Superconcentrated aqueous electrolytes have recently emerged as a new class of electrolytes, called water-in-salt electrolytes. They are distinguished, in both weight and volume, by a quantity of salt greater than water. Currently, these electrolytes are attracting major interest, particularly for application in aqueous rechargeable batteries. These electrolytes have only a small amount of free water due to an ultrahigh salt concentration. Consequently, the electrochemical stability window of water is wider than the predicted thermodynamic value of 1.23 V. Hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) have been shown to be shifted to more negative and positive potentials, respectively. The decrease in free water population is recognized as being involved in the increase in the electrochemical stability window of water. Here, we study the quantitative contribution of the decrease in the free water molecule concentration to the permittivity of the solution and of the activity of water to the OER and HER overpotentials when the salt concentration increases. We compare our model with that of Kornyshev and get three types of electrolyte structures: diluted, gradient of water contents, and aggregation. The theoretical calculation of the redox potentials of the OER and HER is compared with the experimentally determined electrochemical properties of aqueous LiTFSI electrolytes.
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