Dynamical evolution of CO₂ and H₂O on garnet electrolyte elucidated by ambient pressure X-ray spectroscopies

Garnet-type Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLZO) is considered a promising solid electrolyte, but the surface degradation in air hinders its application for all-solid-state battery. Recent studies have mainly focused on the final products of the LLZO surface reactions due to lacking of powerful in situ characterization methods. Here, we use ambient pressure X-ray spectroscopies to in situ investigate the dynamical evolution of LLZO surface in different gas environments. The newly developed ambient pressure mapping of resonant Auger spectroscopy clearly distinguishes the lithium containing species, including LiOH, Li₂O, Li₂CO₃ and lattice oxygen. The reaction of CO₂ with LLZO to form Li₂CO₃ is found to be a thermodynamically favored self-limiting reaction. On the contrary, the reaction of H₂O with LLZO lags behind that of CO₂, but intensifies at high pressure. More interestingly, the results provide direct spectroscopic evidence for the existence of Li⁺/H⁺ exchange and reveal the importance of the initial layer formed on clean electrolyte surface in determining their air stability. This work demonstrates that the newly developed in situ technologies pave a new way to investigate the oxygen evolution and surface degradation mechanism in energy materials.