Water doping sodium battery electrolyte controls nanostructure, interactions, and electrochemical properties

Salt-in-ionic liquids (SiILs) are promising electrolytes for batteries. This study reveals how water affects the nanostructure, surface forces, and electrochemical properties of sodium-SiILs with bis(trifluoromethanesulfonyl)imide ([TFSI]^-) using experiments and molecular dynamics simulations. Dry sodium-SiILs exhibit long-range repulsive forces that deviate from classical electrostatics and are influenced by surface-induced aggregation of nanoscale ionic clusters. Addition of water reduces cluster size and order, yielding force profiles more similar to neat ILs. Atomic force microscopy shows water-induced cluster reorganization near negatively charged surfaces. Water-in-SiILs exhibit increased capacitance and a shift from camel- to bell-shaped profiles, indicating a fundamental change in the double layer, while enhancing conductivity and maintaining a wide electrochemical stability window. These findings underscore the sensitivity of the SiIL nanostructure to hydration from bulk to interface and its critical role in electrochemical properties. Advances in the understanding of the interplay between the nanostructure and screening are essential for the rational design of the solid electrolyte interphase, a crucial component dictating battery performance and safety.