Effect of Pore Connectivity on Li Dendrite Propagation within LLZO

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The Effect of Pore Connectivity on Li Dendrite Propagation Within LLZO Electrolytes Observed with Synchrotron X-ray Tomography Fengyu Shen, Marm Dixit, Xianghui Xiao, and Kelsey Hatzell ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00249 • Publication Date (Web): 30 Mar 2018 Downloaded from http://pubs.acs.org on April 1, 2018

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ACS Energy Letters

The Effect of Pore Connectivity on Li Dendrite Propagation Within LLZO Electrolytes Observed with Synchrotron X-Ray Tomography Fengyu Shen1,2*, Marm B. Dixit2*, Xianghui Xiao3, and Kelsey B. Hatzell1,2,4§ 1

Interdisciplinary Department of Material Science, Vanderbilt University, Nashville, 37235, USA Department of Mechanical Engineering, Vanderbilt University, Nashville TN, 37235, USA 3 X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne IL, 60439, USA 4 Department of Chemical and Biomolecular Engineering, Vanderbilt University, Nashville TN, 37235, USA § Correspondence: [email protected] *Equal Contributions Abstract 2

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 structure-processing relationships for practical design of solid electrolytes, the present study tracks structural transformations in solid electrolytes processed at three different temperatures (1000, 1050 and 1150 °C) using synchrotron x-ray tomography. A sub volume 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.

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The elasticity, morphology, and structure of solid|solid interfaces are important for understanding the fundamental mechanisms that promote dendrite formation and propagation across ceramic electrolytes1-2. All-solid-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 298K)7 and a high shear moduli (60 GPa)8. Theoretical studies have predicted that solid electrolytes with shear moduli greater than 8.5 GPa can negate lithium filament initiation9. However, experimental studies on ceramic-based materials have contradicted these findings and revealed preferential lithium growth along grains1, 3. While there is a significant interest in decreasing the interfacial resistance in all-solid state batteries with rationally designed coatings4, 10, there is an increasing need to understand how lithium dendrites grow and plate within ceramic electrolytes11-14. 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 electrolytes, grain boundary size and


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orientation affects Li-ion conductivity and dendrite migration19. This degradation pathway initiates at current densities between ~0.05-0.2 mA cm-2 and recent studies have suggested that Li infiltration depends on the surface morphology of the solid electrolyte14. The most utilized techniques available for imaging lithium within solid electrolytes are exsitu scanning electron and optical microscopy3, 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 x-ray tomography to track structural transformations in LLZO electrolytes. Our 3-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 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 engineering21. The X-ray intensity through a sample exponentially drops following the Beer-Lambart law, 𝐼 𝑑 = 𝐼$ 𝑒 &'( , 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 contrast22. Synchrotron sources allow a nearcontinuous selection of monochromatic beam energies or a polychromatic white beam with a broad band energy spectrum (Fig. 1a)23. Due to the presence of heavy Z-elements in LLZO, the



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attenuation length for the monochromatic beams (at APS-2BM) is less than 200 μm (Fig. S1a)24. A 35 keV monochromatic beam cannot image a thick solid electrolyte (1000 μm) because no transmission occurs in the sample (Fig. 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 ensures good contrast between the ceramic and void region (pores and Li metal) (Fig. 1c). The shift to the polychromatic beam is necessary because the cut-off energy of the monochromatic beam is 35 keV at the APS 2-BM beamline.

Figure 1. A diagram of the synchrotron X-ray tomography setup. The selection of the right beam energy and experimental set-up is important for resolving the morphological characteristics of a solid ceramic electrolyte (a). Garnet electrolytes dominated by heavy elements cannot attenuate X-rays and thus imaging is impossible. In contrast using high-energy x-rays at APS (white beam)


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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).

(a)

(c)

(b)

50 μm

25 μm

100 μm

50 μm

200 μm

100 μm

300 μm

150 μm

(d)

400 μm

200 μm

500 μm

250 μm

(e)

Figure 2. Field of view optimization for 3D reconstructions and solid electrolyte characterization. Small reconstruction volumes (50𝜇𝑚) cannot capture the heterogeneity of the material system and large reconstruction volumes (500𝜇𝑚) are computationally intense. The standard deviation between the experimentally measured density and porosity (tomography) is minimized for



reconstructions ~300𝜇𝑚 (a). The 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 sub volumes between 50 and 500𝜇𝑚. Temperature dependent ionic conductivities (d) and transference number measurements for pristine LLZO (e). 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 (Fig. S4). Previous reports have suggested a critical relative density of >93% can eliminate the formation of dendrites in LLZO electrolytes15, 25. However, further sintering at temperatures greater than 1150 °C did not yield any observable increase in density. Three-dimensional images of each solid electrolyte was obtained from synchrotron x-ray tomography (details in supporting information). Binarization of the tomography dataset enables quantitative analysis of the internal microstructure of the ceramic electrolyte (Fig 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 highenergy 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.


