Understanding Degradation Mechanisms in Water‐In‐Salt Electrolyte: Part 1—In Depth Soaking Investigation by Means of Multiprobe Techniques of LiFePO ₄ versus TiS ₂

Water‐based liquid electrolytes for Li‐ion batteries offer the promise of improved safety and lower cost, but the energy density remains too low due to the narrow electrochemical stability window of water. Switching to the water‐in‐salt electrolyte approach appears to be an ideal solution as the electrochemical stability window of water is extended, thereby increasing the overall energy density. To date, despite an increase in electrochemical stability window, hydrogen evolution reaction (HER) and oxygen evolution reaction still occur during cycling, resulting in poor electrochemical performance. Most articles report that this phenomenon is intrinsically related to the change in potential within the cell. In the present work, we carry out a complete surface‐to‐bulk investigation of two well‐known electroactive materials used in the water‐in‐salt system, LiFePO ₄ and TiS ₂ . The aim in this first part is to understand the role of soaking the composite electrode in the water‐in‐salt electrolyte and to see if degradation occurs prior to any electrochemical measurement. We show that LiFePO ₄ is a robust material that develops a surface layer rich in LiF, whereas TiS ₂ decomposes at the top surface into a mixture of TiO ₂ /TiS ₂ or oxysulfide byproduct.