Structural Features and the Li-Ion Diffusion Mechanism in Tantalum-Doped Li7La3Zr2O12 Solid Electrolytes

Solid electrolytes with high values of lithium-ion conductivity are required for the creation of high-energy lithium and lithium-ion power sources, and compounds with a garnet structure based on Li7La3Zr2O12 (LLZO) are one of the candidate materials for this purpose. In the present work, solid electrolytes of the Li7–xLa3Zr2–xTaxO12 system with x = 0.0–2.0 were synthesized using the sol–gel method. According to X-ray diffraction analysis, all of the compounds with x ≥ 0.1 have the same cubic modification with the space group Ia3̅d. However, an increase in Ta concentration affects the short-range order crystal structure of these materials, resulting in higher local distortions, which was shown by pair distribution function (PDF) analysis. Particularly, the PDF data indicate an increase in the probability of Li ions to locally occupy not only two typical positions, Li1─96 h and Li2─24 d, but also a third one, Li3─48 g. The maximum value of lithium-ion conductivity in the studied system was observed for the Li6.4La3Zr1.4Ta0.6O12 compound (i.e., x = 0.6) and had the value of 1.4 × 10–4 S cm–1 at 25 °C. This is consistent with the results of density functional theory (DFT) modeling, which confirmed that a moderate Ta-doping (up to x < 1.0) is most suitable for enhancing Li diffusion in LLZO materials. A combination of DFT modeling, structural characterization of the short and average structures, and conductivity measurements in this work allowed getting insight into this important class of Li-conducting oxides and ideas on improving their properties.