Effect of Pore Connectivity on Li Dendrite Propagation within LLZO Electrolytes Observed with Synchrotron X‑ray Tomography Fengyu Shen,†,‡,# Marm B. Dixit,‡,# Xianghui Xiao,§ and Kelsey B. Hatzell*,†,‡,∥ †
Interdisciplinary Department of Material Science, Vanderbilt University, Nashville, Tennessee 37235, United States Department of Mechanical Engineering, Vanderbilt University, Nashville, Tennessee 37235, United States § X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States ∥ Department of Chemical and Biomolecular Engineering, Vanderbilt University, Nashville, Tennessee 37235, United States ‡
S Supporting Information *
ABSTRACT: Li7La3Zr2O12 (LLZO) is a garnet-type material that demonstrates promising characteristics for all-solid-state battery applications due to its high Li-ion conductivity and its compatibility with Li metal. The primary limitation of LLZO is the propensity for short-circuiting at low current densities. Microstructure features such as grain boundaries, pore character, and density all contribute to this shorting phenomenon. Toward the goal of understanding processing-structure relationships for practical design of solid electrolytes, the present study tracks structural transformations in solid electrolytes processed at three different temperatures (1050, 1100, and 1150 °C) using synchrotron X-ray tomography. A subvolume of 300 μm3 captures the heterogeneity of the solid electrolyte microstructure while minimizing the computational intensity associated with 3D reconstructions. While the porosity decreases with increasing temperature, the underlying connectivity of the pore region increases. Solid electrolytes with interconnected pores short circuit at lower critical current densities than samples with less connected pores.
T
lytes, the grain boundary size and orientation affects Li-ion conductivity and dendrite migration.19 This degradation pathway initiates at current densities between ∼0.05 and 0.2 mA cm−2, and recent studies have suggested that Li infiltration depends on the surface morphology of the solid electrolyte.14 The most utilized techniques available for imaging lithium within solid electrolytes are ex situ scanning electron and optical microscopy.3,5,14,20 It is challenging to image metallic lithium within the bulk electrolyte because the electrolyte contains high Z-element materials (i.e., zirconium). Even with high-energy X-rays, it is difficult to achieve good phase contrast between metallic lithium and ceramic materials. The present study aims to overcome this challenge by using synchrotron Xray tomography to track structural transformations in LLZO electrolytes. Our three-dimensional reconstructions show an increase in the X-ray transparent region upon failure, which suggests that Li-metal accumulates and expands within the pores. Isolated deposition of lithium metal within the ceramic can mechanistically occur by gaining an electron from the
he elasticity, morphology, and structure of solid|solid interfaces are important for understanding the fundamental mechanisms that promote dendrite formation and propagation across ceramic electrolytes.1,2 Allsolid-state lithium ion batteries (LIBs) are promising candidates for electric vehicle applications because they may enable the use of metallic lithium as an anode. Li metal is thermodynamically unstable in conventional liquid electrolytes but is stable against some garnet-type solid electrolytes (i.e., Li7La3Zr2O12 or LLZO).3−6 Doped LLZO materials have fast ionic conductivities (∼1 mS cm−1 at 298 K)7 and a high shear modulus (60 GPa).8 Theoretical studies have predicted that solid electrolytes with shear moduli greater than 8.5 GPa can negate lithium filament initiation.9 However, experimental studies on ceramicbased materials have contradicted these findings and revealed preferential lithium growth along grains.1,3 While there is significant interest in decreasing the interfacial resistance in allsolid-state batteries with rationally designed coatings,4,10 there is an increasing need to understand how lithium dendrites grow and plate within ceramic electrolytes.11−14 Dendrite formation and propagation in garnet electrolytes is microstructure-driven (i.e., solid electrolyte density, grain size, etc.)15−18 and is independent of the electrolyte composition. In LLZO electro© 2018 American Chemical Society
Received: February 11, 2018 Accepted: March 30, 2018 Published: March 30, 2018 1056
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ACS Energy Letters
Figure 1. Diagram of the synchrotron X-ray tomography setup. The selection of the right beam energy and experimental setup is important for resolving the morphological characteristics of a solid ceramic electrolyte (a). Garnet electrolytes dominated by heavy elements attenuate Xrays, and thus, imaging is impossible. In contrast, using high-energy X-rays at APS (white beam), the pore phase and ceramic phase can be identified independently (b). The porosity, grain, and textural details can be extracted using high-energy X-rays (c).
