The effects of water concentration (C H2O ) on the coulombic efficiency (CE) of lithium (Li) deposition/dissolution reactions in tetraglyme-based electrolytes are examined. The CE increases from approximately 50% to 80% with increasing C H2O to 1000 ppm, but decreases with further increase in C H2O . X-ray photoelectron spectroscopy analyses of the formed Li deposits reveal that the increase in CE at C H2O values of <1000 ppm is associated with the increase in the composition ratio of lithium oxide in the deposits. In contrast, the reduction of the CE at C H2O > 1000 ppm is associated with the formation of resistive lithium hydroxide (LiOH). In addition, scanning electron microscopic analysis of the deposits reveals that large Li-aggregates with the size of several tens of μm are formed when the C H2O exceeds 1000 ppm. Taken together, these results indicate that optimized amount of water is beneficial for reversible Li deposition/dissolution reactions.