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

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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+ (Li7-3x-z=6.4Ga x La3Zr2-z Ta z 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 Li6.4Ga0.133La3Zr1.8Ta0.2O12 garnet with enhanced ionic conductivity (6.141 × 10-4 S cm-1) and improved high-density microstructure (RD: ∼95.7%) was obtained.