An Overcrowded Water-Ion Solvation Structure for a Robust Anode Interphase in Aqueous Lithium-Ion Batteries

The water-in-salt electrolyte (WISE) features intimate interactions between a cation and anion, which induces the formation of an anion-derived solid electrolyte interphase (SEI) and expands the aqueous electrolyte voltage window to >3.0 V. Although further increasing the salt concentration (even to >60 molality (m)) can gradually improve water stability, issues about cost and practical feasibility are concerned. An alternative approach is to intensify ion–solvent interactions in the inner solvation structure by shielding off outward electrostatic attractions from nearby ions. Here, we design an “overcrowded” electrolyte using the non-polar, hydrogen-bonding 1,4-dioxane (DX) as an overcrowding agent, thereby achieving a robust LiF-enriched SEI and wide electrolyte operation window (3.7 V) with a low salt concentration (<2 m). As a result, the electrochemical performance of aqueous Li4Ti5O12/LiMn2O4 full cells can be substantially improved (88.5% capacity retention after 200 cycles, at 0.57 C). This study points out a promising strategy to develop low-cost and stable high-voltage aqueous batteries.