Microscopic Understanding of the Ionic Networks of “Water-in-Salt” Electrolytes

“Water-in-salt” electrolytes with excellent electrochemical and physical properties have been extensively investigated. However, the structural understanding of the lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) in water is still lacking. Here, we perform synchrotron X-ray scattering to systemically study the structural variation of TFSI anions in an aqueous solution under a variety of concentrations and temperatures. There are two different solvation structures in the solution: TFSI^- solvated structure and TFSI^- network. As the concentration increases, the TFSI^- solvated structure gradually disappears while the TFSI^- network gradually forms. Even at relatively low concentrations, the TFSI^- network can be observed. Our experimental results show that these two structures can coexist at a particular concentration, and temperature changes will lead to one structure’s formation or disappearance. Also, the TFSI^- network is the key to obtain a stable electrochemical window under relatively high temperatures.