The oxidative stability of glyme molecules is enhanced by the complex formation with alkali metal cations. Clear liquid can be obtained by simply mixing glyme (triglyme or tetraglyme) with lithium bis(trifluoromethylsulfonyl)amide (Li[TFSA]) in a molar ratio of 1:1. The equimolar complex [Li(triglyme or tetraglyme)(1)][TFSA] maintains a stable liquid state over a wide temperature range and can be regarded as a room-temperature ionic liquid consisting of a [Li(glyme)(1)](+) complex cation and a [TFSA](-) anion, exhibiting high self-dissociativity (ionicity) at room temperature. The electrochemical oxidation of [Li(glyme)(1)][TFSA] takes place at the electrode potential of ~5 V vs Li/Li(+), while the oxidation of solutions containing excess glyme molecules ([Li(glyme)(x)][TFSA], x > 1) occurs at around 4 V vs Li/Li(+). This enhancement of oxidative stability is due to the donation of lone pairs of ether oxygen atoms to the Li(+) cation, resulting in the highest occupied molecular orbital (HOMO) energy level lowering of a glyme molecule, which is confirmed by ab initio molecular orbital calculations. The solvation state of a Li(+) cation and ion conduction mechanism in the [Li(glyme)(x)][TFSA] solutions is elucidated by means of nuclear magnetic resonance (NMR) and electrochemical methods. The experimental results strongly suggest that Li(+) cation conduction in the equimolar complex takes place by the migration of [Li(glyme)(1)](+) cations, whereas the ligand exchange mechanism is overlapped when interfacial electrochemical reactions of [Li(glyme)(1)](+) cations occur. The ligand exchange conduction mode is typically seen in a lithium battery with a configuration of [Li anode|[Li(glyme)(1)][TFSA]|LiCoO(2) cathode] when the discharge reaction of a LiCoO(2) cathode, that is, desolvation of [Li(glyme)(1)](+) and insertion of the resultant Li(+) into the cathode, occurs at the electrode-electrolyte interface. The battery can be operated for more than 200 charge-discharge cycles in the cell voltage range of 3.0-4.2 V, regardless of the use of ether-based electrolyte, because the ligand exchange rate is much faster than the electrode reaction rate.

Certain glymeLi salt complexes, which are composed of equimolar mixtures of a glyme and a Li salt, are liquid under ambient conditions with physicochemical properties such as high thermal stability, wide potential window, high ionic conductivity, and high Li + transference number and can be regarded as a new family of room-temperature ionic liquids.Room-temperature ionic liquids (RTILs), which are liquid at room temperature and composed entirely of ions, have attracted much attention because of their unique properties such as nonflammability, low-volatility, high chemical stability, and high ionic conductivity.1 RTILs are expected to be applied to electrochemical devices, including electric double-layer capacitors, 2 fuel cells, 3 dye-sensitized solar cells, 4 and lithium ion batteries (LIBs).5 Most of the RTILs reported to date can be classified as combinations of weakly Lewis-acidic cations and weakly Lewis-basic anions, which leads to ionic dissociation without strong coordination of solvent molecules around each ion. Thus, the most common compositions of RTILs are combinations of onium cations such as imidazolium cations, quaternary ammonium cations, and quaternary phosphonium cations and soft anions such as bis(trifluoromethylsulfonyl)-amide (TFSA ¹ ), tetrafluoroborate (BF 4 ¹ ), and hexafluorophosphate (PF 6 ¹ ). There are few reports of RTILs consisting of strongly Lewis-acidic cations such as Li + and Na + and strongly Lewis-basic anions such as F ¹ and Cl ¹ . Melting points of salts consisting of strongly Lewis-acidic cations and strongly Lewisbasic anions are generally much higher than room temperature, resulting in the formation of ionic crystals at room temperature. So far, we have reported the preparation of lithium ionic liquids consisting of lithium salts of borates having electron-withdrawing groups, to reduce the anionic basicity, and lithium coordinating ether-ligands, to dissociate the lithium cations from the anionic centers.6 However, possibly due to the strong Lewis acidity of Li + , the viscosity and ionicity (dissociativity) of the lithium ionic liquids at room temperature are as high as 500 mPa s and as low as 0.10.2, respectively, resulting in a low ionic conductivity of 10 ¹5 S cm ¹1 at its maximum. Weakly Lewis-basic anions such as BF 4 ¹ and PF 6 ¹ are prepared by the reactions between Lewis acids (BF 3 and PF 5 ) and a Lewis base (F ¹ ) by forming coordination bonds. However, the preparation of weakly Lewis-acidic cations for RTILs by the reaction between a Lewis acid and a Lewis base has not been proposed. It is anticipated that weakly Lewis-acidic cations can be prepared by the combination of alkali metal cations (Lewis acid) and suitable ligands (Lewis base).Ethers are relatively strong Lewis bases, and alkali metal cations are strongly coordinated with ethers. It is well-known that particular molar ratio mixtures of Li salts and oligoethers such as crown ethers, triglyme (G3), and tetraglyme (G4) form complexes. Henderson et al. have conducted a systematic study of glymeLi salt...

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