Physical and Electrochemical Properties of Dialkyl Ethers
Authors: Suzuki Shohei , Ogasawara Masaru , Nambu Noritoshi 
Published: February 2020

1,2-Dimethoxyethane (DME, ethylene glycol dimethyl ether) and 1,2-diethoxyethane (DEE, ethylene glycol diethyl ether) are dialkyl ethers of ethylene glycol. DME shows a high boiling point as compared to diethyl ether and tetrahydrofuran. DME is also used as a reagent in organometallic chemistry and as a solvent in some electrolyte solutions for lithium batteries.

The high electron-pair donability of oxygen atoms of the C-O-C moieties in DME molecules causes the chelation of a metal ion. The chelating nature and low viscosity of DME contribute to the high conductivity of the blended electrolyte solutions. The solubility of lithium salts in DEE is much less than that in DME. Partial fluorination of DEE can improve the solubility of lithium salts. In contrast, the polyfluorinated and perfluorinated organic solvents show very low polarity. The individual dipole moments of the C−F bonds are canceled in the molecules resulting in low relative permittivities. These solvents are the so-called fluorous media and are often immiscible with organic solvents as well as water.

We synthesized a series of fluorinated DEE: trifluorinated, tetrafluorinated, pentafluorinated, and hexafluorinated DEEs (ETFEE, FETFEE, DFETFEE, and BTFEE, respectively). We report the physical and electrochemical properties of these dialkyl ethers.

Relative permittivity reflects the ease of progress of dielectric polarization. The relative permittivity is a measure of the relative effect a solvent has on the force with which two oppositely charged plates attract each other. The relative permittivities (ε
r) of ETFEE, FETFEE, DFETFEE, and BTFEE single solvents decreased linearly with an increase in temperature (θ). The relative permittivity of FETFEE was higher than those of DFETFEE and ETFEE: FETFEE > DFETFEE > BTFEE > ETFEE > DEE. The finding suggests that the amount of the cooperative orientation polarization decreases in that order. The number of hydrogen atoms that are bound to the same terminal carbon atom as fluorine atoms decreases in the following descending order: FETFEE (2) > DFETFEE (1) > BTFEE (0) = ETFEE (0) = DEE (0). The attraction between FETFEE molecules and the attraction between DFETFEE molecules can be based on nonconventional weak intermolecular hydrogen bonding (CF−H···O or C−H···F−C). The weak hydrogen-bonding system does not exchange its proton and therefore it is no more a genuine hydrogen bond; it is an electrostatic attraction between positive charge on the hydrogen and negative charge on the organic fluorine or the organic oxygen.

Conductivity of an electrolyte solution is a key factor determining the internal resistance and rate performance of lithium batteries. The conductivities (κ) of electrolyte solutions in ETFEE, FETFEE, and DFETFEE increased with an increase in temperature (θ). We used LiPF6 (1 mol dm−3 at 25 ºC) as the electrolyte. The solubility of LiPF6 in DEE and BTFEE was much less than 1 mol dm−3. The viscosities of the fluorinated DEEs decreased rapidly with an increase in temperature and approached that of DEE at high temperatures.

Interestingly, the conductivity of LiPF6 solution in DFETFEE was higher than that in ETFEE: DFETFEE > FETFEE > ETFEE. The partial fluorination can increase the ability of a solvent to form a hydrogen bond. Therefore, the acceptability of an electron-pair of a donor atom from a solute molecule becomes high. This effect may result in the increased solvation of PF6
− ions and, consequently, in the higher degree of ionic dissociation. In contrast, the polyfluorination can decrease the electron-pair donability of an oxygen atom of the C-O-C moiety. A trifluoromethyl group is a strong electron-withdrawing substituent. The degree of ionic dissociation may become lower especially in BTFEE. The anodic stability of DFETFEE was slightly higher than that of ETFEE: DFETFEE > FETFEE > ETFEE.