The structural and electrochemical properties of lithium (Li) ion complexes in concentrated electrolytes based on acetonitrile (AN) and tris­(2,2,2-trifluoroethyl) phosphate (TFEP) as solvents and LiTFSA [TFSA: bis­(trifluoromethanesulfonyl)­amide] as a Li salt were investigated by employing electrochemical measurements, vibrational spectroscopy, and high-energy X-ray total scattering (HEXTS) with all-atom molecular dynamics (MD) simulations. Via electrochemical measurements, reversible Li-ion insertion/deinsertion into/from the graphite electrode was observed in concentrated LiTFSA/AN solutions but not in concentrated LiTFSA/TFEP solutions. The experimental radial distribution functions [G exp(r)] derived from HEXTS were successfully represented by the corresponding MD-derived values [G MD(r)] for both AN- and TFEP-based electrolyte systems. We found that (1) in the dilute system, Li ions were solvated with only solvent molecules in AN-based solutions to form a completely dissociated [Li­(AN)4]+ complex, while contact ion pairs exhibiting Li+···TFSA– interactions were formed in the TFEP-based solutions. (2) In the concentrated system, a specific Li+···Li+ correlation was observed for shorter r values (∼3 Å) in the AN-based solutions, suggesting ordered ionic structure formation based on multinuclear Li-ion complexes. However, no ordered ionic structure formation was found in the TFEP-based solutions. We discussed the relation between the ordered ionic structure and graphite electrode reaction at the molecular level, particularly focusing on the solvent size; that is, the smaller AN more easily forms a compact solution structure (ordered structure) in the concentrated solutions, while bulky TFEP causes steric repulsion among the coordinated species (TFEP and TFSA) in the Li-ion complexes, preventing such ordered formation.

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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 6 Li/ 7 Li and H/D. The Li + local structure changes with the increase of the LiTFSA concentration; the oligomer ([Li p (TFSA) q ] (p-q)+ (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 (H 2 O) 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 H 2 O), a variety of TFSA − and H 2 O coordination manners would exist in this solution; in particular, the oligomer is formed in which the monodentate TFSA cross-links Li + .