An efficient multi-doping strategy to enhance Li-ion conductivity in the garnet-type solid electrolyte Li₇La₃Zr₂O₁₂

Lithium-ion (Li⁺) batteries suffer from problems caused by the chemical instability of their organic electrolytes. Solid-state electrolytes that exhibit high ionic conductivities and are stable to lithium metal are potential replacements for flammable organic electrolytes. Garnet-type Li₇La₃Zr₂O₁₂ is a promising solid-state electrolyte for next-generation solid-state Li batteries. In this study, we prepared mono-, dual-, and ternary-doped lithium (Li) garnets by doping tantalum (Ta), tantalum-barium (Ta-Ba), and tantalum-barium-gallium (Ta-Ba-Ga) ions, along with an undoped Li₇La₃Zr₂O₁₂ (LLZO) cubic garnet electrolyte, using a conventional solid-state reaction method. The effect of multi-ion doping on the Li⁺ dynamics in the garnet-type LLZO was studied by combining joint Rietveld refinement against X-ray diffraction and high-resolution neutron powder diffraction analyses with the results of Raman spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and multinuclear magic angle spinning nuclear magnetic resonance. Our results revealed that Li⁺ occupancy in the tetrahedrally coordinated site (24d) increased with increased multi-ion doping in LLZO, whereas Li⁺ occupancy in the octahedrally coordinated site (96h) remained constant. Among the investigated compounds, the ternary-doped garnet structure Li_6.65Ga_0.05La_2.95Ba_0.05Zr_1.75Ta_0.25O₁₂ (LGLBZTO) exhibited the highest total ionic conductivity of 0.72 and 1.24 mS cm^-1 at room temperature and 60 °C, respectively. Overall, our findings revealed that the dense microstructure and increased Li⁺ occupancy in the tetrahedral-24d_Li1 site played a key role in achieving the maximum room-temperature Li-ion conductivity in the ternary-doped LGLBZTO garnet, and that the prepared ternary-doped LGLBZTO was a potential solid electrolyte for Li-ion batteries without polymer adhesion.