Ionic Liquid-Induced Solvation Structure Reconfiguration and Li⁺ Chains in Water-in-Salt Electrolytes

Water-in-salt electrolytes (WiSEs) hold broad prospects in the field of electrochemical energy storage due to their advantages such as high safety and wide electrochemical windows. Recently, it was reported that the addition of ionic liquids (ILs) into WiSE can effectively improve the solubility of lithium salts and electrochemical stability. However, the related mechanism and microstructure remain unclear. In this work, four kinds of ILs with the same cation but different anions ([Bmim][TFSI], [Bmim][OTf], [Bmim][CF₃COO], and [Bmim][CH₃COO]) were selected to study the influence of IL hydrophilicity on the microstructure of WiSE (H₂O in Li[TFSI]) by molecular dynamics (MD) simulations. It was found that after the introduction of IL into WiSE, the solvation structure of Li⁺ was reconstructed, where IL anions competitively displaced a fraction of H₂O and [TFSI]^- in the first solvation shell of Li⁺. Meanwhile, these anions, acting as connecting nodes, coordinate with Li⁺ to form a bridging lithium-chain structure within the primary solvation shell. The hydrophilicity of the IL anions was positively correlated with the shortening of the Li⁺-Li⁺ distance, which promoted the formation of an extended lithium-chain structure. In addition, the displaced water molecules were reorganized into nanoscale water clusters. This solvation structural change may be correlated with an increased solubility of lithium salts and an expanded electrochemical stability window, which is consistent with reported experimental results. These findings provide new insights into the design of an advanced electrolyte of lithium-ion batteries.