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

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Abstract

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)1][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 [Li|[Li(G3)x][TFSA]|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)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].

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