Elucidating the Importance of Quaternary Dopants in Stabilizing Cubic LLZO at Low Temperature

Future-generation solid-state Li batteries could benefit from the use of garnet-type Li₇La₃Zr₂O₁₂ (LLZO) as an electrolyte. However, attaining a stable and highly ionic conducting LLZO garnet at low temperature is a big challenge. In this work, cubic phase stabilized LLZO with enhanced ionic conduction was achieved at low temperature by “quaternary substitution,” in specific substitution of Ga–Mg and Ta–Nb at Li and Zr sites, respectively. Investigations on the structural, morphological, and electrochemical properties were conducted to study the impact of the quaternary dopants. Structural analysis revealed that quaternary substitution in LLZO forms a direct cubic phase Li₇La₃Zr₂O₁₂ without the need for high-temperature sintering. Morphological studies demonstrated that quaternary substitution promotes grain growth and reduces the size of the grain boundaries. Impedance measurements proved that cubic LLZO achieved by substituting quaternary dopants show an ionic conduction of ∼1.5923 × 10^–4 S/cm, which is four orders higher than tetragonal LLZO (∼2.9945 × 10^–8 S/cm). Galvanostatic stripping and plating profiles assured the appropriateness of LLZO as a solid-state electrolyte for solid-state batteries. Additionally, band valence site energy analysis was used to theoretically justify the improvement in ionic conduction achieved by quaternary dopants.