One greener alternative to current Li-ion batteries are aqueous batteries. Unfortunately, water is thermodynamically stable in a very narrow potential window of only 1.23 V, resulting in poor energy efficiency. Using concentrated aqueous electrolytes (Water-In-Salt aqueous Electrolytes (WISEs)), a significant increase in the potential window of aqueous Li batteries up to 3 V can be achieved. Yet, aqueous batteries based on WISE electrolytes suffer from several issues leading to electrochemical failure such as self-discharge, pH evolution, parasitic reactions and instable interfaces layers. There is thus a strong need to understand the reactivity in concentrated electrolytes. In the frame of the ANR project AQUABATT, we will address these issues using a comprehensive approach combining different advanced characterization techniques. The PhD student will address these limitations by providing a comprehensive approach of the reactivity as function of salt concentration. The student will combine electrochemical measurements with infrared and NMR spectroscopies to elucidate the solvation structure of the solutions. The nature of the interface between the electrolyte and the electrode and bulk redox mechanisms in electrodes will be investigated by means of synchrotron operando infrared spectroscopy and operando X-Ray Absorption Spectroscopy (XAS) respectively.