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.