Determination of Water Reduction Rates and Activation Energies in Water-in-Salt Electrolytes

Abstract

The introduction of aqueous superconcentrated electrolyte, namely Water-in-salt electrolytes (WiSE), reinvigorated the field of aqueous electrolyte for Li-ion batteries (LIB). The electrochemical stability window of water-based electrolyte was shown to be expanded owing to the formation of a solid electrolyte interphase (SEI) at the negative electrode. However, questions remain regarding the efficacy of the SEI protection against the hydrogen evolution reaction (HER), namely upon repeating cycling and as function of temperature. In this paper, by coupling electrochemical characterizations, operando gas measurements and differential scanning calorimetry (DSC), the role of the HER on the cell lifetime is studied. Combining these measurements, the evolution of the electrolyte concentration is estimated by calculating the rates of water reduction during cycling and resting period, and therefore the battery end-of-life is predicted. The activation energy of the HER was equally determined during cycling and resting period and confronted with that of other aqueous or organic systems. We found a predominance of the HER in the early end-of-life of WiSE-based batteries even though parasitic reactions such as corrosion must be considered.