Ionic selectivity is of significant importance in both fundamental science and practical applications. For instance, an ion-selective material allows the passage of a particular kind of ions while blocking the others, which could be used for purification of materials. Herein, the Li-ion-selectivity of a garnet-type solid electrolyte is discussed by comparing the difference of activation energy between different ions migrating in solids. The high ion-selectivity is confirmed by harvesting high-purity metallic lithium (99.98 wt%) from low-lithium-purity sources (80 wt%) at a moderate temperature (190 ℃). This gives it huge potential in separating lithium with impurities especially alkali and alkali-earth elements. The cost of metallic lithium production is only 25% of the international lithium price. The proposed electrochemical metallic lithium separating method is advantageous compared with the traditional process in terms of efficiency, safety, and cost.