Can an Inorganic Coating Serve as Stable SEI for Aqueous Superconcentrated Electrolytes?

Developing a stable, conformal solid electrolyte interphase (SEI) for aqueous-based Li-ion batteries has been a long-awaited dream to support the development of nontoxic and eco-friendly energy storage technologies. Toward that goal, aqueous superconcentrated electrolytes were recently introduced as their unique solvation structure allows for forming a LiF-rich SEI layer at the negative electrode, imparting the stability to the interface. However, the intrinsic stability of such LiF-rich SEI was never measured, despite growing evidence of poor passivation properties and water reduction upon operation. In this work, LiF conformal layers were coated onto lithium electrodes, and their reactivity toward superconcentrated aqueous electrolytes was studied by combining solubility measurements, in situ microscopy, and gas chromatography. We demonstrate that the use of superconcentrated electrolytes drastically reduces the solubility of LiF. However, such layer is intrinsically unstable in aqueous environments, but stable in organic electrolytes, owing to the absence of self-passivation. Comparing different interfaces, we conclude that an artificial SEI made of an inorganic coating is not suitable for preventing water reactivity in aqueous systems.