<p>Oxide-based garnet-type solid electrolytes are promising for next-generation all-solid-state batteries owing to their high lithium-ionic conductivity and excellent chemical stability. However, high-temperature sintering is typically required for sufficient densification, which can cause lithium loss, phase instability, and reduced conductivity. In this study, the effects of two typical lithium precursors, lithium carbonate (LC) and lithium hydroxide monohydrate (LH), on the powder characteristics, phase formation, microstructure, and electrochemical performance of Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZT) were systematically compared. LH-based LLZT demonstrated high-phase purity, fine and uniform particles, and good sinterability. In contrast, LC-based LLZT exhibited larger particles and broader distribution, leading to poor densification. To address this, a sintering additive was applied to LC-based LLZT, achieving a high relative density of 97.0% at 1100 ℃, comparable to that of LH-based LLZT. Its ionic conductivity reached 4.40 × 10^–4 S/cm at 25 ℃, with a low electronic conductivity (7.59 × 10^–9 S/cm), confirming good insulation. This work demonstrates that the choice of lithium precursor and sintering additives significantly influences the phase stability and microstructure of garnet-type electrolytes. A practical approach is suggested to enhance the electrolyte performance using cost-effective LC precursors, which can support the scalable production of oxide-based solid electrolytes for safer all-solid-state batteries.</p>