Aqueous electrolytes based on superconcentrated solutions have recently attracted a lot of interest and used in rechargeable batteries and electrochemical capacitors. Superconcentrated solutions, also called water-in-salt electrolytes, with lithium bis(trifluoromethylsulphonyl)imide (LiTFSI) as a representative and widely investigated systems, typically contain a larger amount (mass and moles) of salt with respect to water. An interesting feature of these aqueous solutions is their wide electrochemical stability window that is significantly larger than the thermodynamic value of 1.23 V and that enables to achieve cell voltage that can reach 3 V despite the presence of water in the electrolyte. The wider electrochemical stability window and larger cell voltage is believed to be due to the lower amount of free water that is present in the electrolyte and the formation of a solid electrolyte interface chemical at the electrode/electrolyte interface. In this talk, the electrochemical behavior of superconcentrated aqueous solutions will be presented. We have observed a significant increase of the electrochemical stability window of classical electrodes such as platinum, gold and glassy carbon as well as stainless steel when the concentration of the salt is increased from 1 to 21 molal. Furthermore, a highly concentrated (21 molal) bis(trifluoromethylsulfonyl)imide anions aqueous solution is stable when the potential range that the electrode is cycled is limited to potential values prior to the onset of the hydrogen evolution reaction. In addition, their behavior in electrochemical energy storage systems will be also presented and discussed.

Superconcentrated aqueous electrolytes: Electrochemistry and applications in rechargeable batteries

2019 Fall National Meeting

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