Physical and Electrochemical Properties of Fluorinated Dialkyl

Received: June 2, 2016 Electrochemistry Accepted: August 10, 2016 Published: October 5, 2016

The Electrochemical Society of Japan http://dx.doi.org/10.5796/electrochemistry.84.776 Communication Electrochemistry, 84(10), 776–778 (2016) Physical and Electrochemical Properties of Fluorinated Dialkyl Ethers Noritoshi NAMBU,a,* Yuya MATSUSHITA,a Masahiro TAKEHARA,b and Yukio SASAKIa a Department of Life Science and Sustainable Chemistry, Faculty of Engineering, Tokyo Polytechnic University, 1583 Iiyama, Atsugi, Kanagawa 243-0297, Japan b Mitsubishi Chemical Group Science and Technology Research Center, Inc., 8-3-1 Chuo, Ami, Inashiki, Ibaraki 300-0332, Japan * Corresponding author: [email protected]

ABSTRACT Dialkyl ethers show high relative permittivities and low viscosities as compared to the corresponding linear carbonates. We have synthesized 1-(2-fluoroethoxy)-2-(2,2,2-trifluoroethoxy)ethane (FETFEE). The relative permittivity and viscosity of FETFEE were higher than those of 1,2-diethoxyethane (DEE, ethylene glycol diethyl −3 ether). The conductivity of 1 mol dm LiPF6 solution in FETFEE was higher than that in 1-ethoxy-2-(2,2,2- trifluoroethoxy)ethane (ETFEE). The use of FETFEE as a co-solvent improved the discharge capacity of a Li | LiCoO2 coin cell. © The Electrochemical Society of Japan, All rights reserved.

Keywords : Dialkyl Ether, Fluorinated Solvent, Relative Permittivity, Conductivity

1. Introduction 2. Experimental

1,2-Dimethoxyethane (DME, ethylene glycol dimethyl ether), 1- The synthesis of EFEE, ETFEE, and DEE was described in the ethoxy-2-methoxyethane (EME, ethylene glycol ethyl methyl ether), previous paper.3 We synthesized FETFEE from 2-fluoroethyl p- and 1,2-diethoxyethane (DEE, ethylene glycol diethyl ether) are toluenesulfonate and 2-(2,2,2-trifluoroethoxy)ethanol in acetonitrile dialkyl ethers of ethylene glycol. They show high boiling points as in the presence of sodium hydroxide in a yield of about 23%. compared to diethyl ether and tetrahydrofuran. DME is used as a 2-fluoroethyl p-toluenesulfonate was prepared by reaction of 2- reagent in organometallic chemistry such as Grignard reactions and fluoroethanol with p-toluensulfonyl chloride in pyridine in a yield of as a solvent in some electrolyte solutions for lithium batteries.1 about 70%. FETFEE was purified under reduced pressure first by DME, EME, and DEE show high relative permittivities and low simple distillation and then by fractional distillation. The purity of viscosities as compared to dimethyl carbonate (DMC), ethyl methyl FETFEE was determined to be more than 99.9% by the use of gas carbonate (EMC), diethyl carbonate (DEC) in spite of the similar chromatograph equipped with a FID detector (Shimadzu Corp., molecular mass and the near boiling points. GC-1700). The distilled FETFEE was dehydrated by purified The high electron-pair donability of oxygen atoms of the C-O-C molecular sieves (4A) before use. Ethylene carbonate (EC) was used moieties in a DME molecule causes the chelation of a metal ion. as received (Kishida Chemical Co., Ltd., LBG grade). DME forms chelate complexes with cations and acts as a bidentate The apparatus and techniques for measurements are essentially ligand. The chelating nature and low viscosity of DME contribute the same as those previously reported.3,5–7 We measured relative to the high conductivity of the blended electrolyte solutions. The permittivities (¾r) or relative dielectric constants by the use of a LF conductivity of an electrolyte solution is a dominant factor impedance analyzer (Hewlett Packard, 4192A) that was connected governing the internal resistance and rate performance of lithium to a thermostat (Ando Denki, TO-9). The electrostatic capacitances batteries. In contrast, the solubility of lithium salts in DEE is of air (C0) and a sample (Csample) were measured separately at a much less than those in DME and EME. Partial fluorination of frequency of 1 MHz. The frequency is involved in an electrical DEE increased the polarity and improved the solubility of lithium region. The effect of the dielectric dispersion and the effect of the salts.2 dielectric absorption can be ignored with regard to the dielectric Fluorine is the most electronegative of all the elements, and the polarization of normal liquids at less than 107 Hz. The relative size is small next to hydrogen. Fluorine atoms show very low permittivity of a vacuum is defined as unity, but for practical polarizability and high enthalpy of ionization. Partially fluorinated purposes measurements were made in air. The relative permittivity solvents exert the polar effect on the physical and electrochemical can be adequately approximated by the ratio of Csample to C0: ¾r µ properties such as relative permittivity, viscosity, electrolytic Csample/C0. The samples were sufficiently deaerated by bubbling Ar conductivity, and electrochemical stability.3 In contrast, polyfluori- gas (99.9%) before the measurement. The relative permittivities nated and perfluorinated organic solvents show low polarity. The were investigated over a temperature range from 10°C to 70°C. individual dipole moments of the C–F bonds are canceled in the Conductivities of electrolyte solutions were measured by the use molecule resulting in low relative permittivity.4 of a conductometer (Toa Electronics Inc., Model CM-60S) that was In the present paper, we describe the physical and electrochem- equipped with the cell (Model CGT-511B). The conductivities were ical properties of three fluorinated dialkyl ethers: 1-(2-fluoroethoxy)- investigated over a temperature range from ¹5°C to 60°C. 2-(2,2,2-trifluoroethoxy)ethane (FETFEE), 1-ethoxy-2-(2,2,2-trifluoroethoxy)ethane (ETFEE), 1-ethoxy-2-(2-fluoroethoxy)ethane 3. Results and Discussion (EFEE). EFEE, ETFEE, and FETFEE correspond to monofluori- nated, trifluorinated, and tetrafluorinated DEEs. 3.1 Physical properties Relative permittivity reflects the ease of progress of dielectric

