Effect of Simultaneous Substitution of Y and Ta on the Stabilization of Cubic Phase, Microstructure, and Li⁺ Conductivity of Li₇La₃Zr₂O₁₂ Lithium Garnet

Garnet-type lithium stuffed oxide Li₇La₃Zr₂O₁₂ (LLZ) in the cubic phase has received significant attention because of its high Li⁺ conductivity at room temperature and excellent stability against lithium metal anodes. In addition to the high Li⁺ conductivity, the dense microstructure is also a critical issue for the successful application of LLZ as a solid electrolyte membrane in all-solid-state lithium and lithium–air batteries. The stabilization of LLZ in the cubic phase with dopants indicated a reduction in sintering temperature with La³⁺ site doping and improved conductivity by doping the Zr⁴⁺ site. However, there are only a few reports regarding the simultaneous substitution on the La³⁺ and on the Zr⁴⁺ site in LLZ. In the present study, systematic investigations have been carried out on Li_7–<i>x</i>La_3–<i>y</i>Y_<i>y</i>Zr_2–<i>x</i>Ta_<i>x</i>O₁₂ (<i>x</i> = 0.4, <i>y =</i> 0, 0.125, 0.25, and 0.5) to understand the effect of simultaneous substitution of Y³⁺ for La³⁺ and Ta⁵⁺ for Zr⁴⁺ in LLZ on the stabilization of the high conductive cubic phase, microstructure, and Li⁺ conduction behavior. Powder X-ray diffraction (PXRD) revealed the stabilization of a cubic-like garnet structure for the entire selected compositional range of Li_7–<i>x</i>La_3–<i>y</i>Y_<i>y</i>Zr_2–<i>x</i>Ta_<i>x</i>O₁₂ (<i>x</i> = 0.4, <i>y</i> = 0, 0.125, 0.25, and 0.5) samples sintered at 750 °C. However, the Raman spectra revealed that the cubic phase stabilized at around 750 °C for the Li_7–<i>x</i>La_3–<i>y</i>Y_<i>y</i>Zr_2–<i>x</i>Ta_<i>x</i>O₁₂ (<i>x</i> = 0.4, <i>y</i> = 0, 0.125, 0.25, and 0.5) samples is different from the high Li⁺ conductive cubic phase (<i>Ia</i>3̅<i>d</i>), and the transformation to the high Li⁺ conductive cubic phase with a distorted lithium sublattice (<i>Ia</i>3̅<i>d</i>) is observed only for the samples sintered at elevated temperature. Preliminary thermogravimetric (TG), Raman, and Fourier transform infrared (FTIR) studies indicated that the observed low temperature cubic phase of the investigated samples sintered at 750 °C might result from insertion of water vapor from the humid atmosphere into the crystal lattice and subsequent replacement of the lithium ions by protons to form O–H bonds. The AC impedance analysis indicated that the optimal Y substitution in Li_7–<i>x</i>La_3–<i>y</i>Y_<i>y</i>Zr_2–<i>x</i>Ta_<i>x</i>O₁₂ (<i>x</i> = 0.4, <i>y</i> = 0.125 and 0.25) helps to reduce the grain boundary resistance in a major way and also helps to reduce the bulk resistance slightly. Among the investigated compositions, Li_6.6La_2.75Y_0.25Zr_1.6Ta_0.4O₁₂ sintered at 1200 °C exhibits a maximized room temperature total (bulk + grain boundary) Li⁺ conductivity of 4.36 × 10^–4 S cm^–1 along with the improved ceramic density