Interfacial Stability of Li-metal/Solid Electrolyte Elucidated via In Situ Electron Microscopy

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1 Supplementary Information Interfacial Stability of Li-metal/Solid Electrolyte Elucidated via In Situ Electron Microscopy C. Ma, a Y. Cheng, b K. Yin, a J. Luo, c A. Sharafi, d J. Sakamoto, d J. Li, e K. L. More, e and N. J. Dudney, e M. Chi a, * a Center of Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA. b Chemical and Engineering Materials Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA. c Department of NanoEngineering, University of California at San Diego, La Jolla, California 92093, USA. d Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA. e Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA. Methods Materials and macroscopic characterizations. The preparation of c-llzo and t-llzo ceramics is described elsewhere 1. For c-llzo, a small amount of Al was used to stabilize the cubic structure 1, resulting in a composition of (Li 6.25 Al 0.25 )La 3 Zr 2 O 12. On the other hand, since the tetragonal structure is thermodynamically stable for Al-free compositions 2, 3, the doping was not performed for t-llzo. Therefore, its composition remained as Li 7 La 3 Zr 2 O 12. In order to acquire dense ceramics, hot-pressing was performed, and the relative density was measured to be above 97%. According to the X-ray diffraction experiments, these ceramics are all single-phase with the expected garnet structures. Theoretical calculations. Ab initio calculations were performed with the Vienna Ab initio Simulation Package (VASP) 4. The Projector Augmented Wave (PAW) method 4, 5 was used to 1

2 describe the effects of core electrons, and Perdew-Burke-Ernzerhof (PBE) 6 implementation of the Generalized Gradient Approximation (GGA) for the exchange-correlation functional. Energy cutoff was 500eV for the plane-wave basis of the valence electrons. The cubic and tetragonal phases were calculated for two compositions, Li 6.25 Al 0.25 La 3 Zr 2 O 12 and Li 6.75 Al 0.25 La 3 Zr 2 O 12, to compare the potential energy of the relaxed configurations. The lattice parameters and atomic coordinates for the initial structure were generated according to literature 2, 7, 8. The 2 Al atoms randomly occupy the 24d sites (for the cubic phase) or 8a sites (for the tetragonal phase). To obtain statistical information on such disordered systems, ten different configurations for each phase at each composition were generated. For each configuration, ab initio molecular dynamics simulation was first conducted to anneal the structure at 600K for 1ps (time step 1fs) with the box size/shape fixed, so that any unrealistic local distribution of Li can be removed and the system will not be trapped in an instantaneous local potential energy minimum. The structure after annealing was then fully relaxed to the potential energy minimum using the conjugate gradient algorithm. Energy tolerance is 0.001eV per atom for the electronic structure calculation, and 0.01eV per atom for the potential energy minimization. It should be noted that entropy is not considered in the calculation. At a finite temperature, the balance may be shifted due to the different entropies of the two systems and their dependences on the Li content. However, the shift is expected to be small at room temperature 2, so the trend is unlikely to be affected. Electron microscopy study. The c-llzo specimen used for the in situ STEM study was prepared using a Hitachi NB-5000 dual (Focused Ion/Electron) beam microscope from a freshly ground surface of the c-llzo ceramic. When the specimen was properly prepared, it was 2

3 transferred into a pumping station using an air-protection holder and stored at 10-5 Torr. Before the in situ STEM study, the c-llzo specimen was mounted to a Nanofactory scanning probe TEM holder in a glove bag filled with dry helium. Within the same glove bag, Li metal was scratched from a freshly cut Li foil using a W tip, which was also transferred to the Nanofactory holder. Afterwards, the holder with separated c-llzo and Li metal was inserted into the microscope, and then the electron beam was used to sculpt Li, removing any possible lithium oxides on the metal surface. The absence of contamination was confirmed for both c-llzo and Li via EELS (Supplementary Fig. S2). During the in-situ STEM observation, the Li-coated W tip was driven to contact the c-llzo using the piezomanipulator on the Nanofactory holder. The STEM/EELS study was carried out on an aberration-corrected FEI Titan S TEM/STEM equipped with a Gatan Image Filter Quantum-865 operated at 300 kv. Z-contrast HAADF-STEM imaging was performed with a probe convergence angle of 30 mrad and a large inner collection angle of 65 mrad. EELS data were collected in STEM mode using a 5 mm aperture and a spectrometer collection angle of 40 mrad. The spectrum imaging in Fig. 1 and individual spectra in Fig. 2 were acquired at a dispersion of 0.25 and 0.1 ev per channel, respectively. The specimen thickness was calculated from the EELS data using the log-ratio method 9. 3

4 Figure S1 In situ STEM configuration used in the present study. At the beginning of the experiment, c-llzo and Li metal were separated. The two materials made contact to form the interface in situ in the microscope, during which the real-time structure and chemistry evolutions were monitored with high spatial resolution. 4

5 a O-K b C-K Li 2 O Li Li 2 CO 3 c-llzo Figure S2 Possible contamination of Li and LLZO was successfully prevented. a, EELS data of the Li metal used for the in-situ observation. The O-K edge of the standard Li 2 O powder was displayed for comparison. No O signal was detected in the Li metal. b, EELS data of the LLZO specimen used for the in-situ observation. The C-K edge of the standard Li 2 CO 3 powder was displayed for comparison. No C signal was detected in our c-llzo specimen. 5

6 a Zr-L 3 ZrO 2 Zr b Li-K Interfacial layer Zr-L 2 Li 2 O c-llzo Figure S3 Verifying the possibility of decomposition through EELS. a, Standard Zr-L 2,3 edges of ZrO 2 and Zr in literature 10. b, Li-K edges of the interfacial layer, Li 2 O, and c-llzo. The red, blue, and black dashed lines indicated the energy loss where the Li-K edge of the interfacial layer, Li 2 O, and pristine c-llzo peaked, respectively. 6

7 Table S1 Mean atomic numbers of LLZO and the possible decomposition products. Chemical formula Mean atomic number Pristine material (Li 6.25 Al 0.25 )La 3 Zr 2 O Possibility I 11 La 2 O Zr 40 Li 8 ZrO Zr 40 Decomposition Possibility II 12 La 2 O products Li 2 O 4.67 Possibility III 12, Zr 3 O La 2 O Li 2 O

8 a b t = 0.10 λ = 14.9 nm Figure S4 Specimen thickness obtained from EELS analysis. a, EELS data collected from a nm 2 region at the edge of the LLZO specimen used in the present study. b, Plasmon peak of the spectrum in a. The log-ratio method 14 was used to evaluate the relative specimen thickness t/λ from the intensities of the plasmon peak and zero-loss peak (t and λ are the actual specimen thickness and inelastic mean free path, respectively). The Iakoubovskii method 14 was used to calculate the inelastic mean free path λ for LLZO. From the values of t/λ and λ, the actual specimen thickness t was obtained as 14.9 nm. 8

9 Figure S5 Verifying the possibility of decomposition through Z-contrast images. a, HAADF-STEM image showing that a portion of the c-llzo specimen was in-situ buried within the Li foil. The region highlighted with red dashed lines was scrutinized at a higher magnification in b and c. b,c, High-magnification HAADF-STEM images of c-llzo before and after the 30-min Li treatment. The region buried within Li was delineated with blue dashed lines. 9

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