Study of Codoping Effects of Ta 5+ and Ga 3+ on Garnet Li 7 La 3 Zr 2 O 12 .

Garnet Li 7 La 3 Zr 2 O 12 (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 Ta 5+ and Ga 3+ (Li 7-3 x - z =6.4 Ga x La 3 Zr 2- z Ta z O 12 ) 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 (LiO 6 ) 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 6.4 Ga 0.133 La 3 Zr 1.8 Ta 0.2 O 12 garnet with enhanced ionic conductivity (6.141 × 10 -4 S cm -1 ) and improved high-density microstructure (RD: ∼95.7%) was obtained.