| Summary: | Li<sub>7</sub>La<sub>3</sub>Zr<sub>2</sub>O<sub>12</sub> garnet (LLZO) belongs to the most promising solid electrolytes for the development of solid-state Li batteries. The stability of LLZO upon exposure to air is still a matter of discussion. Therefore, we performed a comprehensive study on the aging behavior of Al-stabilized LLZO (space group (SG) <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>I</mi><mi>a</mi><mover accent="true"><mn>3</mn><mo>¯</mo></mover><mi>d</mi></mrow></semantics></math></inline-formula>) and Ga-stabilized LLZO (SG <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>I</mi><mover accent="true"><mn>4</mn><mo>¯</mo></mover><mn>3</mn><mi>d</mi></mrow></semantics></math></inline-formula>) involving 98 powder and 51 single-crystal X-ray diffraction measurements. A Li<sup>+</sup>/H<sup>+</sup> exchange starts immediately on exposure to air, whereby the exchange is more pronounced in samples with smaller particle/single-crystal diameter. A slight displacement of Li from interstitial Li2 (96<i>h</i>) toward the regular tetrahedral Li1 (24<i>d</i>) sites occurs in Al-stabilized LLZO. In addition, site occupancy at the 96<i>h</i> site decreases as Li<sup>+</sup> is exchanged by H<sup>+</sup>. More extensive hydration during a mild hydrothermal treatment of samples at 90 °C induces a structural phase transition in Al-LLZO to SG <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>I</mi><mover accent="true"><mn>4</mn><mo>¯</mo></mover><mn>3</mn><mi>d</mi></mrow></semantics></math></inline-formula> with a splitting of the 24<i>d</i> site into two independent tetrahedral sites (i.e., 12<i>a</i> and 12<i>b</i>), whereby Al<sup>3+</sup> solely occupies the 12<i>a</i> site. Li<sup>+</sup> is preferably removed from the interstitial 48<i>e</i> site (equivalent to 96<i>h</i>). Analogous effects are observed in Ga-stabilized LLZO, which has SG <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>I</mi><mover accent="true"><mn>4</mn><mo>¯</mo></mover><mn>3</mn><mi>d</mi></mrow></semantics></math></inline-formula> in the pristine state.
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