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

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.