Li⁺ solvation in glyme–Li salt solvate ionic liquids

Abstract

Certain molten complexes of Li salts and solvents can be regarded as ionic liquids. In this study, the local structure of Li(+) ions in equimolar mixtures ([Li(glyme)]X) of glymes (G3: triglyme and G4: tetraglyme) and Li salts (LiX: lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]), lithium bis(pentafluoroethanesulfonyl)amide (Li[BETI]), lithium trifluoromethanesulfonate (Li[OTf]), LiBF4, LiClO4, LiNO3, and lithium trifluoroacetate (Li[TFA])) was investigated to discriminate between solvate ionic liquids and concentrated solutions. Raman spectra and ab initio molecular orbital calculations have shown that the glyme molecules adopt a crown-ether like conformation to form a monomeric [Li(glyme)](+) in the molten state. Further, Raman spectroscopic analysis allowed us to estimate the fraction of the free glyme in [Li(glyme)]X. The amount of free glyme was estimated to be a few percent in [Li(glyme)]X with perfluorosulfonylamide type anions, and thereby could be regarded as solvate ionic liquids. Other equimolar mixtures of [Li(glyme)]X were found to contain a considerable amount of free glyme, and they were categorized as traditional concentrated solutions. The activity of Li(+) in the glyme-Li salt mixtures was also evaluated by measuring the electrode potential of Li/Li(+) as a function of concentration, by using concentration cells against a reference electrode. At a higher concentration of Li salt, the amount of free glyme diminishes and affects the electrode reaction, leading to a drastic increase in the electrode potential. Unlike conventional electrolytes (dilute and concentrated solutions), the significantly high electrode potential found in the solvate ILs indicates that the solvation of Li(+) by the glyme forms stable and discrete solvate ions ([Li(glyme)](+)) in the molten state. This anomalous Li(+) solvation may have a great impact on the electrode reactions in Li batteries.

Design and Materialization of Ionic Liquids Based on an Understanding of Their Fundamental Properties

“…8(a)]. 99 This stabilization has also been validated by PGSE-NMR measurements of these mixtures, as shown in Fig. 8(b).…”

“…[89][90][91][92] We explored a variety of such combinations and found that, in certain combinations, these materials have similar properties to conventional aprotic ILs. [93][94][95][96][97][98][99][100][101][102][103][104][105][106][107][108][109] We conclude that these mixtures can be categorized as "solvate ILs," i.e., liquids consisting of a salt and a solvent that strongly coordinates with cations or anions to form stable solvate ions. 110 Detailed studies of the solvate IL on the transport properties, [93][94][95][96][97][98][99] spectroscopic, [99][100][101][102][103] crystallographic, [100][101][102][103] electrochemical, [94][95][96]104,105 quantum mechanical characterizations, [106][107][108]…”

“…8(b). 97,99 This figure Electrochemistry, 84(9), 642-653 (2016) increase, the D G /D Li ratio approaches unity. Furthermore, as the ratio approaches unity, the amount of free glyme diminishes rapidly.…”

“…110 Detailed studies of the solvate IL on the transport properties, [93][94][95][96][97][98][99] spectroscopic, [99][100][101][102][103] crystallographic, [100][101][102][103] electrochemical, [94][95][96]104,105 quantum mechanical characterizations, [106][107][108][109] and possible applications [111][112][113][114][115][116][117][118][119][120] have been investigated in collaboration with many researchers, especially with Y. Umebayashi at Niigata University, K. Dokko at Yokohama National University, and Tsuzuki. The formation of solvate ILs is affected by the structure of the lithium salt anions 97,99 and glyme molecules. 98 Furthermore, the transport properties of solvate ILs are much better than conventional Li-doped ILs 113 and can be further improved by dilution with solvents that do not disrupt the solvate structures.…”

“…121,122 Figure 8 shows how the anionic structures of lithium salts affect the formation of solvate ILs. 99 The amount of free glyme, determined by Raman spectroscopy, strongly depends on the anionic structures [ Fig. 8(a)].…”