Water-in-Bi-Salt Electrolyte with Record Salt Concentration and Widened Electrochemical Stability Window.

Abstract

Water-in-salt and water-in-bi-salt electrolytes have attracted much attention recently due to their expanded electrochemical stability windows. The concentration limit of such electrolytes is constrained by the solubility of the lithium salts employed, ca. 21 m (mol kg-1) for LiTFSI (lithium bis(trifluoromethanesulfonyl)imide). By adding a second lithium salt, the total salt concentration can be increased, but the hydrogen evolution keeps limiting the application of such systems in batteries with low potential anodes. Herein we report a water-in-bi-salt electrolyte with a record salt concentration (31.4 m LiTFSI + 7.9 m Li[N(CH3)2((CH2)3SO3)((CH2)4SO3)]) in which the bulky anion completely prevents the crystallization even at such low water contents. Although the hydrogen evolution reaction is not completely suppressed, the expanded electrochemical stability window allows for low potential reactions such as aluminum-lithium alloying. The high salt concentration favors the formation of a suitable passivation layer that can be further engineered by modifying the anion structure.