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.
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) x ][TFSA] electrolytes containing excess G3 (x > 1). This cointercalation took place in the voltage range 1.5–0.2 V of the [Li|[Li(G3) x ][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].
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-Vclass 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 Li 2 S m is very low, and highly efficient charge/discharge of the Li−S battery is possible without severe side reactions.
An equimolar mixture of lithium bis(trifluoromethylsulfonyl)amide (Li [TFSA]) and triglyme (G3) or tetraglyme (G4) yields the stable molten complexes, [Li(G3)] [TFSA] or [Li(G4)][TFSA], respectively, classified into solvate ionic liquids (SILs). The Li-conducting SIL electrolytes have favorable thermal and electrochemical properties, but their intrinsic high viscosities and low ionic conductivities impede widespread application. In this study, SILs were diluted with organic solvents, such as toluene, hydrofluoroether (HFE) and propylene carbonate (PC), to enhance their ionic conductivity. Subsequently, the performance of a battery consisting of diluted SILs, LiCoO 2 , and graphite electrodes was evaluated. The electrochemical stability and charge/discharge behavior of the LiCoO 2 cathode and graphite anode were greatly influenced by the stability of the complex cations, [Li(G3)] + or [Li(G4)] + , in the diluted SILs. Unfavorable ligand exchange between the glyme and PC occurred in PC-diluted SILs. Oxidative decomposition of the uncoordinated glyme and pitting corrosion of Al current collector deteriorated the battery performance of LiCoO 2 half-cell with PC-diluted SILs. We demonstrate that toluene-and HFE-diluted SILs, which do not contain chemicals such as carbonate solvent and LiPF 6 used in commercialized Li-ion batteries, allow both LiCoO 2 cathode and graphite anode to operate stably. Because of their high specific energy density, Li-based secondary batteries have been the most attractive candidates among a variety of energy storage systems. Research aimed at the development of Li-ion battery technologies has focused mainly on positive and negative electrode material, 1,2 while Li-conducting electrolytes have played a "supporting" role. Indeed, the major components of the electrolyte in commercialized Li-ion batteries, i.e., a mixture of polar ethylene carbonate (EC) and low-viscous linear carbonates such as diethyl carbonate (DEC) with ∼1 mol dm −3 of lithium hexafluorophosphate (LiPF 6 ), 3 have been in use since these batteries were first developed for practical application in 1991.The standard electrolyte (1 mol dm −3 LiPF 6 in EC/DEC) has been selected for the mature Li-ion battery technology, for several reasons, [3][4][5][6][7] and the battery reaction relies on each electrolyte component. The polar EC not only assists the supporting Li-salt to dissociate to a higher degree, but also forms a good solid electrolyte interphase (SEI) on the graphite anode. The SEI is formed via sacrificial initial decomposition and it allows reversible intercalation/deintercalation reactions of Li ions during charge/discharge. Less viscous linear carbonates such as DEC are mixed with the more viscous EC to improve the ionic conductivity of the electrolyte without compromising the high degree of salt dissociation and formation of the SEI. Despite its poor chemical stability, LiPF 6 is used as the supporting salt because of its high degree of dissociation. Moreover, LiPF 6 plays an important role in suppressing the corr...
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