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

Abstract

Hydro­thermally treated Al- and Ga-stabilized stuffed Li garnets, nominally Li ₇La ₃Zr ₂O ₁₂, have been investigated by single-crystal X-ray diffraction to determine structural and site-occupation alterations due to Li ^I/H ^I exchange.

Abstract

Single crystals of an Li-stuffed, Al- and Ga-stabilized garnet-type solid-state electrolyte material, Li ₇La ₃Zr ₂O 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 d structure to the acentric I 3 d subtype. Li ^I ions at the inter­stitial octa­hedrally (4 + 2-fold) coordinated 48 e site are most easily extracted and Al ^III ions order onto the tetra­hedral 12 a site. Deep hydration induces a distinct depletion of Li ^I at this site, while the second tetra­hedral site, 12 b, 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 48 e 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.