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Reconstructing the entire field of view of the ceramic electrolyte (1.6x1.4mm2) is computationally intense, so a geometric analysis was completed on varying sub volumes between 50 and 500 µm3 (Fig 2a). Small (50 µm3) sub volumes 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 sub volume of 300 µm3 was found to be optimum and was used in all subsequent analyses in this work. It should be noted that the estimation of the geometric parameters is strongly dependent on the binarization step, image processing, and analysis (details in 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 the resolution of the imaging technique. Scanning electron micrographs corroborate this finding (Fig. S5). The pristine solid electrolyte’s porosities are 6.59%, 5.61% and 4.52% (at 1050 °C, 1100 °C, and 1150 °C). These porosities match well with the density experiments (Fig. S4). The sample sintered at 1150 °C shows a wider pore size distribution than the sample sintered at 1050 °C (Fig. 2b). Porosity calculations are estimated from the binarized dataset 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 (Fig. S2). The LLZO grains grew from ~1 µm (1000 °C) to nearly ~150 µm (1150 °C) after sintering (Fig. S5a-d). Lithium ion diffusion kinetics along the grain boundaries are a large contributing factor in the formation of dendrites and short-circuiting events in solid state batteries. Moreover, it has



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been suggested that the morphology of the interface also plays a role in filament initiation14. The ionic conductivity for each sample increases from 25-100 °C and is proportional to the sintering temperature (Fig. 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 (Fig. 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), 0.41 eV (1150 °C)27-28. The electronic and ionic transference numbers are calculated as follows29: 𝑡3 4 =

5 64 57879:

and 𝑡;< = = 1 −

5 64 57879:

(1)

The electrolyte transference number increases with sintering temperature and nearly achieves a value of 1 at 1100 °C.


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Figure 3 Galvanostatic cycling of Li/LLZO/Li symmetric cells at ~20 °C with LLZO electrolytes sintered at 1050 °C (a), 1100 °C (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 surface and cross section (d). While the density of the electrolyte increases with sintering temperature, the critical current density (CCD) decreases (Fig. 3a-c). The critical current density represents the highest current the electrolyte can withstand before experiencing a shorting event. The grains and grain boundaries are more visible in samples sintered above 1050°C. Li metal dendrites can penetrate solid LLZO along the grain boundaries and cause a short circuit2, 5-6, 8, 16, 30. At 1000 °C the CCD exceeds 0.08 mA cm-2 (Fig. S6). The CCD decreases to ~0.025 mA cm-2 at 1050 °C (Fig. 3a) and ~0.013 mA cm-2 at 1100 °C and 1150 °C (Fig. 3b and 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 characteristic 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 the accelerated Li deposition and lower CCD values (Fig. 3d). The morphological transformations in the bulk solid electrolyte can be observed with Xray tomography (Fig. 4). The use of a 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 elements 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.


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Figure 4 Morphological changes in LLZO solid electrolytes before and after the critical current density (CCD) is achieved. X-ray tomographic reconstructions of void phase in the interior of LLZO electrolytes sintered at (a) 1050 °C (b) 1100 °C and (c) 1150 °C. The change in pore size distribution between the pristine and failed electrolytes are shown in (d)-(f) for 1050 °C, 1100 °C and 1150 °C respectively.

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 enable 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 (Fig. 4d-f). A significant change in the x-ray transparent region can be seen when evaluating across the entire thickness of the electrolyte (Fig. 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 is uniform, while there are large deviations in porosity observed in the cycled electrolytes (Fig. 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 ten largest connected pore structures within the electrolyte (Fig. 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


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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 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 sub-micron 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 which is governed by the interfacial resistance and non-uniform 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 which 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, density, grain



size, and ionic conductivity of LLZO electrolytes increases 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 critical current densities. Supporting Information 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; Porosity distribution, void structures and fraction of X-ray transparent region of LLZO pellets sintered at different temperatures; 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-AC02-06CH11357. 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. References: (1) Ren, Y.; Shen, Y.; Lin, Y.; Nan, C.-W. Direct Observation of Lithium Dendrites Inside GarnetType Lithium-Ion Solid Electrolyte. Electrochem. Commun. 2015, 57, 27-30.


