Operando Electrical Transport Spectroscopy Determination of Hydration Phase Diagrams of Alkali Metal Cations in Nanoconfined Spaces

The solvation of alkali metal cations (AM⁺) in nanoconfined spaces plays a crucial role across a wide range of fields, including biological systems, ion sieving, desalination, and electrochemical energy storage. Achieving a comprehensive understanding of AM⁺ solvation behavior is central for controlling these processes. We exploit operando electrical transport spectroscopy to decipher AM⁺ desolvation dynamics upon intercalation in the nanoconfined space of MXene thin films and establish the corresponding hydration phase diagrams. Our findings show that hydrated Li⁺ cations partially shed their secondary solvation shell when entering MXene interlayers, undergoing further desolvation during negative polarization until only the primary solvation shell remains. Larger cations exhibit a shrinking phase region for the secondary hydration shell. Na⁺ retains only the primary shell at high concentrations while partially retaining its secondary hydration at low concentrations but losing it under negative polarization. K⁺, in contrast, maintains only the primary hydration shell throughout the entire phase diagram. These phase diagrams elucidate the intricate interplay among cation size, concentration, and electrochemical potential in governing solvation status, offering a foundational framework for the rational design of advanced materials and interfaces in electrochemical devices.