Abstract
Large single crystals of garnet-type Li6La3ZrTaO12 (LLZTO) were grown by the Czochralski method and analysed using neutron diffraction between 2.5 and 873 K in order to fully characterize the Li atom distribution, and possible Li ion mobility in this class of potential candidates for solid-state electrolyte battery material. LLZTO retains its cubic symmetry (space group Ia 3 d) over the complete temperature range. When compared to other sites, the octahedral sites behave as the most rigid unit and show the smallest increase in atomic displacement parameters and bond length. The La and Li sites show similar thermal expansion in their bond lengths with temperature, and the anisotropic and equivalent atomic displacement parameters exhibit a distinctly larger increase at temperatures above 400 K. Detailed inspection of nuclear densities at the Li1 site reveal a small but significant displacement from the 24d position to the typical 96h position, which cannot, however, be resolved from the single-crystal X-ray diffraction data. The site occupation of LiI ions on Li1 and Li2 sites remains constant, so there is no change in site occupation with temperature.
Highlights
Modern and high-performance energy storage devices based on the Li ion battery technology require the choice of a safe electrolyte to overcome known problems with the electrochemical and thermodynamic stability of metallic Li and/or the formation of dendrites when using liquid electrolytes
Total conductivities of 0.4 m S cmÀ1 were observed at room temperature, e.g. in the compound Li7La3Zr2O12 (LLZO), only in the cubic modification
This study further investigates these initial findings with an in-depth look at the crystal structure of high-quality Czochralski-grown single crystals of Li6La3ZrTaO12 (Stanje et al, 2017) over a wide temperature range, with special emphasis on the Li distribution, using combined single-crystal neutron and X-ray diffraction methods
Summary
Modern and high-performance energy storage devices based on the Li ion battery technology require the choice of a safe electrolyte to overcome known problems with the electrochemical and thermodynamic stability of metallic Li and/or the formation of dendrites when using liquid electrolytes Such ion-conducting electrolytes should have exceptionally high ionic conductivities at room temperature coupled with negligible electronic conductivities, high stability and resistance to chemical reaction with both the anode and cathode, especially elemental Li, a high electrochemical decomposition voltage and, last but not least, should be environmentally friendly and of low cost (Thangadurai et al, 2014; Ramakumar et al, 2017; Knauth, 2009; Wang et al, 2020). Total conductivities of 0.4 m S cmÀ1 were observed at room temperature, e.g. in the compound Li7La3Zr2O12 (LLZO), only in the cubic modification The latter can be stabilized by doping with aliovalent cations such as AlIII (Buschmann et al, 2011; Geiger et al, 2011) instead of LiI at the tetrahedral site, thereby introducing a degree of Li disorder. A companion paper deals with the stability of LLZTO in the presence of moisture and different wet environments such as distilled water and acetic acid (Redhammer et al, 2021)
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