Mechanism of Li Ion Desolvation at the Interface of Graphite Electrode and Glyme–Li Salt Solvate Ionic Liquids

Li⁺ intercalation into graphite electrodes was investigated in electrolytes consisting of triglyme (G3) and Li­[TFSA] [TFSA = bis­(trifluoromethanesulfonyl)­amide]. Li⁺-intercalated graphite was successfully formed in an equimolar molten complex, [Li­(G3)₁]­[TFSA]. The desolvation of Li⁺ ions took place at the graphite/[Li­(G3)₁]­[TFSA] interface in the electrode potential range 0.3–0 V vs Li. In contrast, the cointercalation of G3 and Li⁺ (intercalation of solvate [Li­(G3)₁]⁺ cation) into graphite occurred in [Li­(G3)_<i>x</i>]­[TFSA] electrolytes containing excess G3 (<i>x</i> > 1). This cointercalation took place in the voltage range 1.5–0.2 V of the [Li|[Li­(G3)_<i>x</i>]­[TFSA]|graphite] cell. X-ray diffraction showed that the [Li­(G3)₁]⁺-intercalated graphite forms staged phases in the voltage range 1.5–0.3 V. However, exfoliation of the graphite is caused by further intercalation at voltages lower than 0.3 V. [Li­(G3)₁]⁺ intercalation was reversible in the voltage range 1.5–0.4 V. The cointercalation process was studied using cyclic voltammetry, and it was found that the electrode potential for cointercalation depends on the [Li­(G3)₁]⁺ activity, irrespective of the presence of free (uncoordinated) G3. In contrast, the electrode potential for the formation of Li⁺-intercalated graphite (desolvation of solvate [Li­(G3)₁]⁺ cation) changes greatly, depending on the activities of not only the solvate [Li­(G3)₁]⁺ cation but also free G3 in the electrolyte. In extremely concentrated electrolytes, the activity of the free solvent becomes very low. Raman spectroscopy confirmed a very low concentration of free G3 in [Li­(G3)₁]­[TFSA]. Consequently, the electrode potentials for the formation of Li⁺-intercalated graphite were higher than that for cointercalation, and the cointercalation of G3 was inhibited in [Li­(G3)₁]­[TFSA]

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 (<i>D</i>_sol/<i>D</i>_Li) 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, <i>D</i>_sol/<i>D</i>_Li 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 <i>D</i>_sol/<i>D</i>_Li ∼ 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 <i>D</i>_sol/<i>D</i>_Li. 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 (Λ_imp/Λ_NMR) of [Li­(glyme or THF)_<i>x</i>]­[TFSA] exhibited a maximum at an [O]/[Li⁺] ratio of 4 or 5

Solvent Activity in Electrolyte Solutions Controls Electrochemical Reactions in Li-Ion and Li-Sulfur Batteries

Solvent–ion and ion–ion interactions have significant effects on the physicochemical properties of electrolyte solutions for lithium batteries. The solvation structure of Li⁺ and formation of ion pairs in electrolyte solutions composed of triglyme (G3) and a hydrofluoroether (HFE) containing 1 mol dm^–3 Li­[TFSA] (TFSA: bis­(trifluoromethanesulfonyl)­amide) were analyzed using pulsed-field gradient spin–echo (PGSE) NMR and Raman spectroscopy. It was found that Li⁺ is preferentially solvated by G3 and forms a [Li­(G3)]⁺ complex cation in the electrolytes. The HFE scarcely participates in the solvation because of low donor ability and relatively low permittivity. The dissociativity of Li­[TFSA] decreased as the molar ratio of G3/Li­[TFSA] in the solution decreased. The activity of G3 in the electrolyte diminishes negligibly as the molar ratio approaches unity because G3 is involved in 1:1 complexation with Li⁺ ions. The negligible activity of G3 in the electrolyte solutions has significant effects on the electrochemical reactions in lithium batteries. As the activity of G3 diminished, the oxidative stability of the electrolyte was enhanced. The corrosion rate of the Al current collector of the positive electrode was suppressed as the activity of G3 diminished. The high oxidative stability and low corrosion rate of Al in the G3/Li­[TFSA] = 1 electrolyte enabled the stable operation of 4-V-class lithium batteries. The activity of G3 also has a significant impact on the Li⁺ ion intercalation reaction of the graphite electrode. The desolvation of Li⁺ occurs at the interface of graphite and the electrolyte when the activity of G3 in the electrolyte is significantly low, while the cointercalation of Li⁺ and G3 takes place in an electrolyte containing excess G3. The activity of G3 influenced the electrochemical reaction process of elemental sulfur in a Li–S battery. The solubility of lithium polysulfides, which are reaction intermediates of the sulfur electrode, decreased as the activity of G3 in the electrolyte decreased. In the G3/Li­[TFSA] = 1 electrolyte, the solubility of Li₂S_<i>m</i> is very low, and highly efficient charge/discharge of the Li–S battery is possible without severe side reactions