Mechanism of Li Ion Desolvation at the Interface of Graphite Electrode and Glyme–Li Salt Solvate Ionic Liquids

Li+ intercalation into graphite electrodes was investigated in electrolytes consisting of triglyme (G3) and LiTFSA TFSA = bis(trifluoromethanesulfonyl)amide. Li+-intercalated graphite was successfully formed in an equimolar molten complex, Li(G3)1TFSA. The desolvation of Li+ ions took place at the graphite/Li(G3)1TFSA interface in the electrode potential range 0.3–0 V vs Li. In contrast, the cointercalation of G3 and Li+ (intercalation of solvate Li(G3)1+ cation) into graphite occurred in Li(G3)xTFSA electrolytes containing excess G3 (x > 1). This cointercalation took place in the voltage range 1.5–0.2 V of the Li|[Li(G3)xTFSA|graphite] cell. X-ray diffraction showed that the Li(G3)1+-intercalated graphite forms staged phases in the voltage range 1.5–0.3 V. However, exfoliation of the graphite is caused by further intercalation at voltages lower than 0.3 V. Li(G3)1+ intercalation was reversible in the voltage range 1.5–0.4 V. The cointercalation process was studied using cyclic voltammetry, and it was found that the electrode potential for cointercalation depends on the Li(G3)1+ activity, irrespective of the presence of free (uncoordinated) G3. In contrast, the electrode potential for the formation of Li+-intercalated graphite (desolvation of solvate Li(G3)1+ cation) changes greatly, depending on the activities of not only the solvate Li(G3)1+ cation but also free G3 in the electrolyte. In extremely concentrated electrolytes, the activity of the free solvent becomes very low. Raman spectroscopy confirmed a very low concentration of free G3 in Li(G3)1TFSA. Consequently, the electrode potentials for the formation of Li+-intercalated graphite were higher than that for cointercalation, and the cointercalation of G3 was inhibited in Li(G3)1TFSA.