A series of binary mixtures composed of glymes (triglyme, G3; tetraglyme, G4; pentaglyme, G5) and alkali-metal bis(trifluoromethanesulfonyl)amide salts (M[TFSA]; M = Li, Na, and K) were prepared, and the correlation between the composition and solvate stability was systematically investigated. Their phase diagrams and Raman spectra suggested complexation of the glymes with M[TFSA] in 1:1 and/or 2:1 molar ratio(s). From isothermal stability measurements, it was found that the formation of structurally stable complexes in the solid state did not necessarily ensure their thermal stability in the liquid state, especially in the case of 2:1 complexes, where uncoordinating or highly exchangeable glyme ligands existed in the molten complexes. The phase-state-dependent Raman spectra also supported the presence of free glymes in certain liquid complexes. The effect of the electric field induced by the alkali-metal cations on the oxidative stability of certain glyme complexes was examined by linear sweep voltammetry and quantum chemical calculations. Although the actual oxidative stability of complexes did not necessarily reflect the calculated HOMO energy levels of the glymes, the strong electric field induced by the smaller M(+) cations and proper coordination structures impart high stability to the glyme complexes. The results of thermogravimetry of complexes with different M(+) cations revealed that a balance of competitive interactions of the M(+) ions with the glymes and [TFSA](-) anions predominates the thermal stability.

We prepared a series of binary mixtures composed of selected Na salts and glymes (tetraglyme, G4, and pentaglyme, G5) with different salt concentrations and anionic species ([X](-): [N(SO2CF3)2](-) = [TFSA](-), [N(SO2F)2](-) = [FSA](-), ClO4(-), PF6(-)) and studied the effects of concentration, anionic structure, and glyme chain length on their phase diagrams and solvate structures. The phase diagrams clearly illustrate that all the mixtures form 1:1 complexes, [Na(G4 or G5)1][X]. The thermal stability of the equimolar mixtures was drastically improved in comparison with those of diluted systems, indicating that all the glyme molecules coordinate to Na(+) cations to form equimolar complexes. Single-crystal X-ray crystallography revealed that [Na(G5)1][X] forms characteristic solvate structures in the crystalline state irrespective of the paired anion species. A comparison of the solvate structures of the glyme-Na complexes with those of the glyme-Li complexes suggests that the ionic radii of the coordinated alkali-metal cations have substantial effects on the resulting solvate structures. The Raman bands of the complex cations were assigned by quantum chemical calculations. Concentration dependencies of cationic and anionic Raman spectra show good agreement with the corresponding phase diagrams. In addition, the Raman spectra of the 1:1 complexes strongly suggest that the glymes coordinate to Na(+) cation in the same way in both liquid and crystalline states. However, the aggregated structure in the crystalline state is broken by melting, which is accompanied by a change in the anion coordination.

The physicochemical properties of pentaglyme (G5) and sodium bis(trifluoromethanesulfonyl)amide (Na[TFSA]) binary mixtures were investigated with respect to salt concentration and temperature. The density, viscosity, ionic conductivity, self-diffusion coefficient, and oxidative stability of a series of binary mixtures were measured, and the mixtures were examined as electrolytes for Na secondary batteries. An equimolar mixture of G5 and Na[TFSA] formed a low melting solvate, [Na(G5)1][TFSA], which exhibited an ionic conductivity of 0.61 mS cm(-1) at 30 °C. The ionicity (Λimp/Λideal) of the glyme-Na[TFSA] mixture was estimated from the molar conductivity of electrochemical impedance measurements (Λimp) and the Walden plot (Λideal). [Na(G5)1][TFSA] possessed a high ionicity of 0.63 at 30 °C, suggesting that [Na(G5)1][TFSA] is highly dissociated into a [Na(G5)1](+) cation and a [TFSA](-) anion, regardless of the extreme salt concentration in the liquid. The oxidative stabilities of G5-Na[TFSA] mixtures were investigated by linear sweep voltammetry, and the higher concentration resulted in higher oxidative stability due to the lowering of the HOMO energy level of G5 by complexation with the Na(+) ion. In addition, battery tests were performed using the mixtures as electrolytes. The [Na|[Na(G5)1][TFSA]|Na0.44MnO2] cell showed good charge-discharge cycle stability, with a discharge capacity of ca. 100 mA h g(-1), while the [Na(G5)1.25][TFSA] system, containing excess G5, showed poor stability.

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Room temperature ionic liquids (RTILs) are desirable electrolyte materials for advanced batteries because of their unique properties such as non-volatility, non-flammability and high thermal and electrochemical stability. We have reported that glyme-Li salt complexes, comprising equimolar mixtures of a glyme (CH3O(CH2CH2O) n CH3) and a Li salt, have similar properties to RTILs and classified as solvate ionic liquids. Certain glyme-Li salt complexes such as lithium bis(trifluoromethylsulfonyl)amide (Li[TFSA]) and tetraglyme (G4) or triglyme (G3) are liquid at room temperature and show high thermal stability, high lithium ion transference number, and high lithium ion concentration. We demonstrated reversible charge-discharge of lithium secondary batteries using glyme-Li salt complex as an electrolyte. Recently, the similar results were obtained with equimolar mixtures of pentaglyme (G5) and Na[TFSA] abbreviated as [Na(G5)1][TFSA] was able to use as an electrolyte of sodium secondary battery. Elemental sulfur has been studied as the post-lithium ion battery cathode material for its high theoretical capacity of 1672 mAh g-1. However, the major drawback of this material is the poor cycle stability because of dissolution of lithium polysulfides into typical organic electrolyte. One facile approach to overcome this problem is trapping the sulfur inside a polymer network such as poly(acrylonitrile) (PAN) to prevent the dissolution. By mixing PAN and excess amount of sulfur and heating at around 300 °C, cyclization and dehydrogenation of PAN is promoted by sulfur and results in sulfur contained p-conjugated polymer (PAN-S). The PAN-S has been studied as cathode active material with high specific capacity, good efficiency and cycling stability. However, most reports of PAN-S dealt with lithium batteries and not so many reports have been done in PAN-S sodium system. In this study, we tested the charge-discharge property of PAN-S composite cathode with [Na(G5)1][TFSA] electrolyte at 30 °C. The PAN-S was synthesized by mixing PAN and elemental surfur in 1:4 weight ratio and heated at 350 °C for 6 hours under Ar atmosphere. The composite cathode was fabricated by mixing PAN-S, AB, PVA in the weight ratio of 70:20:10 with NMP used as homogenizer to obtain slurry and pasted onto Al sheet. The electrolyte was prepared by mixing pentaglyme (G5) and sodium bis(trifluoromethylsulfonyl)amide (Na[TFSA]) in 1:1 molar ratio ([Na(G5)1][TFSA]) and mixed with a hydrofluoroether (HFE) in 1:4 molar ratio in order to improve ionic conductivity. Anode material Na15Sn4was prepared by mixing sodium metal and Sn powder in 15:4 molar ratio by ball mill for 5 hours, 510 rpm. The result of charge discharge test is shown in the figure. The initial discharge capacity was about 500 mA h g(PANS) -1 and charge-discharge capacity after that was around 400 mA h g(PANS) -1with excellent coulombic efficiency for 30 cycles. Although there are capacity decrease as the cycle number increase, the cycle stability is much improved compared to that of elemental sulfur composite cathode and sodium metal anode system. References T.Tamura et al., Chem. Lett., 2010, 39, 753. K.Yoshida et al.,J. Am. Chem. Soc., 2011, 133, 13121. X. Yu et al., J. Electroanal. Chem., 2004, 573,121-128