Proton Bulk Diffusion in Cubic Li₇La₃Zr₂O₁₂ Garnets as Probed by Single X-ray Diffraction

Ceramic electrolytes, characterized by a very high ionic conductivity as it is the case for Al-stabilized cubic Li₇La₃Zr₂O₁₂ (Al:LLZO), are of utmost interest to develop next-generation batteries that can efficiently store electrical energy from renewable sources. If envisaged not as a solid electrolyte but as a protecting layer in lithium-metal batteries with liquid electrolytes, the ceramic should allow Li⁺ to pass through but block out other species such as H⁺. Protons, for example, originating from the decomposition of electrolyte solvent molecules, will form detrimental LiH that severely affects the performance and lifetime of such batteries. Although Li-ion dynamics in Al:LLZO has been the topic of many studies, until today, little information is available about macroscopic proton diffusion in LLZO. Here, we used single-crystal X-ray diffraction to study the Li⁺/H⁺ exchange rate in AL:LLZO over a period of about 3 years. Rietveld refinements reveal that H solely exchanges on the 96h site. The Li/H portion significantly changes from the anhydrous pristine sample to Li_4.21:H_0.66 after 17 days of altering in humid air and finally to Li_2.55:H_2.32 after 960 days. Considering the change of the Li/H portion and the probing depth of X-rays into Al:LLZO, we applied a spherical diffusion model to estimate the proton diffusion coefficient of D₀ ≈ 10^-17 m² s^-1. Such a proton diffusion coefficient value is sufficiently high to have significant impact on cell performance and safety if Al:LLZO is going to be used to protect the Li-metal anode from reaction with the liquid electrolyte. In particular, during Li plating, such a high H⁺ penetration rate may accelerate the formation of LiH, giving rise to safety problems of these types of batteries.