Electrochemical behavior of platinum, gold and glassy carbon electrodes in water-in-salt electrolyte

Superconcentrated electrolytes, including water-in-salt solutions, are currently attracting major interest in the field of rechargeable batteries. These electrolytes could potentially find application in other electrochemical systems. Therefore, it is relevant to investigate the electrochemical behavior of common electrode materials such as platinum, gold and glassy carbon in these electrolytes. Preliminary cyclic voltammetry results revealed that the electrochemical stability window of the electrodes increased by about 0.5 V for platinum and gold and 1 V for glassy carbon when the lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) concentration is increased from 1 to 5 M (mol/L). This increase is mainly due to the shift of the oxygen evolution potential to a more positive value in the more concentrated solution. In the latter, the H2O/Li+ molar ratio of 2.6 is insufficient for full solvation of Li+ cations. The positively charged electrode is therefore compensated mainly by TFSI− anions because the electrolyte contains a negligible amount of free water molecules, leading to an enhancement of the oxidative stability of the electrolyte.