| Summary: | The cubic garnet Li<sub>7</sub>La<sub>3</sub>Zr<sub>2</sub>O<sub>12</sub> (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 Li<sub>2</sub>CO<sub>3</sub> formation 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 Li<sub>7</sub>La<sub>3</sub>Zr<sub>2</sub>O<sub>12</sub> (Al-LLZO), Al-Ta-doped Li<sub>7</sub>La<sub>3</sub>Zr<sub>2</sub>O<sub>12</sub> (Al-LLZTO), and Al-Nb-doped Li<sub>7</sub>La<sub>3</sub>Zr<sub>2</sub>O<sub>12</sub> (Al-LLZNO). It was found that Al-LLZTO and Al-LLZNO are less reactive with air than Al-LLZO. The morphology of Li<sub>2</sub>CO<sub>3</sub> on 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 Li<sub>2</sub>O and, thus, the next step product of Li<sub>2</sub>CO<sub>3</sub> following the reactions of Li<sub>2</sub>O + H<sub>2</sub>O → LiOH and LiOH + CO<sub>2</sub> → Li<sub>2</sub>CO<sub>3</sub>.
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