Solid–Electrolyte Interphase of Molecular Crowding Electrolytes

Molecular crowding electrolytes extend the stability window of aqueous batteries with water-miscible/soluble polymers at a low concentration of lithium salts [2 m lithium bis(trifluoromethane sulfonyl)imide (LiTFSI)]. Water decomposition [especially hydrogen evolution reaction (HER)] is effectively suppressed, enabling the use of numerous negative electrodes which cannot work in traditional aqueous electrolytes. However, the mechanism underlying the cathodic stability of molecular crowding electrolytes is not yet fully understood. Here, we compare the HER suppression effect in molecular crowding electrolytes with LiTFSI and lithium perchlorate and correlate their distinct cathodic stability to the difference in the solid–electrolyte interphase (SEI). This work reveals the critical role of LiF in developing a stable SEI and suppressing the HER in molecular crowding electrolytes, providing a design path for safe, low-cost, and high-voltage aqueous batteries.