Diverse Microstructures and Quasi-Ionic Liquid-like\nTransport Mechanisms in Concentrated “Water-in-Salt”\nLithium Salt Electrolytes: A Molecular Dynamics Study

“Water-in-salt”(WIS)\nelectrolytes as potential green\nand nonflammable electrolytes are currently applied in various energy\nstorage devices, such as lithium-ion batteries and supercapacitors.\nHowever, the microstructure at molecular scale and fast ion transport\nmechanism in such aqueous electrolytes are still under heavy debate\ndue to the complex interactions among ions and water. Here, molecular\ndynamics simulations are used to study the microstructure and ion\ntransport behaviors from the very dilute LiTFSI/water solution to\nthe highly concentrated WIS electrolytes. It revealed that the diverse\nmicrostructures such as completely hydrated ions, ion complexes, and\nbridge-water molecules are jointly responsible for the electrochemical\nstability of WIS electrolytes. Diffusion model analysis showed that\nthe Li<sup>+</sup> ions exhibit a vehicular transport mechanism with\nfirst shell water molecules and structural diffusion mechanism with\nTFSI<sup>–</sup> anions. The lithium ion and its first hydration\nshell act as a single cationic entity. The entity forms a quasi-ionic\nliquid-like dynamic transport structure with associated anions. Our\nstudy challenges previous findings that the high transport dynamics\nof lithium ions arises from their transport in water-rich nanodomains\nin high-concentration WIS systems.