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

Abstract 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.