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1 Electronic Supplementary Information The Effect of Pore Connectivity on Li Dendrite Propagation Within LLZO Electrolytes Observed with Synchrotron X-Ray Tomography Fengyu Shen 1,2*, Marm Dixit 2*, Xianghui Xiao 3, and Kelsey B. Hatzell 1,2,4 1 Interdisciplinary Department of Material Science, Vanderbilt University, Nashville, 37235, USA 2 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: Kelsey.B.Hatzell@vanderbilt.edu *Equal Contributions Experimental Methods 1. Material Synthesis A conventional solid-state-reaction method was employed to prepare LLZO powder, using LiOH, La 2 O 3, ZrO 2, and Al 2 O 3 as the starting materials 1. Briefly, 2.5 g LiOH (Pre-dried at 200 C for 6 h) g La 2 O 3 (Pre-dried at 900 C for 12 h), g ZrO 2, and g Al 2 O 3 were mixed and ball milled for 2 h at 500 rpm. The mixture was then annealed at 800 C for 12 h. Cubic LLZO was obtained (Figure S3). The powder was ball milled again for 2 h at 500 rpm and pressed into pellets for the second annealing at C for 6 h. The pellets were covered with the same mother powder during annealing. 2. Characterization The crystal phase was detected by powder X-ray diffraction (Rigaku Smartlab, Tokyo, Japan) with a step of The microstructures of sintered LLZO pellets were examined by fieldemission scanning electron microscope (Zeiss Merlin, Oberhochem, Germany) with an Au conductive coating applied before imaging. 1

2 3. Electrochemical Studies The ionic conductivity was measured using an impedance analyzer (Bio-logic, VMP 3) with frequency from 1 MHz to 1 Hz at AC amplitude of 50 mv. Ag paste was deposited on both sides of the finely polished pellets with Ag wires and dried at 175 C for 0.5 h before test. The symmetrical battery cell was assembled by melting Li foils on both sides of the LLZO pellet. Charging/discharging cycling of the symmetric cells was conducted at a periodic increased current. EIS studies were carried out on the assembled cells before and after the charge/discharge cycling. 4. Tomography Studies, Reconstruction and Data Analysis Synchrotron X-ray tomography was carried out at Beamline 2-BM-B of Advanced Photon Source at the Argonne National Laboratory. Filtered white beam X-rays were used to allow for X-ray penetration through the LLZO pellets projections with an exposure time of 100 ms each were recorded evenly during 180 rotation of the sample. A pco.edge5.5 camera coupled with a mitutoyo long-working distance 10x magnification lens was used that gives a voxel size of 0.65 μm and a total FoV of ~ 1.6 x 1.4 mm 2. Under these experimental conditions, a single tomographic scan took approximately 15 to 20 minutes. The raw data obtained was processed with TomoPy 2. Wavelet-Fourier filtering based ring removal 3 and Paganin phase retrieval 4 were applied to the normalized projection images, and gridrec algorithm 5-6 was used for tomographic reconstruction. The reconstructed images were binarized using autothresholding routines available in Fiji/ImageJ 7. Pore size distribution plugin 8 was used on the binarized image sets to identify and isolate solid phase in the sample. This is carried out by fitting the pore region (dark regions of Figure S1b, Figure 2c) of the binarized 3D data set with spheres of different radii. The 2

3 histogram of the sphere radii acts as the continuous pore size distribution of the sample. The pore size distribution analysis of the failed samples was carried out on larger subvolumes (500 µm x 1.1 mm x 1.1 mm) to remove possible user bias. Figure S1 Attenuation lengths in LLZO for the monochromatic beam (a) and binarized image of LLZO sintered at 1150 C (b). Figure S2 Ten largest connected pore structures for LLZO samples sintered at 1050 C (a) 1100 C (b) and 1150 C (c) 3

4 Figure S3 XRD patterns of LLZO powder sintered at 800 C for 12 hand LLZO pellets sintered at 1000 C, 1050 C, 1100 C and 1150 C (a) and the magnified (211) peak (b). Figure S4 Density of LLZO pellet with sintering temperature. 4

5 Figure S5 Microstructures of LLZO pellets sintered at different temperatures (a) 1000, (b)1050, (c) 1100, and (d) Figure S6 EIS before and after galvanostatic cycling of Li/LLZO/Li symmetric cells at ~20 C with LLZO electrolytes sintered at (a) 1050 C, (b) 1100 C, (c) 1150 C. 5

6 Figure S7 Galvanostatic cycling of Li/LLZO/Li symmetric cells at ~20 C with LLZO electrolyte sintered at 1000 C. Figure S8 Sample field of View for sub volumes analyzed in terms of z-axis pore size distributions (a). Void structures obtained for failed samples (b) and fraction of X-ray transparent region for failed and pristine sample sintered at three temperatures (c). References: (1) Zheng, J.; Tang, M. X.; Hu, Y. Y. Lithium Ion Pathway within Li7la3zr2o12-Polyethylene Oxide Composite Electrolytes. Angew Chem Int Edit 2016, 55,

7 (2) Gürsoy, D.; De Carlo, F.; Xiao, X.; Jacobsen, C. Tomopy: A Framework for the Analysis of Synchrotron Tomographic Data. J. Synchrotron Radiat. 2014, 21, (3) Münch, B.; Trtik, P.; Marone, F.; Stampanoni, M. Stripe and Ring Artifact Removal with Combined Wavelet Fourier Filtering. Opt. Express 2009, 17, (4) Paganin, D.; Mayo, S.; Gureyev, T. E.; Miller, P. R.; Wilkins, S. W. Simultaneous Phase and Amplitude Extraction from a Single Defocused Image of a Homogeneous Object. J. Microscopy 2002, 206, (5) Rivers, M. L. In Tomorecon: High-Speed Tomography Reconstruction on Workstations Using Multi-Threading, Proc. SPIE, 2012; p 85060U. (6) Dowd, B. A.; Campbell, G. H.; Marr, R. B.; Nagarkar, V. V.; Tipnis, S. V.; Axe, L.; Siddons, D. P. In Developments in Synchrotron X-Ray Computed Microtomography at the National Synchrotron Light Source, Developments in X-ray Tomography II, International Society for Optics and Photonics: 1999; pp (7) Schneider, C. A.; Rasband, W. S.; Eliceiri, K. W. Nih Image to Imagej : 25 Years of Image Analysis. Nat. Methods 2012, 9, (8) Munch, B.; Holzer, L. Contradicting Geometrical Concepts in Pore Size Analysis Attained with Electron Microscopy and Mercury Intrusion. J. Am. Ceram. Soc. 2008, 91,