Solvation-controlled lithium-ion complexes in a nonflammable solvent containing ethylene carbonate: structural and electrochemical aspects

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 (nEC), TFEP (nTFEP), and TFSA− (nTFSA) in the first solvation sphere of the Li-ion. We found that the nEC value linearly increased with increasing EC mole fraction (xEC), whereas the nTFEP and nTFSA values gradually decreased with increasing nEC. The ionic conductivity and viscosity (Walden plots) indicated that mainly Li+⋯TFSA− ion pairs formed in neat TFEP (xEC = 0). This ion pair gradually dissociated into positively charged Li-ion complexes as xEC 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 xEC ≥ 0.25. The same was not observed in the lower xEC cases. We discussed the relation between Li-ion solvation and electrode reaction behaviors at the molecular level and proposed that nEC plays a crucial role in the electrode reaction, particularly in terms of solid electrolyte interphase formation on the graphite electrode.