Unraveling the Nature of Interfacial Behavior in the LiTFSI‐LiCl Aqueous Biphasic System

Aqueous biphasic systems (ABSs) with water-in-salt electrolytes are gaining significant attention for their role in aqueous biphasic interphase studies, particularly in energy storage devices. Aqueous salt-salt biphasic electrolytes are considered a promising alternative to replace traditional liquid electrolytes commonly used in battery technologies, for example, membrane-less redox flow batteries, owing to their low cost and high ionic conductivity. However, the stability of the interphase over time must be considered, as it can impact the long-term electrochemical performance in various applications. This study reports the unstable interfacial behavior of lithium bis(trifluoro-methanesulfonyl)imide and lithium chloride (LiTFSI-LiCl) system using an optical microscope and electrochemical properties. These observations reveal the liquid-liquid instability phenomenon at the interphase over time. Moreover, this research discovers and analyzes the unwanted solid phase formation at the LiTFSI-LiCl interphase. This study not only contributes to fundamental knowledge in interfacial science but also holds significant implications for developing novel applications reliant on ABSs stability.