Deep hydration of an Li 7-3x La 3 Zr 2 M III x O 12 solid-state electrolyte material: a case study on Al- and Ga-stabilized LLZO.

Single crystals of an Li-stuffed, Al- and Ga-stabilized garnet-type solid-state electrolyte material, Li 7 La 3 Zr 2 O 12 (LLZO), have been analysed using single-crystal X-ray diffraction to determine the pristine structural state immediately after synthesis via ceramic sintering techniques. Hydrothermal treatment at 150 °C for 28 d induces a phase transition in the Al-stabilized compound from the commonly observed cubic Ia-3d structure to the acentric I-43d subtype. Li I ions at the interstitial octahedrally (4 + 2-fold) coordinated 48e site are most easily extracted and Al III ions order onto the tetrahedral 12a site. Deep hydration induces a distinct depletion of Li I at this site, while the second tetrahedral site, 12b, suffers only minor Li I loss. Charge balance is maintained by the incorporation of H I , which is bonded to an O atom. Hydration of Ga-stabilized LLZO induces similar effects, with complete depletion of Li I at the 48e site. The Li I /H I exchange not only leads to a distinct increase in the unit-cell size, but also alters some bonding topology, which is discussed here.