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

Abstract

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.

Interfacial Modification of Ga-Substituted Li7La3Zr2O12 against Li Metal via a Simple Doping Method

“…Therefore, this result confirms the successful substitution of Zr 4+ with Sb 5+ . The lattice parameter of composition without Sb 5+ doping is 12.9721 Å, which is similar to that of previous reports on Ga-doped LLZO. , …”

“…The lattice parameter of composition without Sb 5+ doping is 12.9721 Å, which is similar to that of previous reports on Ga-doped LLZO. 12,40 Figure 2 illustrates the cross-sectional SEM images of Ga,Sb-LLZO (x = 0−0.4) ceramics sintered at 1100 °C for 12 h. In the case of Ga 3+ single-doped LLZO (Figure 2a), clear grain boundaries and irregular voids between grains can be observed. Additionally, the average size of the grains is larger than 200 μm, indicating a serious overgrowth grain problem.…”

“…It can be observed that the impedance spectra of all samples exhibit an incomplete semicircular arc in the high-frequency region and a tail in the low-frequency region, similar to those of previous reports. 40,42 The impedance data were analyzed using the Zview2 software, and the fitting results are shown in Figure 3a and Table S1. The total ionic conductivities for Ga,Sb-LLZO (x = 0−0.4) ceramics were calculated and are presented in Figure 3b and Table S2.…”

Review of the Developments and Difficulties in Inorganic Solid-State Electrolytes

“…Nb doping can form contractile Nb-O bonds in LLZO, which can stabilize the cubic phase and speed up the sintering process [ 36 , 37 ]. Doping Ta makes more Li + occupy Li2 sites and improves the conductivity between grain boundaries; however, it leads to low sinterability; Ga co-doping enhances the characteristics by providing good sinterability [ 38 ]. Wu et al doped Rb to replace La and obtained Rb and Ga co-doping sample with a conductivity of 1.62 × 10 −3 S·cm −1 at room temperature, which is the highest reported conductivity so far [ 39 ].…”