Ethyl acetate (EA) shows low viscosity for its relative permittivity. Monofluorinated organic solvents exert the polar effect on the various properties. We have investigated the effect of position isomerism on the physical and electrochemical properties of two monofluorinated carboxylates: 2-fluoroethyl acetate (2FEA) and ethyl fluoroacetate (EFA). Relative permittivity of 2FEA was lower than that of EFA, whereas viscosity of 2FEA was higher. Electrolytic conductivity of a LiPF 6 solution in 2FEA was lower than that in EFA, but higher than that in EA at high temperatures. The use of 2FEA as a co-solvent improved cycling efficiency and suppressed fading of discharge capacity of a Li|LiCoO 2 coin cell at high cycle numbers.

Methyl propyl carbonate (MPC) shows a liquid temperature range that is wider than that of diethyl carbonate (DEC). Monofluorinated organic solvents exert the strong polar effect on the physical and the electrochemical properties. 2-Fluoropropyl methyl carbonate (2FPMC) and ethyl 2-fluoroethyl carbonate (E2FEC) are isomeric with each other. The relative permittivity and viscosity of 2FPMC were higher than those of E2FEC, MPC, and DEC. The ionic conductivity of 1 mol dm −3 LiPF 6 solution in 2FPMC was lower than those in E2FEC, MPC, and DEC. The use of 2FPMC as a co-solvent greatly improved cycling efficiency of a lithium anode.

A trifluoromethyl group (CF 3 -) is a strong electron-withdrawing substituent. The trifluoromethyl is much larger than a methyl group. We have investigated the physical and electrolytic properties of trifluorinated linear ethers as solvents and their application to lithium secondary batteries. We synthesized 1-(2,2,2-trifluoroethoxy)-2-methoxyethane (TFEME) as a trifluorinated diether and 1-(2,2,2-trifluoroethoxy)-2-(2-methoxyethoxy)ethane (TFEMEE) as a trifluorinated triether. Three fluorine atoms of TFEME and TFEMEE are situated on the terminal carbon atom of the ethyl group. The mass densities, relative permittivities, viscosities, and anodic stabilities of TFEME and TFEMEE were higher than those of the corresponding nonfluorinated linear ethers (1-ethoxy-2-methoxyethane (EME) and 1-ethoxy-2-(2-methoxyethoxy)ethane (EMEE)). The presence of the trifluomethyl group can weaken the attractive forces between molecules. The kinematic viscosities of TFEME and TFEMEE were as low as those of the corresponding nonfluorinated linear ethers especially at high temperatures. The use of an EC−TFEME or an EC−TFEMEE binary mixture improved the cycling efficiency of a lithium anode and the discharge capacity of a Li | LiCoO 2 coin cell.

The use of partially fluorinated compounds as alternative solvents can increase the solubility of electrolytes and improve the performance of electric double-layer capacitors (EDLCs). The solubility of triethylmethylammonium tetrafluoroborate (TEMABF 4 ) in fluoroethylene carbonate (FEC) was 3.0 M (M = mol dm

The use of tetramethylammonium ions (TMA + ) at moderately high concentrations of the electrolytes can increase the number of ions accumulated in pores of an activated carbon electrode and bring about high capacitance of an electric double-layer capacitor (EDLC). The solubility of tetramethylammonium difluoro(oxalato)borate (TMADFOB) in propylene carbonate (PC) was 2.0 M (M = mol dm −3 ) or more. The maximal ionic conductivity of a TMADFOB solution in PC was observed at about 1.6 M at 25°C. The increase in the concentration of the electrolyte resulted in the increase in the gravimetric capacitance. The gravimetric capacitance found for TMADFOB was slightly higher than that for triethylmethylammonium tetrafluoroborate (TEMABF 4 ).