Study of Codoping Effects of Ta⁵⁺ and Ga³⁺ on Garnet Li₇La₃Zr₂O₁₂

Garnet Li₇La₃Zr₂O₁₂ (LLZO) is a promising solid electrolyte for all-solid-state Li-ion batteries because of its outstanding performance. However, LLZO exists in two polymorphic phases, tetragonal (∼10^-3 mS cm^-1) and cubic (1-10^-1 mS cm^-1), where the cubic phase exhibits higher Li-ion conductivity but is thermodynamically unstable at ambient room temperature. To stabilize the cubic phase with high ionic conductivity, we fabricated mono- and codoped garnet with Ta⁵⁺ and Ga³⁺ (Li_7-3Ga La₃Zr_2- Ta O₁₂) and investigated the doping effects. The doping effects on the crystal structure and ionic conductivity were systematically investigated using X-ray diffraction, Raman scattering, scanning electron microscopy, alternative current (AC) impedance, and direct current (DC) polarization methods. The characterization results revealed that Ta-doping favors higher occupation of Li-ions on the mobile octahedral (LiO₆) site and improves ionic conductivity of the grain boundary. However, it showed poor total ionic conductivity (2.044 × 10^-4 S cm^-1 at 1100 °C for 12 h) due to the low sinterability [relative density (RD): ∼80.3%]. On the other hand, Ga-doping provides better sinterability (RD: ∼93.1%) and bulk conductivity. Considering the beneficial effects of Ga- and Ta-doping, codoped Li_6.4Ga_0.133La₃Zr_1.8Ta_0.2O₁₂ garnet with enhanced ionic conductivity (6.141 × 10^-4 S cm^-1) and improved high-density microstructure (RD: ∼95.7%) was obtained.