de Beer, E.; Notten, P. In Situ SEM Study of the Interfaces in Plastic Lithium Cells. J. Power Sources 1999, 81-82, 918-921. (2) Brissot, C.; Rosso, M.; Chazalviel, J. N.; Lascaud, S. Dendritic Growth Mechanisms in Lithium/Polymer Cells. J. Power Sources 1999, 81–82, 925-929. (3) Nagao, M.; Hayashi, A.; Tatsumisago, M.; Kanetsuku, T.; Tsuda, T.; Kuwabata, S. In Situ SEM Study of a Lithium Deposition and Dissolution Mechanism in a Bulk-Type SolidState Cell with a Li2S-P2S5 Solid Electrolyte. Phys. Chem. Chem. Phys. 2013, 15, 1860018606. (4) Monroe, C.; Newman, J. The Impact of Elastic Deformation on Deposition Kinetics at Lithium/Polymer Interfaces. J. Electrochem. Soc. 2005, 152, A396-A404. (5) Ren, Y.; Chen, K.; Chen, R.; Liu, T.; Zhang, Y.; Nan, C.-W. Oxide Electrolytes for Lithium Batteries. J. Am. Ceram. Soc. 2015, 98, 3603-3623. 28


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Figure 1. (a)Particle size distribution and (b-e) SEM images for xTa-LLZO powders. (b)0.125Ta, (c)0.25Ta, (d)0.6Ta, (e)0.8Ta.



Figure 2 Variation of (a) relative density and (b) room-temperature total ionic conductivity with Ta doping level and sintering temperature for xTa-LLZO pellets. The number in parenthesis beside each data point in (b) is the activation energy (in eV) of the corresponding xTa-LLZO pellet. 169x66mm (600 x 600 DPI)


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Figure 3 DC polarization test at 0.5 mA/cm2 and room temperature on Li/xTa-LLZO/Li cells with different garnet pellets: (a) xTa-LLZO sintered at 1150 ℃, (b) 0.8Ta-LLZO sintered at different temperatures, (c) 0.8Ta-LLZO sintered at 1150 ℃. The upper inset in (a) and the inset in (b) show the initial impedance spectra of the corresponding cells. The lower inset in (a) shows the initial parts of the polarization curves in (a). (d) Impedance spectra of the Li/0.8Ta-LLZO/Li cell measured at different time points corresponding to the dots marked as 1-5 in (c): 1 - before polarization test, 2 - immediately after the first polarization cut-off, 3 - after resting at open circuit for 44 hrs, 4 - after replacement of anodic lithium metal, 5 - immediately after the polarization test. The inset in (d) shows the enlarged view of the high frequency parts of the impedance spectra. 169x119mm (600 x 600 DPI)



Figure 4 (a)(c)(e)(g) Surface and (b)(d)(f)(h) cross-sectional SEM images of xTa-LLZO pellets sintered at 1150 ℃: (a)(b) 0.125Ta, (c)(d) 0.25Ta, (e)(f) 0.6Ta, (g)(h) 0.8Ta. Inset in (h) shows the enlarged view. Arrows in (a)(c)(e) indicate holes.


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Figure 5 (a) cross-sectional SEM images in secondary electron (SE) and back scatter electron (BSE) modes of the short-circuited 0.25Ta-LLZO pellet sintered at 1150 ℃; (b) EDX line scan along the yellow line in a local region covered by lithium dendrite.



Figure 6 (a) SEM image and EDX mapping of a grain surface on the cross section of 0.25Ta-LLZO pellet sintered at 1150 ℃. Cross-sectional SEM images of (b) 0.6Ta-LLZO pellet sintered at 1175 ℃ and (c) 0.8TaLLZO pellet sintered at 1200 ℃. Dotted lines circle out intergranular fracture areas.


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Figure 7 Schematic diagram of the microstructure of (a) lithium-dendrite-free xTa-LLZO pellet and (b) lithium-dendrite-penetrated xTa-LLZO pellet. (blue: garnet; red: Li-Al-O phase)


Microstructure manipulation for enhancing the resistance of garnet

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