Makoto Ue

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

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

A New Concept for Lithium Primary Cells using Poly(carbon Monofluoride) Cathode -The Effect of Quinone Compounds as an Electrolyte Additive-

Lithium primary cells are widely utilized as power sources for electronic instruments, such as electronic control gas meters, memory backup of mobile instruments, etc. Since lithium primary cells using poly(carbon monofluoride), (CF)n, cathode have extremely high energy density and excellent stability performance at high temperatures, long-life usage is possible under severe environmental conditions. However, the lithium cells using (CF)n cathode have peculiarities; the cell voltage drops drastically at high current discharge when the depth of discharge is low, and the recovery of dropped voltage requires many hours. The reason for this phenomenon is presumed to be that the discharge reaction occurs only in some part of the active materials because of the poor electric conductivity of (CF)n and the time taken to unify their potentials. We focused on the fact that the reduction potential of 1,4-benzoquinone (14BQ, 2.9 V vs. Li/Li) is close to the discharge reaction potential of (CF)n with lithium ion (2.8V). By adding a compound whose reduction potential is close to that of the reaction potential of cathode and lithium ion, the reduction of this additive occurs in the vicinity of the cathode material instead of the electrode reaction as shown in Fig. 1. Therefore, the electrode potential becomes the hybrid potential of (CF)n and reduction potential of the additive, so that the returned voltage at the end of rest can be kept as high as expected. Quinone couple is known as a reversible redox substance, however, few examples have been reported as an electrolyte additive. 1) In this work, the effect of some quinone compounds was examined as an electrolyte additive. Table 1 shows the reduction potentials of several quinone compounds, which were determined at 1 wt.% in 1 M LiBF4/γbutyrolactone (GBL) by linear sweep voltammetry, This results show that the reduction potentials of quinone compounds are close to the discharge reaction potential of (CF)n. The test cells were fabricated with (CF)n as the cathode, lithium metal as the anode, and 1 M LiBF4/GBL as the base electrolyte. Fig. 2 shows the minimum discharge voltage in the test of 0.25C pulse discharge at -40C and the returned voltage immediately after the discharge with/without a 1 wt.% quinone compound, respectively. The returned voltages of the cells including quinone compounds became higher than that of the one without additive. Moreover, when 14NQ was added, the minimum discharge voltage also became higher. We will report the comparison of the additive effect of several quinone compounds, and the correlation between their chemical structure and their effects. Fig. 1 Concept of quinone compounds assisting discharge.