Chelate Effects in Glyme/Lithium Bis(trifluoromethanesulfonyl)amide Solvate Ionic Liquids. I. Stability of Solvate Cations and Correlation with Electrolyte Properties

Abstract

To develop a basic understanding of a new class of ionic liquids (ILs), "solvate" ILs, the transport properties of binary mixtures of lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]) and oligoethers (tetraglyme (G4), triglyme (G3), diglyme (G2), and monoglyme (G1)) or tetrahydrofuran (THF) were studied. The self-diffusion coefficient ratio of the solvents and Li(+) ions (Dsol/DLi) was a good metric for evaluating the stability of the complex cations consisting of Li(+) and the solvent(s). When the molar ratio of Li(+) ions and solvent oxygen atoms ([O]/[Li(+)]) was adjusted to 4 or 5, Dsol/DLi always exceeded unity for THF and G1-based mixtures even at the high concentrations, indicating the presence of uncoordinating or highly exchangeable solvents. In contrast, long-lived complex cations were evidenced by a Dsol/DLi ∼ 1 for the longer G3 and G4. The binary mixtures studied were categorized into two different classes of liquids: concentrated solutions and solvate ILs, based on Dsol/DLi. Mixtures with G2 exhibited intermediate behavior and are likely the borderline dividing the two categories. The effect of chelation on the formation of solvate ILs also strongly correlated with electrolyte properties; the solvate ILs showed improved thermal and electrochemical stability. The ionicity (Λimp/ΛNMR) of [Li(glyme or THF)x][TFSA] exhibited a maximum at an [O]/[Li(+)] ratio of 4 or 5.

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

“…[8][9][10] Of particular interest are the stoichiometric mixtures of glymemetal salts because they often form a low-melting complex with long-lived complex cation through the chelate effect. 11,12 Many glyme-metal salt complexes have been studied, ranging from alkali metal salts (Li, 13 Na, 14 and K 15 ) to alkali earth metal salts (Mg 16 and Ca 17 ). However, most of the complexes yielding solvate ILs comprise bis(trifluoromethensulfonyl) amide (TFSA) salts owing to their good thermal and electrochemical stabilities, their ability to lower the melting point, and weakly coordinating property that ensures strong glyme-metal ion interactions.…”

Categorizing Molten Salt Complexes as Ionic Liquids and Their Applications to Battery Electrolytes

“…A structurally related series of concentrated mixtures of Li[TFSA] and oligoethers, including G4, G3, diglyme (G2), and monoglyme (G1) or tetrahydrofuran (THF), were studied to clarify the chelate effect on the formation of the stable complex cation in the molten state. 25 In Fig We also found that the presence or absence of the free glyme in [Li(glyme) n ]X strongly correlated with the thermal and electrochemical properties. Because the thermal and oxidative stability of the glymes were enhanced by the strong complex formation, the increase of T d and the electrochemical oxidative stability were less pronounced for [Li(glyme) n ]X classified as the concentrated solutions.…”

“…Because the thermal and oxidative stability of the glymes were enhanced by the strong complex formation, the increase of T d and the electrochemical oxidative stability were less pronounced for [Li(glyme) n ]X classified as the concentrated solutions. 25,26 Again, [Li(G3)]X and [Li(G4)]X with perfluorinated sulfonylamide anions met the criteria related to the structure and properties of the solvate ILs.…”

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

“…[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]…”

“…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. 117,122 Consequently, solvate ILs are suitable for use as electrolytes in lithium ion batteries [111][112][113][114] and lithium-sulfur batteries.…”