Abstract
Single crystals of an Li-stuffed, Al- and Ga-stabilized garnet-type solid-state electrolyte material, Li7La3Zr2O12 (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. LiI ions at the interstitial octahedrally (4 + 2-fold) coordinated 48e site are most easily extracted and AlIII ions order onto the tetrahedral 12a site. Deep hydration induces a distinct depletion of LiI at this site, while the second tetrahedral site, 12b, suffers only minor LiI loss. Charge balance is maintained by the incorporation of HI, which is bonded to an O atom. Hydration of Ga-stabilized LLZO induces similar effects, with complete depletion of LiI at the 48e site. The LiI/HI exchange not only leads to a distinct increase in the unit-cell size, but also alters some bonding topology, which is discussed here.
Highlights
The garnet family, X3Y2Z3O12, has been well described mineralogically and crystallographically in recent decades (Novak & Gibbs, 1971), and is of interest to a range of scientists from the fields of geoscience and technology, due to its thermodynamic stability in a variety of geological environments and its flexible structure, which can host $60 different chemical elements as major and minor components (Geiger, 2013; Baxter et al, 2013)
Several studies have investigated the role of LiI/HI exchange under different environmental conditions and found that LLZO-type materials are distinctly unstable in the presence of moisture (Ma et al, 2015; Galven et al, 2011, 2012, 2013; Larraz et al, 2015; Orera et al, 2016; Liu et al, 2019)
Surfaces quickly degrade with the formation of LiOH and Li2CO3, and an increase in unit-cell parameters is observed as LiI/HI exchange progresses
Summary
The garnet family, X3Y2Z3O12, has been well described mineralogically and crystallographically in recent decades (Novak & Gibbs, 1971), and is of interest to a range of scientists from the fields of geoscience and technology, due to its thermodynamic stability in a variety of geological environments and its flexible structure, which can host $60 different chemical elements as major and minor components (Geiger, 2013; Baxter et al, 2013). The high Li-ion conductivity is associated with the ‘standard’ cubic garnet structure with Ia3d symmetry. The latter can be stabilized by various aliovalent substitutions, e.g. by small amounts of AlIII, which – in the first experiments – entered the structure as a contaminant from the corundum crucibles during synthesis (Geiger et al, 2011; Buschmann et al, 2011). We have shown, using diffraction methods, that significant LiI is especially lost from the interstitial sites of the structure in AlIII-, GaIII- and TaV-substituted LLZOs during ageing at room temperature under high humidity (Hiebl et al, 2019; Redhammer et al, 2021a,b). We report on the deep hydration of Al- and Ga-substituted LLZO using hydrothermal treatment of single-crystalline material
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