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

Abstract

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) Ia3̅d 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 I4̅3d from SG Ia3̅d. This symmetry reduction was found to be related to the site occupation behavior of Li at the tetrahedral 12a site in SG I4̅3d. 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.

Aging Behavior of Al- and Ga- Stabilized Li7La3Zr2O12 Garnet-Type, Solid-State Electrolyte Based on Powder and Single Crystal X-ray Diffraction

“…During aging in air, lattice parameters generally expand, which is indicative of Li + -H + exchange reaction as proposed in the literature [14,17,20,24,25,30]. In detail, individual peaks in the PXRD pattern of Al10-LLZO shift to lower Bragg angles (indicative of an increase in lattice parameters) with increased exposure to dry air.…”

“…This is shown for some selected aging times in Figure 2. This development of low angle shoulders was not observed during the aging of Li6La3ZrTaO12 garnet (LLZTO) in the air in the study of [24] but is evident in Al10-LLZO and Ga40-LLZO samples aged in air in this study. Data can be generally refined with a single garnet phase.…”

“…Where this was not possible (aging of small single crystals), uncrushed pellets were packed under Argon and kept in a glove box until use and stored for no longer than 4 weeks. The methodology of aging is very similar to that used for Li 6 La 3 TaZrO 12 [24].…”

“…Based on inelastic neutron scattering data, they propose that it is primary H + , not H 3 O + , which is present in H-LLZO. Very recently, Redhammer et al [24] investigated the stability of Ta-substituted LLZO (Li 6 La 3 ZrTaO 12 ) under wet environments and mild hydrothermal conditions and, indeed, they observed a structural phase transition to the I43d symmetry under deep hydration conditions, accompanied by distinct increases in lattice parameters and loss of Li + mainly at the 96h and 48e positions.…”

Investigating the electrochemical stability of Li7La3Zr2O12 solid electrolytes using field stress experiments

“…While in all previously shown experiments micro-structuring was performed using a combination of photolithography and ion beam etching, the microelectrodes used for the experiment series on Ga:LLZO were prepared via direct sputtering using a Ni shadow mask. Since this process is solvent-free, it can be excluded that contact with protic solutions during photolithography (see experimental), potentially leading to Li + /H + exchange [37][38][39] , is the reason for the observed phenomena.…”