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

"Water-in-salt"(WIS) electrolytes as potential green and nonflammable electrolytes are currently applied in various energy storage devices, such as lithium-ion batteries and supercapacitors. However, the microstructure at molecular scale and fast ion transport mechanism in such aqueous electrolytes are still under heavy debate due to the complex interactions among ions and water. Here, molecular dynamics simulations are used to study the microstructure and ion transport behaviors from the very dilute LiTFSI/water solution to the highly concentrated WIS electrolytes. It revealed that the diverse microstructures such as completely hydrated ions, ion complexes, and bridge-water molecules are jointly responsible for the electrochemical stability of WIS electrolytes. Diffusion model analysis showed that the Li⁺ ions exhibit a vehicular transport mechanism with first shell water molecules and structural diffusion mechanism with TFSI^- anions. The lithium ion and its first hydration shell act as a single cationic entity. The entity forms a quasi-ionic liquid-like dynamic transport structure with associated anions. Our study challenges previous findings that the high transport dynamics of lithium ions arises from their transport in water-rich nanodomains in high-concentration WIS systems.