Unveiling the Critical Influence of EDTA Additives on Modulating Solvation Structure and Solid Electrolyte Interphase Formation in Water-in-Salt Electrolytes for Aqueous Batteries

Water-in-salt (WIS) electrolytes confer a wide voltage window to aqueous batteries. However, the dynamic solid electrolyte interphase (SEI) is adversely affected by LiTFSIprecipitation/dissolution and continuous reforming issues, causing electrolyte dryness. Here, the aminopolycarboxylic (Ethylenediaminetetraacetic acid, EDTA) additive is introduced to WIS electrolytes. An intriguing solvation phenomenon is observed wherein EDTA exhibited insolubility in a low-concentrated (7m) solution while achieving certain solubility in a high-concentrated (21m) one. The assembled full cell with EDTA exhibited good cycling stability at a low 0.5 C. To elucidate the unique solvation phenomenon and unravel the mechanism of SEI formation, experimental characterizations, and simulations are conducted. Molecular Dynamics (MD) and physical measurements disclosed that sufficient Li⁺ acts as a bridge connecting EDTA with TFSI⁻-H₂O. The simulated electrode/electrolyte interface investigated the dynamics, showing the difference in the activity and density of molecules after adding EDTA. Density Functional Theory (DFT) calculations together with physical measurements discovered EDTA- species are prone to facile reduction during cycling, and the products facilitated the formation of a robust fluorine–oxygen–sulfur-based SEI, outstanding critical roles of EDTA in forming the interphase compared with the unstable dynamic SEI. This work directs an alternative way and clear formation mechanism of the interphase for building stable aqueous batteries.