Abstract
“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.
Highlights
One key component of the Lithium-ion batteries (LIBs) is the electrolyte, which works as an indispensable ion conductor and an electron-insular between the anode and cathode [1, 2]
The electrolytes were prepared by dissolving the lithium bis(trifluoromethane sulfonyl)imide (LiTFSI, >99%, Sigma-Aldrich), and sodium bis(trifluoromethane sulfonyl)imide (NaTFSI, >97%, Sigma-Aldrich) in high purity water which conductivity is 18.2 MΩ × cm at 25°C
Based on the Small-angle X-ray scattering (SAXS) profiles of LiTFSI and NaTFSI aqueous solutions, we find that TFSI- salts with cations in the same group have the same behavior as a function of concentration and temperature
Summary
One key component of the Lithium-ion batteries (LIBs) is the electrolyte, which works as an indispensable ion conductor and an electron-insular between the anode and cathode [1, 2]. Borodin et al [9] investigated the ion solvation and transport in LiTFSI aqueous system with combined MD simulation, small-angle neutron scattering (SANS), and a variety of spectroscopic techniques. They found the disproportionation of cation solvation leads to a heterogeneous liquid structure, and the Li+(H2O) domain serves as a percolating channel for fast Li+ transports with a high lithium-transference number. We investigated the solvation structure and dynamics of highly concentrated LiTFSI aqueous electrolyte combining SAXS with MD simulations [18]. High concentration of LiTFSI solution is stable at high temperatures
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