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

Area of Science:

• Materials Science
• Solid-State Electrochemistry
• Crystallography

Background:

• Lithium-ion conductivity in solid-state batteries is crucial for next-generation energy storage.
• Li₇La₃Zr₂O₁₂ (LLZO) exhibits promising ionic conductivity but is susceptible to degradation via protonation.
• Understanding Li⁺/H⁺ exchange mechanisms is vital for stabilizing LLZO-based solid electrolytes.

Purpose of the Study:

• To investigate the impact of Li⁺/H⁺ exchange on the structural integrity and ion transport kinetics of Li₆La₃ZrTaO₁₂ (LLZTO) garnets.
• To elucidate the influence of environmental conditions (moisture, temperature) and composition on LLZTO degradation.
• To provide insights into the site-specific exchange mechanisms and their effect on crystal symmetry and Li⁺ diffusion pathways.

Main Methods:

• Synthesis of single-crystal and polycrystal LLZTO using Czochralski and solid-state reaction methods.
• Exposure of LLZTO samples to various weathering conditions: air, aqueous solutions at room temperature, and aqueous solution at 363 K.
• Structural characterization using X-ray diffraction (XRD) and neutron diffraction (ND) to analyze changes in crystal structure, space group, and site occupancy.

Main Results:

• LLZTO crystallizes in space group Ia3̅d with Li occupying 96h and 24d sites.
• Li⁺/H⁺ exchange preferentially occurs at the 24d site in LLZTO under humid conditions, contrary to previous findings.
• Aging in water at 363 K reduced LLZTO symmetry to I4̅3d, linked to Li occupation at the 12a site, and increased exchange capacity and rate, especially with temperature.

Conclusions:

• The Li⁺/H⁺ exchange process in LLZTO is strongly dependent on environmental conditions, particularly temperature, and specific sample composition.
• Degradation pathways and structural changes vary significantly between different LLZTO compositions, necessitating individual analysis.
• Understanding composition-structure-kinetics interplay is critical for designing stable LLZO-based solid electrolytes for solid-state batteries.