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

[Image: see text] 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(7–3x–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.