Garnet-type solid-state electrolytes (SSEs) are among the most promising electrolytes for solid-state lithium-metal batteries. However, the garnet electrolyte has inadequate stability in air, leading to the formation of lithium carbonate. This reduction in the lithium content in electrolytes can result in decreased ionic conductivity, increased interfacial resistance, and consequently, poor electrochemical performance. Here, we report a fast and effective method for recovering long-term stored garnet oxide. The lithium carbonate in long-term stored Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) was completely removed by immersion in a LiOH solution followed by ultra-fast sintering. As a result, garnet SSEs can obtain higher ionic conductivity and lower electronic conductivity than their counterparts before treatment. In addition, an increased critical current density together with improved electrochemical performance of Li|LiNi_0.8Co_0.1Mn_0.1O₂ half-cells is also achieved for the recovered LLZTO. This work proposes a simple strategy for recycling garnet electrolytes for application in solid-state lithium-metal batteries.