Local Structure of Li⁺ in Superconcentrated Aqueous LiTFSA Solutions

It has been reported that aqueous lithium ion batteries (ALIBs) can operate beyond the electrochemical window of water by using a superconcentrated electrolyte aqueous solution. The liquid structure, particularly the local structure of the Li⁺, which is rather different from conventional dilute solution, plays a crucial role in realizing the ALIB. To reveal the local structure around Li⁺, the superconcentrated LiTFSA (TFSA: bis(trifluoromethylsulfonil)amide) aqueous solutions were investigated by means of Raman spectroscopic experiments, high-energy X-ray total scattering measurements, and the neutron diffraction technique with different isotopic composition ratios of ⁶Li/⁷Li and H/D. The Li⁺ local structure changes with the increase of the LiTFSA concentration; the oligomer ([Li(TFSA)]⁽ (q > 2) forms at the molar fraction of LiTFSA (x_LiTFSA) > 0.25. The average structure can be determined in which two water molecules and two oxygen atoms of TFSA anion(s) coordinate to the Li⁺ in the superconcentrated LiTFSA aqueous solution (LiTFSA)_0.25(H₂O)_0.75. In addition, the intermolecular interaction between the neighboring water molecules was not found, and the hydrogen-bonded interaction in the solution should be significantly weak. According to the coordination number of the oxygen atom (TFSA or H₂O), a variety of TFSA^- and H₂O coordination manners would exist in this solution; in particular, the oligomer is formed in which the monodentate TFSA cross-links Li⁺.