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

Li7La3Zr2O12 (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 Li2CO3 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 Li6La3ZrTaO12 (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) Ia3¯ d with Li located in 96h (Li(2)) and 24d (Li(1)) sites, whereas the latter one is displaced toward the general position 96h 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 24d 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 I4¯ 3d from SG Ia3¯ d. This symmetry reduction was found to be related to the site occupation behavior of Li at the tetrahedral 12a site in SG I4¯ 3d. Moreover, Li+ is exchanged by H+ preferably at the 48e site (equivalent to 96h 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.