Abstract

Abstract 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 Li OH 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.