Storage of Garnet Solid Electrolytes: Insights into Air Stability and Surface Chemistry

Solid-state lithium metal batteries (SSLMBs) have been widely considered as an “enabler” for the next-generation high-energy density batteries and simultaneously solve the safety issues of liquid lithium-ion batteries. Garnet-type Li7La3Zr2O12 (LLZO) is one of the most promising solid electrolyte (SE) materials for SSLMBs. In the past decade, great advances for LLZO have been achieved in terms of material preparation, cation dopants and substitutions, ion conductivity, ion transport mechanisms, and interface engineering, but relatively little attention was focused on its air stability. This issue is of equal importance as it plays a key role in LLZO storage, processing, and practical applications in SSLMBs. A thorough understanding of the origin of its instability and the properties of the air passivation layer (APL) may provide guidance for developing strategies to simplify cell assembly and improve the quality of the SE/Li interface. This study aimed to investigate the air stability-related characteristics of Li6.4La3Zr1.4Ta0.6O12 (LLZTO), including the properties of the APL, and the origin of instability and air passivation mechanism. Fifteen-level X-ray photoelectron spectroscopy depth profiling and thermogravimetric analysis reveal that LiOH·xH2O, Li2CO3, and Li6.4–xHxLa3Zr1.4Ta0.6O12 are the main contaminants in the APL, and the contents increased with exposure time. LiOH·xH2O originating from the Li+/H+ exchange reaction is an important intermediate for APL formation. A feasible heat treatment at 700 °C for 3 h has been demonstrated to be an effective and simple method to completely remove the surficial APL. Our findings broaden the understanding of the air stability of LLZTO and head for practical applications of garnet-based SSLMBs. In addition, this systematic methodology may inspire fresh investigations of air stability-related issues for broader solid electrolytes.