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.

We propose a molecular design for lithium (Li)-ion-ordered complex structures in nonflammable concentrated electrolytes that facilitates the Li-ion battery (LIB) electrode reaction to produce safer LIBs. The concentrated electrolyte, composed of Li bis­(fluorosulfonyl)­amide (FSA) salt and a nonflammable tris­(2,2,2-trifluoroethyl) phosphate (TFEP) solvent, showed no electrode reaction (i.e., no Li-ion intercalation into the negative graphite electrode); however, introducing a small molecular additive (acetonitrile [AN]) into concentrated TFEP-based electrolytes is shown to improve the battery electrode reaction, leading to reversible charge/discharge behavior. Combined high-energy X-ray total scattering experiments incorporating all-atom molecular dynamics simulations were used to visualize Li-ion complexes at the molecular level and revealed that (1) Li ions form mononuclear complexes in a concentrated LiFSA/TFEP (without additives) owing to solvation steric effects arising from the molecular size of TFEP and (2) adding a small-sized additive, AN, reduces the steric effect and triggers a change in Li-ion structures, i.e., the formation of a specific Li-ion-ordered structure linked via FSA anions. These Li-ion-ordered complexes stabilize the energy of the lowest unoccupied molecular orbital (LUMO) on FSA anions, which is key to producing an anion-derived solid electrolyte interphase (SEI) at the graphite electrode. We performed in situ surface-enhanced infrared absorption spectroscopy and discussed the electrode/electrolyte interface and SEI formation mechanisms in TFEP-based concentrated electrolyte systems.

Graphite electrode reaction in coordination-controlled LiFSA/TFEP electrolytes.

We report the characteristics of 2,2,2-trifluoroethyl acetate (TFEAc) as a new type of electrolyte solvent for lithium (Li)-ion batteries. TFEAc-based electrolyte solutions containing 1.0 mol dm–3 LiTFSA salt [TFSA: bis­(trifluoromethanesulfonyl)­amide] exhibited a Li-ion insertion reaction into the negative graphite electrode in the first cycle; however, the performance was noticeably degraded during subsequent cycles due to the lack of solid electrolyte interphase (SEI) formation on the electrode. The electrode reaction was markedly improved when a small amount of ethylene carbonate (EC) was added into the LiTFSA/TFEAc solution, which demonstrated a high-rate charge/discharge performance superior to that of the conventional carbonate-based Li-ion battery electrolyte, that is, 1.0 mol dm–3 LiPF6 in the EC + dimethyl carbonate mixture. Quantitative Raman spectral analysis and density functional theory calculations revealed that TFEAc could be categorized as an organic solvent with low solvation power (i.e., with a predicted Gutmann donor number of 9.1); thus, Li ions mainly formed contact ion-pair complexes, [Li­(TFEAc)2(TFSA)], in binary LiTFSA/TFEAc solutions. Adding EC into the TFEAc electrolyte modified the Li-ion complex structure; namely, Li ions were coordinated with each TFEAc, EC, and TFSA component to yield [Li­(TFEAc)­(EC)­(TFSA)] as the major species, which coexisted with the ion pair [Li­(TFEAc)2(TFSA)]. We discuss the effect of the weak coordination solvent and EC additive on the graphite electrode reaction from the aspects of Li-ion desolvation and SEI formation.