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(2) Yu, S.; Schmidt, R. D.; Garcia-Mendez, R.; Herbert, E.; Dudney, N. J.; Wolfenstine, J. B.; Sakamoto, J.; Siegel, D. J. Elastic Properties of the Solid Electrolyte Li7La3Zr2O12 (LLZO). Chem. Mater. 2015, 28, 197-206. (3) Cheng, E. J.; Sharafi, A.; Sakamoto, J. Intergranular Li Metal Propogation through Polycrystalline Li6.25Al0.25La3Zr2O12 Ceramic Electrolyte. Electrochim. Acta 2017, 223, 85-91. (4) Ma, C.; Cheng, Y.; Yin, K.; Luo, J.; Sharafi, A.; Sakamoto, J.; Li, J.; More, K. L.; Dudney, N. J.; Chi, M. Interfacial Stability of Li Metal–Solid Electrolyte Elucidated via in Situ Electron Microscopy. Nano Lett. 2016, 16, 7030-7036. (5) Sharafi, A.; Haslam, C. G.; Kerns, R. D.; Wolfenstine, J.; Sakamoto, J. Controlling and Correlating the Effect of Grain Size with the Mechanical and Electrochemical Properties of Li7La3Zr2O12 Solid-State Electrolyte. J. Mater. Chem. A 2017, 5, 21491-21504. (6) Sharafi, A.; Meyer, H. M.; Nanda, J.; Wolfenstine, J.; Sakamoto, J. Characterizing the Li– Li7La3Zr2O12 Interface Stability and Kinetics as a Function of Temperature and Current Density. J. Power Sources 2016, 302, 135-139. (7) Thangadurai, V.; Narayanan, S.; Pinzaru, D. Garnet-Type Solid-State Fast Li Ion Conductors for Li batteries: Critical Review. Chem. Soc. Rev. 2014, 43, 4714-4727. (8) Yu, S.; Schmidt, R. D.; Garcia-Mendez, R.; Herbert, E.; Dudney, N. J.; Wolfenstine, J. B.; Sakamoto, J.; Siegel, D. J. Elastic Properties of the Solid Electrolyte Li7La3Zr2O12 (LLZO). Chem. Mater. 2016, 28, 197-206. (9) Monroe, C.; Newman, J. Dendrite Growth in Lithium/Polymer Systems A Propagation Model for Liquid Electrolytes Under Galvanostatic Conditions. J. Electrochem. Soc. 2003, 150, A1377-A1384. (10) Han, X.; Gong, Y.; Fu, K. K.; He, X.; Hitz, G. T.; Dai, J.; Pearse, A.; Liu, B.; Wang, H.; Rubloff, G. Negating Interfacial Impedance in Garnet-Based Solid-State Li Metal Batteries. Nat. Mater. 2017, 16, 572-579. (11) Yulaev, A.; Oleshko, V.; Haney, P.; Liu, J.; Qi, Y.; Talin, A. A.; Leite, M. S.; Kolmakov, A. From Microparticles to Nanowires and Back: Radical Transformations in Plated Li Metal Morphology Revealed via in Situ Scanning Electron Microscopy. 2018. (12) Leite, M. S.; Ruzmetov, D.; Li, Z.; Bendersky, L. A.; Bartelt, N. C.; Kolmakov, A.; Talin, A. A. Insights into Capacity Loss Mechanisms of All-Solid-State Li-Ion Batteries with Al Anodes. J. Mater. Chem. A 2014, 2, 20552-20559. (13) Santhanagopalan, D.; Qian, D.; McGilvray, T.; Wang, Z.; Wang, F.; Camino, F.; Graetz, J.; Dudney, N.; Meng, Y. S. Interface Limited Lithium Transport in Solid-State Batteries. J. Phys. Chem. Lett. 2013, 5, 298-303. (14) Porz, L.; Swamy, T.; Sheldon, B. W.; Rettenwander, D.; Frömling, T.; Thaman, H. L.; Berendts, S.; Uecker, R.; Carter, W. C.; Chiang, Y. M. Mechanism of Lithium Metal Penetration Through Inorganic Solid Electrolytes. Adv. Energy Mater. 2017, 7, 1701003. (15) Cheng, L.; Chen, W.; Kunz, M.; Persson, K.; Tamura, N.; Chen, G.; Doeff, M. Effect of surface microstructure on electrochemical performance of garnet solid electrolytes. ACS Appl. Mater. Interfaces. 2015, 7, 2073-2081. (16) Kim, Y.; Yoo, A.; Schmidt, R.; Sharafi, A.; Lee, H.; Wolfenstine, J.; Sakamoto, J. Electrochemical Stability of Li6.5La3Zr1.5M0.5O12 (M= Nb or Ta) against Metallic Lithium. Front. Energy Res. 2016, 4, 20.



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Effect of Pore Connectivity on Li Dendrite Propagation within LLZO

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