Reaction mechanisms of lithium garnet pellets in ambient air: The effect of humidity and CO2

Li 7 La 3 Zr 2 O 12 (LLZO) has been reported to react in humid air to form Li 2 CO 3 on the surface, which decreases ionic conductivity. To study the reaction mechanism, 0.5-mol Ta-doped LLZO (0.5Ta–LLZO) pellets are exposed in dry (humidity ~5%) and humid air (humidity ~80%) for 6 weeks, respectively. After exposure in humid air, the formation of Li 2 CO 3 on the pellet surface is confirmed experimentally and the room-temperature ionic conductivity is found to drop from 6.45×10 −4 S cm −1 to 3.61×10 −4 S cm −1 . Whereas for the 0.5Ta–LLZO samples exposed in dry air, the amount of formed Li 2 CO 3 is much less and the ionic conductivity barely decreases. To further clarify the reaction mechanism of 0.5Ta–LLZO pellets with moisture, we decouple the reactions between 0.5Ta–LLZO with water and CO 2 by immersing 0.5Ta–LLZO pellets in deionized water for 1 week and then exposing them to ambient air for another week. After immersion in deionized water, Li + /H + exchange occurs and LiOH H 2 O forms on the surface, which is a necessary intermediate step for the Li 2 CO 3 formation. Based on these observations, a reaction model is proposed and discussed.