Garnet-type Li_6.4La₃Zr_1.4Ta_0.6O₇ (LLZTO) is regarded as a highly competitive next-generation solid-state electrolyte for all-solid-state lithium batteries owing to reliable safety, a wide electrochemical operation window of 0-6 V versus Li⁺/Li, and a superior stability against Li metal. Nevertheless, insufficient interface contacts caused by pores, along with Li dendrite growth at these voids and grain boundary regions, have hindered their commercial application. Herein, we suggest a method to produce high-quality LLZTO using LiAlO₂ (LAO) as a chemical additive that leads to an improved microstructure with larger grain size (∼25 μm), a high relative density (∼96%), lower porosity (∼3.7%), and continuous secondary phases in grain boundary regions. This improved structure results in (i) improved Li-ion conductivity and enhanced interfacial resistance between Li metal and LLZTO by a denser structure with fewer pores and (ii) suppression of Li dendrite penetration in the electrolyte by secondary phases in grain boundary regions.