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

Li7La3Zr2O12 (LLZO) has been reported to react in humid air to form Li2CO3 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 similar to 5%) and humid air (humidity similar to 80%) for 6weeks, respectively. After exposure in humid air, the formation of Li2CO3 on the pellet surface is confirmed experimentally and the room-temperature ionic conductivity is found to drop from 6.45x10(-4)Scm(-1) to 3.61x10(-4)Scm(-1). Whereas for the 0.5Ta-LLZO samples exposed in dry air, the amount of formed Li2CO3 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 CO2 by immersing 0.5Ta-LLZO pellets in deionized water for 1week and then exposing them to ambient air for another week. After immersion in deionized water, Li+/H+ exchange occurs and LiOH H2O forms on the surface, which is a necessary intermediate step for the Li2CO3 formation. Based on these observations, a reaction model is proposed and discussed.