Herein, we report on a structural study for characterizing unique solution structures in the salt-concentrated electrolytes, which are promising new lithium (Li)-ion battery electrolytes. A combination of high-energy X-ray total scattering (HEXTS) experiments with all-atom molecular dynamics (MD) simulations was performed on the salt-concentrated electrolytes that were based on Li bis­(trifluoromethanesulfonyl)­amide (LiTFSA) and N,N-dimethylformamide (DMF). The radial distribution functions obtained from the HEXTS and MD approaches were in good agreement in the current LiTFSA/DMF solutions. We found that in the local structure: (1) the Li-ions were coordinated with both the DMF molecules and the TFSA anions in the concentrated solutions and (2) specific Li+···Li+ correlations were present in the radial distribution function over the r range of 3 Å–15 Å. The Li+···Li+ correlations originated from the extended multiple Li-ion complexes, that is, polymerized [Li+···TFSA–···Li+] n complexes so that they were highly structurally ordered. We concluded that this type of an ion-ordered structure plays a crucial role in the electrochemical stability and the ion-conducting mechanism, resulting in a unique LIB performance employing these salt-concentrated electrolytes.

We report the characteristics of nonflammable tris(2,2,2-trifluoroethyl) phosphate (TFEP) as a nonaqueous solvent for lithium (Li)-ion battery electrolytes at a molecular level, particularly focusing on the solvent parameters, such as the electron pairdonating ability and molecular bulkiness of TFEP, and the solvation behavior around Li ions. The binding energy for the interactions between a Li ion and several solvents using density functional theory (DFT) calculations indicated that TFEP can be categorized as a nonaqueous solvent with weak electron pair-donating ability, resulting in the prediction of a small Gutmann's donor number (the D N value is approximately 12.9). The Walden plots based on ionic conductivity and viscosity suggested that Li ions tend to exist as contact ion pairs (Li + ••• TFSA − ) in the TFEP-based solutions, irrespective of the Li salt concentration c Li . Infrared and Raman spectroscopic studies demonstrated that Li ions are solvated by two TFEP molecules and one TFSA anion to form the [Li(TFEP) 2 (TFSA)] complex as a major species in the solutions. This result was in good agreement with the DFT calculations for Li-ion interaction complexes: (1) The interaction energy for the [Li(TFEP) n ] complexes linearly decreased with increasing n up to 2. (2) However, the steric repulsion among the solvated TFEP molecules occurs in complexes with n > 3 due to the molecular bulkiness of TFEP. We found that, in the current system, [Li(TFEP) 2 (TFSA)] is a more energy-stable complex than the solvated complexes [Li(TFEP) n ].

We investigated the solvation structure of Mg ions in a diglyme (G2)-based electrolyte solution for Mg ion batteries. The Walden plots based on ionic conductivity and viscosity of the Mg(TFSA)/G2 [TFSA: bis(trifluoromethanesulfonyl)amide] solutions indicated that the dissociativity of Mg(TFSA) gradually increased, even with increasing salt concentration ( c). This behavior is similar to that of the analogous triglyme (G3)-based solutions. Infrared (IR) spectroscopy revealed that Mg ions were coordinated by two G2 molecules to form an octahedral [Mg(G2)] complex in the c range examined herein (≤0.92 M). The detailed coordination geometry of the [Mg(G2)] complex was evaluated using density functional theory calculations. We found that G2 molecules coordinated in a tridentate ligand fashion to form an octahedral [Mg(tri-G2)] complex. This result was different from that of the G3 system; i.e., G3 molecules acted in three ligand modes (bidentate, tridentate, and tetradentate) such that multiple solvation complexes such as [Mg(tri-G3)] and [Mg(bi-G3)(tetra-G3)] complexes were formed. This difference between the G2 and G3 systems might be related to an entropy contribution in the liquid state; i.e., only one coordination structure exists for [Mg(tri-G2)] in the G2 system, whereas more coordination complex structures can be formed in the G3 system.

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.

The structural and electrochemical properties of lithium-ion solvation complexes in a nonflammable organic solvent, tris(2,2,2-trifluoroethyl)phosphate (TFEP) containing ethylene carbonate (EC), were investigated using vibrational spectroscopic and electrochemical measurements. Based on quantitative Raman and infrared (IR) spectral analysis of the Li bis(trifluoromethanesulfonyl)amide (TFSA) salt in TFEP + EC electrolytes, we successfully evaluated the individual solvation numbers of EC (n), TFEP (n), and TFSA (n) in the first solvation sphere of the Li-ion. We found that the n value linearly increased with increasing EC mole fraction (x), whereas the n and n values gradually decreased with increasing n. The ionic conductivity and viscosity (Walden plots) indicated that mainly LiTFSA ion pairs formed in neat TFEP (x = 0). This ion pair gradually dissociated into positively charged Li-ion complexes as x increased, which was consistent with the Raman/IR spectroscopy results. The redox reaction corresponding to an insertion/desertion of Li-ion into/from the graphite electrode occurred in the LiTFSA/TFEP + EC system at x ≥ 0.25. The same was not observed in the lower x cases. We discussed the relation between Li-ion solvation and electrode reaction behaviors at the molecular level and proposed that n plays a crucial role in the electrode reaction, particularly in terms of solid electrolyte interphase formation on the graphite electrode.