On High-Temperature Thermal Cleaning of LLZO Solid-State Electrolytes

The deployment of Li₇La₃Zr₂O₁₂ (LLZO) garnets as Li-ion solid-state electrolytes in Li-metal solid-state batteries is partially hindered by their chemical reaction with water and carbon dioxide, leading to the formation of a Li₂CO₃/LiOH contamination layer on the LLZO surface.^{[1, 2]} The latter causes high Li/LLZO interfacial resistance, voltage polarization at the Li anode, and induces the formation of Li dendrites.^[3] To address this issue, in this work, we investigate the applicability of high temperature annealing (600 – 900 °C) as a method for thermal cleaning of LLZO. Through an in-depth analysis of the LLZO surface using X-ray photoelectron spectroscopy, hard X-ray photoelectron spectroscopy, <i>in-situ</i> grazing incidence synchrotron X-ray diffraction and Raman spectroscopy, we have found that high-temperature thermal cleaning has an inherent issue. On the one hand, heat-treatment allows a significant reduction in Li₂CO₃/LiOH contamination. On the other hand, the use of high temperatures of 600 −900 °C leads to the formation of the La₂Zr₂O₇ phase on the LLZO surface as a result of Li losses. The effect of both factors on the Li/LLZO interfacial resistance, voltage polarization and critical current density of Li/LLZO/Li symmetrical cells have been thoroughly investigated by electrochemical impedance spectroscopy and galvanostatic cycling measurements.<br/><br/><b>References</b><br/>[1] R.H. Brugge, A.K.O. Hekselman, A Cavallaro, F.M. Pesci, R.J. Chater, J.A. Kilner, A. Aguadero, <i>Chem. Mater. </i><b>2018</b>, 30, 3704-3713.<br/>[2] R. Dubey, J. Sastre, C. Cancellieri, F. Okur, A. Forster, L. Pompizii, A. Priebe, Y. E. Romanyuk, L. P. H. Jeurgens, M. V. Kovalenko, K. V. Kravchyk, <i>Adv. </i><i>Energy Mater</i>. <b>2021</b>, 2102086.<br/>[3] T. Krauskopf, F.H. Richter, W.G. Zeier, J. Janek, <i>Chem. Rev. </i><b>2020</b>, 120, 7745-7794.