yield any observable increase in density. Three-dimensional images of each solid electrolyte was obtained from synchrotron X-ray tomography (details in the Supporting Information). Binarization of the tomography data set enabled quantitative analysis of the internal microstructure of the ceramic electrolyte (Figure S1b). The binarization revealed two distinct regions. The first region is identified as the X-ray transparent region, which is composed of pores and/or lithium deposits within the electrolyte. This region is transparent to X-rays because pores (air) and lithium metal both have low attenuation coefficients and thus are invisible to the high-energy X-rays. The second region is the LLZO ceramic electrolyte, which attenuates X-rays strongly. Herein, our aim is to monitor changes in the X-ray transparent region (pores and/or lithium metal) within the bulk of the solid electrolyte. This provides an indirect way to track structural transformations in solid electrolytes after failure. Reconstructing the entire field of view of the ceramic electrolyte (1.6 × 1.4 mm2) is computationally intense; therefore, geometric analysis was completed on varying subvolumes between 50 and 500 μm3 (Figure 2a). Small (50 μm3) subvolumes cannot effectively capture the average microstructure and thus show very large standard deviations. In contrast, large (500 μm3) reconstructed images capture the structural heterogeneities in the ceramic electrolyte. A subvolume of 300 μm3 was found to be optimum and was used in all subsequent analyses in this work. It should be noted that estimation of the geometric parameters is strongly dependent on the binarization step, image processing, and analysis (details in the Supporting Information).26 Each solid electrolyte, except for the electrolyte processed at 1000 °C, demonstrates a biphasic structure composed of voids and ceramic regions. The average pore size within the solid electrolyte processed at 1000 °C is less than 0.65 μm, which falls outside of the resolution of the imaging technique. Scanning electron micrographs corroborate this finding (Figure S5). The pristine solid electrolyte’s porosities are 6.59, 5.61,
oxygen backbone of the ceramic or through electron propagation through the electrolyte. These findings are important and motivate the need for manufacturing and processing methods that can ensure high-density electrolytes and enable grain boundary engineering.21 The X-ray intensity through a sample exponentially drops following the Beer−Lambart law, I(d) = I0e−μd, where μ = μ(E) is the attenuation coefficient, E is the beam energy, and d is the distance of propagation. Good contrast is necessary for effective image reconstruction and requires optimization of the sample transmission properties. The beam energy and sample thickness can affect the transmission and ultimate contrast.22 Synchrotron sources allow a near-continuous selection of monochromatic beam energies or a polychromatic white beam with a broadband energy spectrum (Figure 1a).23 Due to the presence of heavy Z-elements in LLZO, the attenuation length for the monochromatic beams (at APS-2BM) is less than 200 μm (Figure S1a).24 A 35 keV monochromatic beam cannot image a thick solid electrolyte (1000 μm) because no transmission occurs in the sample (Figure 1b). However, at high beam energies (>40 eV), it is possible to distinguish between the ceramic phase (high attenuation coefficient) and void/lithium metal regions (low attenuation coefficient). The high incident energies attainable with the polychromatic white beam ensure good contrast between the ceramic and void region (pores and Li metal) (Figure 1c). The shift to the polychromatic beam is necessary because the cutoff energy of the monochromatic beam is 35 keV at the APS 2-BM beamline. The LLZO solid electrolyte was processed at different temperatures between 1000 and 1150 °C to systematically alter the microstructure and grain size of the solid pellets. The relative density of the ceramic electrolyte increases from 75% at 1000 °C to 92% at 1150 °C (Figure S4). Previous reports have suggested that a critical relative density of >93% can eliminate the formation of dendrites in LLZO electrolytes.15,25 However, further sintering at temperatures greater than 1150 °C did not 1057
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Figure 2. Field of view optimization for 3D reconstructions and solid electrolyte characterization. Small reconstruction volumes (50 μm3) cannot capture the heterogeneity of the material system, and large reconstruction volumes (500 μm3) are computationally intense. The standard deviation between the experimentally measured density and porosity (tomography) is minimized for reconstructions of ∼300 μm3 (a). Pore size distribution for pristine LLZO samples sintered at different temperatures (b) and 3D representations of the X-ray transparent region (or pores) of the pristine LLZO (c). Different colors represent different reconstruction subvolumes between 50 and 500 μm3. Temperature-dependent ionic conductivities (d) and transference number measurements for pristine LLZO (e).
