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 (110H). In addition, scanning electron microscopic analysis of the deposits reveals that large Li-aggregates with the size of several tens of pm 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. (C) 2017 Elsevier B.V. All rights reserved.