Using concentrated aqueous electrolytes (Water-In-Salt aqueous Electrolytes), a significant increase in the potential window of aqueous Li batteries can be achieved. This is due to the absence of free water molecules while the interfaces seem to play a crucial role. While WISEs pave the way for sustainable systems, the Li-based solutions use expensive and toxic salts. To look for more sustainable elements, one can envision sodium, which is less expensive and more abundant. While the systems based on Li imide salts have been quite thoroughly investigated, a fundamental understanding of the reactions at stake in sodium batteries based on WISEs, particularly at the interfaces, is needed. The project aims at identifying the reactions occurring at the interphase between the electrodes and the electrolyte in aqueous water-in-salt Na-ion batteries as well as the redox behavior of electrodes in these solutions. To do so, we propose to use cutting-edge in situ/operando techniques, namely infrared spectroscopy (IR) and X-ray absorption spectroscopy (XAS) at SOLEIL. The PhD student will first develop/adapt dedicated cells to perform these measurements. He/she will then conduct in-depth investigation of
the electrodes and interfaces reactivity in aqueous water-in-salt Na-ion batteries. This will provide insights into electrodes behavior in the bulk and also on the chemical composition of the interfaces, on their formation mechanisms and their stability upon cycling.