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

Abstract

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(SO 2 CF 3 ) 2 /DMSO, whereas the use of a salt-concentrated solution (e.g., 3.2 mol dm -3 LiN(SO 2 CF 3 ) 2 /DMSO) or a binary solution (e.g., 1.0 mol dm -3 LiN(SO 2 CF 3 ) 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.

Ionic Conductivity and Viscosity of Solvate Ionic Liquids Composed of Glymes and Excess Lithium Bis(Trifluoromethylsulfonyl)Amide

“…1 Recently, the electrolytes containing lithium salts at high concentrations have attracted attention due to the distinct characteristics emerged from unique coordination environment of lithium ion and/or unusually low activity of free solvent molecules. [2][3][4][5][6][7][8][9][10][11][12] Some lithium salts are able to be dissolved in glymes (Gn, CH 3 O(CH 2 CH 2 O) n CH 3 ), such as triglyme (G3, n = 3) and tetraglyme (G4, n = 4), at extremely high concentrations (> 3 mol dm ¹3 ). 13,14 It has been known that stoichiometric LiTFSAglyme mixtures (TFSA ¹ = bis(trifluoromethylsulfonyl)amide), of which the mole fractions of LiTFSA, X LiTFSA , are 50 mol%, are able to be regarded as solvate ionic liquids since the mixtures consist of a complex cation of lithium ion and glyme, [Li(glyme)] + , and TFSA ¹ anion according to the following equilibrium.…”

Developing New Functionalities of Superconcentrated Electrolytes for Lithium-ion Batteries

“…From 2010 onward, we successively reported that the Li + intercalation into graphite electrodes was generally observed in various organic solvents at certain high salt concentrations. 10,13,16,17 Interestingly, contrary to common belief, some concentrated electrolytes were found to allow much faster Li + intercalation than a standard 1.0 mol dm ¹3 LiPF 6 /ECbased electrolyte. 13,16 From another point of view, Watanabe and his coworkers researched concentrated mixtures of Li salts and glymes with a focus on their ionic liquid-like behavior, and established "solvate ionic liquids" as a new class of electrolyte.…”

“…We studied the correlation between Li + intercalation behavior and Li + solvation environments in dilute (1.0 mol dm ¹3 ) electrolytes. 10,26 For example, a model study of a graphite electrode in the mixed solvent system of PC and dimethyl carbonate (DMC) suggested that the Li + intercalation behavior was correlated with the solvation number of PC toward Li + ; the cointercalation of PC can be suppressed by decreasing the average solvation number of PC to approximately 1. 26 A similar trend was also found for the mixed solvent system of dimethylsulfoxide (DMSO) and DMC.…”

“…26 A similar trend was also found for the mixed solvent system of dimethylsulfoxide (DMSO) and DMC. 10 We also studied the kinetic aspect of Li + intercalation reactions from the viewpoint of Li + solvation structures. [27][28][29][30] A close examination using several model electrode systems revealed that the solvation strength of Li + dominated the activation barrier of Li + intercalation reactions, suggesting that the de-solvation process is a rate-determining step.…”

“…1), and hence, the highly concentrated region has long been outside the major research focus of battery electrolytes. However, various new functionalities have recently been discovered in the highly concentrated region, [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] which bring about vigorous research worldwide on superconcentrated (highly concentrated) electrolytes. Notably, highly concentrated electrolytes show high stabilities toward both negative and positive electrodes.…”