Electrochemical Lithium Intercalation into Graphite in Dimethyl Sulfoxide-Based Electrolytes: Effect of Solvation Structure of Lithium Ion

The behavior of electrochemical lithium intercalation at graphite has been investigated in dimethyl sulfoxide (DMSO)-based electrolytes: (1) lithium salt-concentrated solutions and (2) binary solutions with dimethyl carbonate (DMC). The intercalation of DMSO-solvated lithium ion (i.e., cointercalation of DMSO) into graphite occurred during charge in 1.0 mol dm(-3) LiN(SO2CF3)(2)/DMSO, whereas the use of a salt-concentrated solution (e.g., 3.2 mol dm(-3) LiN(SO2CF3)(2)/DMSO) or a binary solution (e.g., 1.0 mol dm-3 LiN(SO2CF3)(2)/DMSO: DMC (1:4.8 by vol)) allowed for the intercalation and deintercalation of lithium ion at graphite. Raman spectra of these solutions showed that the solvation number of DMSO molecules toward lithium ion (N-DMSO) decreased from 4.2 in a conventional solution to around 2 in the salt-concentrated solution and the binary solution. A comparison between the behavior of graphite and the N-DMSO values has clarified that the N-DMSO value of 3 is a criterion for determining whether the intercalation of lithium ion or solvated lithium ion occurs. A series of our results suggests a new approach for the suppression of the cointercalation of solvents: the decrease in the solvation number of the relevant solvents toward lithium ion.