and 4.52% (at 1050, 1100, and 1150 °C). These porosities match well with the density experiments (Figure S4). The sample sintered at 1150 °C shows a wider pore size distribution than the sample sintered at 1050 °C (Figure 2b). Porosity calculations are estimated from the binarized data set and thus are not limited by the technique resolution. As the sintering temperature increases, the relative density also increases. However, the interconnected pathway between pores increases as the temperature increases to 1150 °C (Figure S2). The LLZO grains grew from ∼1 μm (1000 °C) to nearly ∼150 μm (1150 °C) after sintering (Figure S5a−d). Lithium ion diffusion kinetics along the grain boundaries are a large contributing factor in the formation of dendrites and shortcircuiting events in solid-state batteries. Moreover, it has been suggested that the morphology of the interface also plays a role in filament initiation.14 The ionic conductivity for each sample increases from 25 to 100 °C and is proportional to the sintering temperature (Figure 2c). XRD peaks demonstrate a shift to higher angles after sintering the green LLZO powders. This shift can occur because of Li evaporation and/or alumina
diffusion from the Al2O3 crucible. However, after sintering, no shifts in the patterns occur and all samples are cubic in structure (Figure S3). As the sintering temperature increases, the solid electrolyte increases in density. The highest room-temperature ionic conductivity is about 1 × 10−4 S cm−1 for the pellet sintered at 1150 °C for 6 h. The activation energy decreases with sintering temperature from 0.59 (1000 °C), 0.52 (1050 °C), 0.36 (1100 °C), and 0.41 eV (1150 °C).27,28 The electronic and ionic transference numbers are calculated as follows29 σ− σ− t e− = e and t Li+ = 1 − e σtotal σtotal (1) The electrolyte transference number increases with sintering temperature and nearly achieves a value of 1 at 1100 °C. While the density of the electrolyte increases with sintering temperature, the critical current density (CCD) decreases (Figure 3a−c). The critical current density represents the highest current that the electrolyte can withstand before experiencing a shorting event. The grains and grain boundaries 1058
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Figure 3. Galvanostatic cycling of Li/LLZO/Li symmetric cells at ∼20 °C with LLZO electrolytes sintered at 1050 (a), 1100 (b), and 1150 °C (c). The black line represents the applied voltage, and the blue line represents the measured current response. Digital images of a cycled LLZO pellet on the surface and cross section (d).
Figure 4. Morphological changes in LLZO solid electrolytes before and after the CCD is achieved. X-ray tomographic reconstructions of void phase in the interior of LLZO electrolytes sintered at (a) 1050, (b) 1100, and (c) 1150 °C. The changes in pore size distribution between the pristine and failed electrolytes are shown in (d−f) for 1050, 1100, and 1150 °C.
are more visible in samples sintered above 1050 °C. Li metal dendrites can penetrate solid LLZO along the grain boundaries and cause a short circuit.2,5,6,8,16,30 At 1000 °C, the CCD exceeds 0.08 mA cm−2 (Figure S6). The CCD decreases to ∼0.025 mA cm−2 at 1050 °C (Figure 3a) and ∼0.013 mA cm−2
at 1100 and 1150 °C (Figure 3b,c). The voltage drops to zero after the CCD, which indicates an electrically shorted cell that cannot be electrochemically cycled. Electrochemical impedance spectroscopy (EIS) was used to confirm the shorting response (Figure S7). The assembled pristine cells show the character1059
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also be caused by the presence of strong, local electric fields at the grain boundaries induced by the strain in the material. While this failure mode is not catastrophic like dendrite formation, it is expected to cause capacity fade due to loss of active material. X-ray tomography is a versatile tool for probing submicron microstructural properties in dense materials. The development of experimental setups that can enable in situ electrochemical measurements during sample rotation (tomography scan) will provide a means for tracking degradation mechanisms in solid electrolytes. Understanding failure mechanisms within these electrolyte systems is critical for designing resilient solid electrolyte systems. Dendrite formation in solid-state batteries is a microstructure-driven event that is governed by interfacial resistance and nonuniform contact between lithium metal and solid electrolyte. High-density electrolytes are favorable for increasing the critical current limit for solid electrolytes without sacrificing ionic conductivity. While lithium filament initiation can be avoided via low interfacial contact resistance, the formation of isolated lithium agglomerates within the interior is potentially a structurally driven phenomenon that is amplified when pore regions are connected within the electrolyte. Excess chemical potentials created within the interior of the electrolyte as a result of grain resistivity can potentially lead to isolated lithium deposition. Furthermore, lithium within the electrolyte can be reduced either via leakage current or by recombination with a donor electron from the oxygen network. Further work on understanding localized deposition events created by the formation of holes, defects, and strain within the electrolyte will be important for achieving high critical current limits. In summary, the density, grain size, and ionic conductivity of LLZO electrolytes increase with sintering temperature. Li penetration is observed using synchrotron X-ray tomography with high-energy X-rays. Solid electrolytes with connected pore regions promote dendrite formation and lower CCDs.
