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

摘要

Li~+ intercalation into graphite electrodes was investigated in electrolytes consisting of triglyme (G3) and Li[TFSA] [TFSA = bis(trifluoromethanesulfonyl)amide]. Li~+- intercalated graphite was successfully formed in an equimolar molten complex, [Li(G3),][TFSA]. The desolvation of Li~+ ions took place at the graphite/[Li(G3)1][TFSA] 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)_x][TFSA] electrolytes containing excess G3 (x > 1). This cointercalation took place in the voltage range 1.5-0.2 V of the [Lil [Li(G3)_x][TFSA]lgraphite] 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)1][TFSA]. 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)1][TFSA].