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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.

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The thermal behavior of a lithium-ion cell during charge and discharge was determined using an isothermal calorimeter. In order to assign the thermal characteristics of the lithium-ion cell to the cathode false(LiCoO2false) and the anode (graphite) material, a LiCoO2/normalLi cell and a graphite/Li cell were prepared. The thermal behaviors were compared with that of a lithium-ion false(LiCoO2 /graphite) cell. The notable thermal characteristics could be attributed to the individual electrode materials. In particular, the discontinuous thermal profiles showed good agreement with the phase change of the host structure of each electrode material. The degradation factor of commercially available lithium-ion cells was determined using these discontinuous thermal profiles as an indicator of the electrode reactions. We found that the decrease in the effective active material of the graphite is the main cause of capacity fading after cycling. © 2002 The Electrochemical Society. All rights reserved.

Electrolyte performance of CH 3 -(OC 2 H 4 ) 3 -CH 3 (TG)/LiN(SO 2 F) 2 (LiFSI) 1:1 molar mixture was investigated for high-safety lithium-ion secondary batteries by various physicochemical and electrochemical measurements. The TG-LiFSI electrolyte formed 1:1 complex, and showed relatively high thermal stability. Stable charge/discharge (intercalation/deintercalation) of lithium ions with both LiFePO 4 positive electrode and graphite negative electrode were enabled with high coulombic efficiency and long cycles. Moreover, long cycle charge/discharge operations of [LiFePO 4 positive electrode|TG-LiFSI electrolyte|graphite negative electrode] cell was achieved with 82% of capacity retention at 100 cycles.