Comparative Study of Stability against Moisture for Solid Garnet Electrolytes with Different Dopants

The cubic garnet Li7La3Zr2O12(c-LLZO) is one of the most promising solid electrolytes due to its high ionic conductivity and large electrochemical window. However, the critical issue of Li2CO3formation on the c-LLZO surface when exposed to air is problematic, which is detrimental to the ionic conductivity and storage. Herein, comparative studies were carried out on the air stability of Al-doped Li7La3Zr2O12(Al-LLZO), Al-Ta-doped Li7La3Zr2O12(Al-LLZTO), and Al-Nb-doped Li7La3Zr2O12(Al-LLZNO). It was found that Al-LLZTO and Al-LLZNO are less reactive with air than Al-LLZO. The morphology of Li2CO3on Al-LLZTO micro-sized powders after air exposure was island-like with ~1.5 μm in thickness. The interfacial resistance of Li/Al-LLZTO was also a factor of ~3 smaller than that of Li/Al-LLZO, leading to the improved cycle stability of Li/Al-LLZTO/Li symmetric cells. The first-principles calculations based on density functional theory (DFT) verified that the decomposition energy of Al-LLZTO was larger than that of Al-LLZO, inhibiting the reaction product of Li2O and, thus, the next step product of Li2CO3following the reactions of Li2O + H2O → LiOH and LiOH + CO2→ Li2CO3.