Wet-Environment-Induced Structural Alterations in Single- and Polycrystalline LLZTO Solid Electrolytes Studied by Diffraction Techniques

ACS Applied Materials & Interfaces, vol. 13, pp. 350–359

Abstract

Li 7 La 3 Zr 2 O 12 (LLZO) is one of the potential candidates for Li metal-based solid-state batteries owing to its high Li + conductivity (≈10 –3 S cm –1 ) at room temperature and large electrochemical stability window. However, LLZO undergoes protonation under the influence of moisture-forming Li 2 CO 3 layers, thereby affecting its structural and transport properties. Therefore, a detailed understanding on the impact of the exchange of H + on Li + sites on structural alteration and kinetics under the influence of wet environments is of great importance. The present study focuses on the Li + /H + exchange in single-crystal and polycrystal Li 6 La 3 ZrTaO 12 (LLZTO) garnets prepared using the Czochralski method and solid-state reactions subjected to weathering in air, aqueous solutions at room temperature, and in aqueous solution at 363 K using X-ray diffraction (XRD) and neutron diffraction (ND) techniques. Based on 36 single-crystal diffraction and 88 powder diffraction measurements, we found that LLZTO crystallizes with space group (SG) Ia 3̅ d with Li located in 96 h (Li(2)) and 24 d (Li(1)) sites, whereas the latter one is displaced toward the general position 96 h forming shorter Li(1)–Li(2) jump distances. The degradation in air, wet air, water, and acetic acid leads to a Li + /H + exchange that preferably takes place at the 24 d site, which is in contrast to previous reports. Higher Li + /H + was observed for LLZTO aged in water at 363 K that reduced the symmetry to SG I 4̅3 d from SG Ia 3̅ d . This symmetry reduction was found to be related to the site occupation behavior of Li at the tetrahedral 12 a site in SG I 4̅3 d . Moreover, Li + is exchanged by H + preferably at the 48 e site (equivalent to 96 h site). We also found that the equilibrium H + concentrations in all media tested remains very similar, which is related to the H + diffusion in the LLZTO-controlled exchange process. Only the increase in temperature led to a significant increase in the exchange capacity as well as in the Li + /H + exchange rate. Overall, we found that the exchange rate, exchange capacity, site occupation behavior of Li + and H +, as well as the structural stability of LLZTO, strongly depend on the composition. These findings suggest that measurements on a single LLZTO variant sample do not lead to a general conclusion for all garnets to guide the field toward better materials. In contrast, each composition has to be analyzed exclusively to understand the interplay of composition, structure, and exchange kinetic properties.

Authors 7

1. Günther Josef Redhammer corresponding

   University of Salzburg

   Affiliation as printed

   Division of Materials Science and Mineralogy, Department of Chemistry and Physics of Materials, University of Salzburg, Jakob-Haringerstr. 2A, Salzburg 5020, Austria

2. Pavan P. Badami

   Arizona State University

   Affiliation as printed

   The Polytechnic School, Ira A. Fulton Schools of Engineering, Arizona State University, Mesa, Arizona 85212, United States

3. Martin Meven Aachen Institute of Crystallography

   Forschungszentrum Jülich · Heinz Maier-Leibnitz Zentrum · RWTH Aachen University

   Affiliation as printed

   Institute of Crystallography, RWTH Aachen University, Jaegerstr. 17/19, Aachen 52056, Germany

   Jülich Centre for Neutron Science (JCNS), Forschungszentrum Jülich GmbH at Heinz Maier-Leibnitz Zentrum (MLZ), Lichtenbergstr. 1, Garching 85748, Germany

4. Steffen Ganschow

   Leibniz Institute for Crystal Growth

   Affiliation as printed

   Leibniz-Institut für Kristallzüchtung, Max-Born-Straße 2, Berlin 12489, Germany

5. Stefan Berendts

   Technische Universität Berlin

   Affiliation as printed

   Institute of Chemistry, Technical University of Berlin, Straße des 17. Juni 135, Berlin 10623, Germany

6. Gerold Tippelt

   University of Salzburg

   Affiliation as printed

   Division of Materials Science and Mineralogy, Department of Chemistry and Physics of Materials, University of Salzburg, Jakob-Haringerstr. 2A, Salzburg 5020, Austria

7. Daniel Rettenwander corresponding

   Graz University of Technology

   Affiliation as printed

   Institute for Chemistry and Technology of Materials, Graz University of Technology, Stremayrgasse 9, Graz 8010, Austria