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

Abstract

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