Physicochemical Properties of Dialkyl Ethers

Abstract

Ethylene glycol diethyl ether (1,2-diethoxyethane: DEE) is dialkyl ethers of ethylene glycol. The solubility of lithium salts in DEE is much less than that in ethylene glycol dimethyl ether (1,2-dimethoxyethane: DME) and ethylene glycol ethyl methyl ether (1-ethoxy-2-methoxyethane: EME). Partial fluorination of DEE increases the polarity. We have investigated the effect of the partial fluorination on the physical and electrochemical properties of DEE in some detail. The mass densities decreased in the order of decreasing the molar mass: tetrafluorinated DEE (FETFEE) > trifluorinated DEE (ETFEE) > monofluorinated DEE (EFEE) (≈ diethyl carbonate (DEC)) > DEE. The molar concentration of FETFEE was higher than that of ETFEE in spite of the large molar size: DEC > EFEE > DEE > FETFEE > ETFEE. The relative permittivity and viscosity of FETFEE were higher than those of ETFEE and EFEE: FETFEE > ETFEE ≈ EFEE > DEE > DEC for the relative permittivity and FETFEE > EFEE > ETFEE > DEC > DEE for the viscosity. The kinematic viscosity of ETFEE was as low as that of DEE above 50 °C. The partial fluorination of DEE improved the solubility of LiPF6. The electrolytic conductivity of 1 mol dm−3 LiPF6/FETFEE was lower than that of 1 mol dm−3 LiPF6/EFEE, but higher than that of 1 mol dm−3 LiPF6/ETFEE. The partial fluorination can increase the ability of the solvent molecules to form hydrogen bonds. The fluoromethyl group may serve as an anion-attracting group. The use of an EC−FETFEE equimolar binary mixture suppressed the discharge capacity fading at high cycle numbers.