Particle Size-Dependent Degradation Kinetics of Garnet-Type Li6.5La3Zr1.5Ta0.5O12 Solid Electrolyte Powders in Ambient Air

Garnet-type Li7La3Zr2O12 (LLZO) is a promising solid electrolyte for the application in solid-state lithium batteries (SSBs). However, its reaction with water and carbon dioxide in ambient air and the resulting formation of insulating lithium carbonate is one of the major obstacles for its large-scale manufacturing and processing. Especially when processed as powder with large surface areas, e.g., for the application in hybrid electrolytes, where LLZO powders are incorporated into a polymer matrix, uncontaminated surfaces are crucial. In this work, the kinetics of the hydration and carbonation mechanism is studied in detail for Ta-doped LLZO powders by time-dependent analyses of morphology, structure, and composition. Common particle sizes for battery applications, i.e., powders with different specific surface areas, are investigated. It is shown that the degradation mechanism follows a two-step consecutive reaction for all particle sizes investigated. It is self-limited by diffusion processes in the reaction layer in accordance with the core shrinking model. The hydration reaction is an essential intermediate step that precedes carbonation, which is demonstrated by systematically adjusting the atmosphere from dry room conditions up to ambient air. Moreover, the reaction rate of the hydration and carbonation depends strongly on the particle size and thus on the surface area. A linear correlation of the reaction rate and the specific surface area is found. Altogether, the novel insights into the degradation mechanism of LLZTO powder scrutinized in this work provide guidance on how to select, handle, and process LLZTO powders according to the surface quality requirements in future battery applications.