Innovation in the design of electrolyte materials is crucial for realizing next-generation electrochemical energy storage devices such as Li–S batteries. The theoretical capacity of the S cathode is 10 times higher than that of conventional cathode materials used in current Li–ion batteries. However, Li–S batteries suffer from the dissolution of lithium polysulfides, which are formed by the redox reaction at the S cathode. Herein, we present simple solvate ionic liquids, glyme–Li salt molten complexes, as excellent electrolyte candidates because they greatly suppress the dissolution of lithium polysulfides. The molten complexes do not readily dissolve other ionic solutes, which leads to the stable operation of the Li–S battery over more than 400 cycles with discharge capacities higher than 700 mAh g-sulfur−1 and with coulombic efficiencies higher than 98% throughout the cycles. Such high performance has not been realized to the best of our knowledge. Furthermore, the addition of a nonflammable fluorinated solvent, which does not break the solvate structure of the glyme–Li salt molten complexes, greatly enhances the power density of the Li–S battery. The strategic design of electrolyte properties provides opportunities for the development of new electrochemical devices with many different electrode materials.
Refractive index, an optical property, and liquid density were measured for 17 types of room-temperature ionic liquids at various temperatures. Ab initio calculations were performed to yield theoretical polarizability of the respective ionic liquid composing ions. A highly linear correlation was found between the experimental refractive index and the predicted polarizability normalized in terms of the molar volume. This suggests that the electronic polarization of the ionic liquid composing ions predominantly contributes to the refractive index of the room-temperature ionic liquids. ■ INTRODUCTIONRoom-temperature ionic liquids (RTILs), which are composed of many types of cation and anion, show a wide range of physicochemical properties because of the particular types of interaction between certain cation and anion species. RTILs have many important physicochemical properties such as low flammability, low volatility, low viscosity, and high electrochemical stability. In recent years, many scientists and engineers have conducted the research and development of RTILs for many basic and applied fields. For example, RTILs have been applied in various electrochemical devices (e.g., lithium secondary batteries, fuel cells, electric double-layer capacitors, dye-sensitized solar cells, and field-effect transistors, etc.), 1 tribology, 2 synthetic production, 3 and as biomass solvent. 4 Their physicochemical properties depend on the combination of their cations and anions. RTILs are so-called "designer solvents" owing to their semi-infinite chemical structures and combinations of cations and anions. However, their physicochemical uniformity has not yet been reported. We have reported a unique phase transition (phosphonium-cation-based RTIL) 5 and the regularity of the ratio between their molecular volumes (i.e., frame volume and van der Waals volume). 6 In this study, we measured the refractive index and density of 17 types of RTILs, which were composed of six cations and ten anions, and studied the cation and anion dependences on the physical properties of these RTILs. We investigated the optical properties of the RTILs by experimental and computational (ab initio) methods. The extractions of universal properties of physicochemical parameters (universalization) are very important for the future molecular design of RTILs because of the semiinfinite cation and anion combinations possible for RTILs.
Electrodeposition of silver was investigated using an impedance technique (separately excited, passive technique) electrochemical quartz crystal microbalance (EQCM) in a room-temperature ionic liquid. The mass changes during silver deposition and dissolution were observed with the current efficiencies of nearly 100% during potential sweep, constant potential step, and constant current step experiments. The product of the viscosity and density of the electrolyte near the electrode, ηnormalLρnormalL , can be estimated by the resonance resistance, which can be monitored simultaneously with the resonance frequency. The change in the ηnormalLρnormalL value during silver deposition was consistent with the change in the calculated concentration of Ag(I) near the electrode. During the outer-sphere electron-transfer reaction between ferrocenium and ferrocene, no significant changes in the mass and the ηnormalLρnormalL value were observed.
The physicochemical (e.g., density, viscosity, ionic conductivity, and self-diffusion coefficient) and electrochemical (e.g., electrochemical window and electric double-layer capacitor performance) properties of two new types of room-temperature ionic liquids, 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (EMIm-FAP) and 1-ethyl-3-methylimidazolium tetracyanoborate (EMIm-TCB), were investigated and compared. EMIm-FAP showed a wider electrochemical window with a carbon working electrode compared with EMIm-TCB, in spite of lower viscosity, larger self-diffusion coefficients and higher ionic conductivity. Also, the electric double-layer capacitor using EMIm-FAP shows better cycle reversibility and higher charge/discharge voltage operation (over 3 V) than that using EMIm-TCB.
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