Experimentally tracing water molecules and mapping high-resolution liquid structure pictures for promising water-in-salt/ionic liquid electrolytes

ABSTRACT Maximizing salt concentration to eliminate the active bulk-like water is the underlying logic of the ‘water-in-salt’ (WiS) strategy in advancing high-voltage aqueous battery chemistries, whereas salts with solubility capable of reaching the WiS regime (typically water-to-salt ratio < 5) are not commonplace. Here, we showcase that, by incorporating certain ionic liquids (ILs), any of the common Zn salts can be formulated into Zn electrolytes that fall into the WiS regime. As a proof-of-concept study, focusing on a ternary Wi(S/IL) system that features a large single-phase liquid region, we demonstrate that the complementary use of infrared/NMR/Raman spectroscopy, electrospray ionization mass spectrometry and synchrotron small-angle X-ray scattering succeed in precisely depicting how the local environment of H2O molecules, primary/secondary solvation sheaths of Zn2+, and other long-range molecular arrangements are structured. Further studies of electrochemical property, Zn cycling and electrolyte/Zn interphasial chemistry performed on a Wi(S/IL) electrolyte with optimal composition reveal that fully eliminating bulk-like water and constructing a weakly bonded anionic primary solvation sheath can promise a 4-V class electrolyte operation window and decent Zn plating/stripping reversibility of 99.7% over long-term cycling. This work implies a revolution of WiS strategy and, more importantly, a new paradigm for experimentally studying the liquid structures of aqueous electrolytes.