Physical and Electrolytic Properties of Partially Fluorinated Organic Solvents and Its Application to Secondary Lithium Batteries

Secondary lithium batteries (lithium ion batteries) are becoming in dispensable power sources for various portable electronic devices and they are also being applied for powering electric vehicles. The safety and reliability of the battery are very important for electric vehicles to be widely applicable. To improve the performance of secondary lithium batteries, much efforts have been focused on the development of the effective solvents (electrolytes) with high energy density, oxidation durability and non-flammability. Fluorinated organic solvents show very different physical properties compared with those of common organic solvents because of very high electronegativity, high ionic potential and low polarizability of the fluorine atom. In particular, partially fluorinated organic solvents show fairly high polarity in comparison with that of perfluoro organic solvents. One of the appropriate methods to find a solvent with good cell performance is the introduction of fluorine atoms into the solvent molecules. The present paper reviews partially fluorinated several important solvents for lithium batteries in view of their physical and electrolytic properties, and charge-discharge characterisics for rechargeability . Table1 shows dielectric constants (e) and viscosities (η) of three kinds of partially fluorinated ethoxymethoxyethane (EME) derivatives, which are fluoroethoxymethoxyethane (FEME), difluoroethoxymethoxyethane (DFEME) and trifluoroethoxymethoxyethane (TFEME). The dielectric constants of these EME derivatives are very higher than that of EME because of high electron withdrawing of fluorine. However, the dielectric constants are not appreciably dependent on the number of fluorine atom. Though these EME derivatives show very high viscosities compared with that of EME, viscosity of TFEME with higher molecular weight becomes small rather than those of FEME and DFEME. It seems that the introduction of three fluorine atoms to EME decreases the molecular interaction in TFEME molecule by steric hindrance and electron repulsion among the fluorine atoms. Figure 1 shows specific conductivities in EME, FEME, DFEME and TFEME solutions in the range of 5°C to 60°C. The solution with low viscosity tends to increase the specific conductivity. However, the specific conductivity of TFEME solution with lower viscosity than those of FEME and DFEME solutions becomes small. This means that the solvation in TFEME is different from those in FEME and DFEME. Figure 2 shows variation of Li electrode cycling efficiencies (charge-discharge coulombic efficiencies for Li electrode) in ethylene carbonate (EC)-based equimolar binary solutions. EC-TFEME electrolyte shows higher efficiency in a high cycle number range than those of other electrolytes. This is a good electrolyte for rechargeable lithium batteries. References 1) J.O.Besenhard and M. Winter, Chem. Phys. Chem., 3, 155 (2002) 2)K.Uneyama, “Organo Fluorine Chemistry” Blackwell Publishing Ltd (2006). 3) Y.Sasaki, Electrochemistry, 76, 2 (2008). Solvent M.W. e η