The Effects of Water Concentration on Lithium Deposition/Dissolution Toward Practical Operation of Lithium-Air Batteries

Lithium (Li)–air batteries are promising devices for the use in sustainable energy management systems as they have the potential to achieve significantly higher energy density than current state-of-the-art Li-ion batteries. 1 However, there are many problems for the application of Li-air batteries. Low coulombic efficiency (C.E.) of Li deposition/dissolution on a Li metal anode caused by various side reactions is one of the critical issues to be overcome. 2 Moisture contaminated from air is known to have influence on the anode reactions and hence on the C.E.. 1 For instance, Zaghib et al. reported the apparent increase of discharge capacity and the formation of a porous lithium hydroxide (LiOH) layer as water (H 2 O) concentration ( C H2O ) increased. 3 Osaka et al. also examined the effects of H 2 O on the C.E. by using carbonate-based electrolytes containing carbon dioxide. The C.E. drastically increased to a maximum of 88.9% with increasing C H2O up to 35 ppm. 4 They concluded from X-ray photoelectron spectroscopy (XPS) results that the solid electrolyte interface (SEI) composed of lithium carbonate and lithium fluoride (LiF), which were formed by reactions involving H 2 O, effectively suppressed side reactions. However, for the purpose of evaluating the C.E. toward the practical application of Li-air batteries, ether-based electrolytes, which are often used in Li-air batteries owing to their higher stability against reactive oxygen species, should be employed. Therefore, in this work, we examined the effects of C H2O on the C.E. by using ether-based electrolytes. In our experiments, coin-type cells composed of Ni foil (as working electrode) and Li foil (as counter electrode) were used with tetraethylene glycol dimethyl ether (TEGDME) containing 1 M lithium bis(fluorosulfonyl)imide (LiFSI) and various concentration of H 2 O as electrolytes. The C.E. was estimated using the cycling protocol proposed by Koch. 5 Figure 1 shows the relationship between the C.E. and C H2O . Although the C.E. was in the range between 40% and 60% in the absence of additional H 2 O, it increased with increasing C H2O and reached a maximum at 80 % at C H2O of 1000 ppm. Then, the C.E. turned to decline when C H2O was further increased. It is known that the morphology of Li deposits significantly affects the C.E.. 2 Therefore, we inspected the morphology of the Li deposits after the initial deposition by scanning electron microscope (SEM). The homogeneity of Li deposits was improv