Li⁺ Solvation and Ionic Transport in Lithium Solvate Ionic Liquids Diluted by Molecular Solvents

Abstract

An equimolar mixture of lithium bis-(trifluoromethanesulfonyl)amide (Li [TFSA]) and either triglyme (G3) or tetraglyme (G4) yielded stable molten complexes: [Li(G3)][TFSA] and [Li(G4)][TFSA]. These are known as solvate ionic liquids (SILs). Glyme-based SILs have thermal and electrochemical properties favorable for use as lithium-conducting electrolytes in lithium batteries. However, their intrinsically high viscosities and low ionic conductivities prevent practical application. Therefore, we diluted SILs with molecular solvents in order to enhance their ionic conductivities. To determine the stabilities of the complex cations in diluted SILs, their conductivity and viscosity, the self-diffusion coefficients, and Raman spectra were measured. [Li(G3)] + and [Li(G4)] + were stable in nonpolar solvents, that is, toluene, diethyl carbonate, and a hydrofluoroether (HFE); however, ligand exchange took place between glyme and solvent when polar solvents, that is, water and propylene carbonate, were used. In acetonitrile (AN) mixed solvent complex cations [Li(G3)(AN)] + and [Li(G4)(AN)] + were formed. [Li(G4)][TFSA] was more conductive than [Li(G3)][TFSA] when diluted with nonpolar solvents due to the greater ionic dissociativity in [Li(G4)][TFSA] mixtures. In view of the stability of the Li−glyme complex cations, the enhanced ionic conductivities, and the intrinsic electrochemical stabilities of the diluting solvents, [Li(G4)][TFSA] diluted by toluene or HFE, can be a candidate for an alternative battery electrolyte.

Redox Active Glyme-Li Salt Solvate Ionic Liquids Based on Tetrabromoferrate(III)

“…37 + complex cations of [Li(G3)][TFSA] was found to remain stable without significant ligand exchange between AN and G3 even in the presence of a large amount of AN. 39 As seen in Fig. 1 .…”

A Design Approach to Lithium-Ion Battery Electrolyte Based on Diluted Solvate Ionic Liquids

“…HFE is not a common organic solvent but can provide non-flammability 39 and high fluidity to the SILs. 31 Similar to the cell with the toluene-diluted SILs, [Li(G4)][TFSA]/HFE showed reversible and stable charge/discharge behaviors with a high Coulombic efficiency (99%) because the glymeLi complex cations also remained stable in HFE. 31 In contrast to the cells using the toluene-diluted SILs, the charge/discharge behavior of the cells using the PC-diluted SILs was very unstable, as shown in Figures 2c and 3.…”

“…31 Similar to the cell with the toluene-diluted SILs, [Li(G4)][TFSA]/HFE showed reversible and stable charge/discharge behaviors with a high Coulombic efficiency (99%) because the glymeLi complex cations also remained stable in HFE. 31 In contrast to the cells using the toluene-diluted SILs, the charge/discharge behavior of the cells using the PC-diluted SILs was very unstable, as shown in Figures 2c and 3. At the beginning of the cycles (1st−5th), the charge capacities exceeded the theoretical capacity of 137 mAh g −1 suggesting unfavorable side reactions occurred in the cell, e.g., the oxidative decomposition of the glyme as discussed in the former section.…”

“…31,34 Except for [Li(G3)][TFSA]/toluene, ionic conductivities were enhanced to a level comparable with that of the standard electrolyte by dilution with less viscous solvents. However, increases in ionic conductivities were not as high as expected given that dilution lowered viscosity values by two orders of magnitude.…”

“…[27][28][29][30] A shortcoming of highly concentrated electrolytes, including SILs, is the poor ionic conductivity resulting from their high viscosity. In previous work, 31 For the LiCoO 2 positive electrode, LiCoO 2 powder (kindly supplied by AGC Seimi Chemical), acetylene black (AB, Denki Kagaku Kogyo), and poly(vinylidene fluoride) (PVDF, Kureha Chemical) were used as an active material, a conduction supporting agent, and a binder, respectively. The mass ratio of LiCoO 2 /AB/PVDF was 85/9/6.…”