istic double-semicircle response, which is representative of the ionic and interfacial resistance of the system. No distinct features are observed in the EIS spectra after the CCD is achieved (failed sample). The large grain boundaries in the samples sintered at 1150 °C can facilitate dendrite growth and propogation and are partially responsible for the low CCD. Furthermore, the presence of low-resistance pathways provides a directed path for ionic current at the solid|solid interface. This leads to accelerated Li deposition and lower CCD values (Figure 3d). The morphological transformations in the bulk solid electrolyte can be observed with X-ray tomography (Figure 4). The use of high-energy X-rays significantly decreases the attenuation contrast sensitivity of the low Z-elements (Li and pore region). At high energies, bulk attenuation occurs because of the high Z-element materials or phases. Thus, it is impossible to identify the lithium deposits, dendrites, and pore regions independently. While we cannot detect the difference between lithium and the pore region, we can track changes in the ceramic phase as well as changes in the void region. Changes in the void region will occur if the pores are broadened because of isolated lithium deposition or if penetrated with a lithium dendrite. To probe structural transformations after failure, we directly compared a pristine ceramic electrolyte with a failed sample. Ex situ comparison between failed and pristine ceramic electrolytes enables quantitative comparisons between microstructural details (i.e., pores, grain, etc.). Any changes in the imaged void phase can be attributed to lithium deposition and/or pore broadening because lithium will be invisible to the high-energy X-rays. Lithium deposition within the solid electrolyte is observed through an increase in the pore size distribution between the failed and pristine cells (Figure 4d−f). A significant change in the X-ray transparent region can be seen when evaluating across the entire thickness of the electrolyte (Figure S7). We see distinct microstructural differences across the thickness of the electrolyte, which is a potential signal of isolated lithium deposition within the electrolyte or an accumulation of lithium metal within a pore. The porosities across the cross section of the pristine ceramic electrolytes are uniform, while there are large deviations in porosity observed in the cycled electrolytes (Figure S7). This heterogeneity in void space across the sample suggests that metallic lithium can be deposited in an isolated form or can accumulate and expand within the pore region. Ionic current at the Li/LLZO interface attempts to maximize flow through low-resistance pathways. This preferential distribution of current leads to the formation of dendrites. In this study, samples sintered at 1150 °C show a higher ionic conductivity and lower CCD than samples sintered at 1050 °C. This suggests that the failure could be microstructure-driven. The larger distribution in pore sizes and larger grain boundary sizes leads to greater connectivity between pores within the solid electrolyte sintered at 1150 °C. The connectivity can be directly observed when imaging the 10 largest connected pore structures within the electrolyte (Figure S2). This connectivity, coupled with the higher ionic conductivity, facilitates lithium transport and dendrite growth within these microstructures and leads to a lower CCD. In contrast, even with a higher porosity, the sample sintered at 1050 °C shows a higher CCD due to the disconnected pore network. Local lithium deposition can result from leakage currents across the electrolyte or through the donation of an electron from the lattice oxygen network. It can
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.8b00249. Experimental procedures and methods; attenuation lengths in LLZO for monochromatic beams and a binarized image of LLZO sintered at 1150 °C; connected pore structures; X-ray diffraction of LLZO powder and pellets; relative density with sintering temperature; SEM images of LLZO sintered at different temperatures; galvanostatic cycling of Li/LLZO(1000 °C)/Li; and porosity distribution, void structures, and fraction of the X-ray transparent region of LLZO pellets sintered at different temperatures (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kelsey B. Hatzell: 0000-0002-5222-7288 Author Contributions #
F.S. and M.B.D contributed equally.
Notes
The authors declare no competing financial interest. 1060
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ACKNOWLEDGMENTS This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC0206CH11357. Experiments were conducted on the synchrotron beamline 2-BM. K.B.H and M.D. were supported by the National Science Foundation under Grant No. 1727863. F.S. acknowledges support from Vanderbilt School of Engineering Start-Up Grants. The authors acknowledge the Vanderbilt Institute of Nanoscience and Engineering (VINSE) for access to their shared characterization facilities.
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