776 Electrochemistry, 84(10), 776–778 (2016)

30 increase in temperature. Consequently, the plots of log10(©/mPa s) ¹1 ¹6 2 ¹1 ¹1 vs. T and the plots of log10(¯/10 m s ) vs. T gave straight lines. The high translational kinetic energy allows intermolecular FETFEE 20 ETFEE attractions to be overcome more easily. The internal friction is ﬁ

r reduced at high temperatures. We can nd the apparent activation ε 9 energy for viscosity (Ea,©) from Andrade’ equation: 10 EFEE DEE E © ¼ A exp a;© ð1Þ © RT

0 The apparent activation energy was determined to be 0 20406080 16.17 kJ mol¹1, 14.36 kJ mol¹1, 12.09 kJ mol¹1, and 10.27 kJ mol¹1 θ / Υ for FETFEE, EFEE, ETFEE, and DEE, respectively. The order of Figure 1. (Color online) Relative permittivities (¾ ) of FETFEE, the apparent activation energy agreed with that of the viscosity. r fi EFEE, ETFEE, and DEE single solvents as a function of temper- The kinematic viscosity is de ned as the ratio of the viscosity to fl ¯ = ©/ ature (ª) from 10°C to 70°C. the mass density of a uid: d. Both the viscosity and the mass density are correlated with the molecular mass. Therefore, the effect of the molecular mass on the kinematic viscosity can become small. polarization. The relative permittivity is a measure of the relative The kinematic viscosity may more clearly reflect the magnitude of effect a solvent has on the force with which two oppositely charged the attractive forces between molecules. The kinematic viscosity of plates attract each other. Figure 1 shows the temperature (ª) ETFEE was as low as that of DEE above 323 K. 1-(2,2,2-Trifluoro- dependence of relative permittivities of FETFEE, EFEE, ETFEE, ethoxy)-2-methoxyethane7 and N,N-dimethyl trifluoroacetamide10 and DEE single solvents. The relative permittivities of FETFEE, also show low kinematic viscosities. The temperature dependency EFEE, and ETFEE decreased linearly with an increase in temper- of kinematic viscosity of FETFEE was larger than that for the other ature. The relative permittivity of FETFEE was higher than those solvents. FETFEE became less viscous at higher temperatures. This of ETFEE and EFEE: FETFEE > ETFEE µ EFEE > DEE. The phenomenon can be attributed to the mobility of stiffness. finding suggests that the amount of the cooperative orientation polarization decreases in that order. The viscosity and kinematic 3.2 Electrolytic conductivities and performance of Li/LiCoO2 viscosity of FETFEE as well as the relative permittivity were higher coin cells than those of the other dialkyl ethers, as described later. The finding The conductivity of an electrolyte solution is a key factor shows that the net dipole moment of FETFEE is larger than those of determining the internal resistance and rate performance of lithium ETFEE and EFEE. batteries. Figure 3(a) shows the temperature (T) dependence of The number of hydrogen atoms that are bound to the same conductivities (¬) of electrolyte solutions in FETFEE, EFEE, and ¹3 terminal carbon atom as fluorine atoms are as given below: FETFEE ETFEE. We used LiPF6 (1 mol dm at 25°C) as the electrolyte. ¹3 (2), EFEE (2), ETFEE (0), and DEE (0). The attraction between The solubility of LiPF6 in DEE was much less than 1 mol dm . FETFEE molecules can be based on nonconventional weak The conductivities increased with an increase in temperature. The intermolecular hydrogen bonding (CF–H£OorC–H£F–C).8 The viscosities of the fluorinated DEEs decreased rapidly with an weak hydrogen-bonding system does not exchange its proton and increase in temperature and approached that of DEE at high therefore it is no more a genuine hydrogen bond; it is an electrostatic temperatures, as shown in Fig. 2(a). attraction between positive charge on the hydrogen and negative Interestingly, the conductivity of LiPF6 solution in FETFEE was charge on the organic fluorine or the organic oxygen. The molar higher than that in ETFEE: EFEE > FETFEE > ETFEE. The partial concentration of FETFEE (6.50 mol dm¹3 at 25°C) was higher than fluorination can increase the ability of the solvent molecules to form that of ETFEE (6.38 mol dm¹3 at 25°C) in spite of the high molar hydrogen bonds. Therefore, the acceptability of an electron-pair of a mass (190.14 g mol¹1 for FETFEE and 172.15 g mol¹1 for ETFEE). donor atom from a solute molecule becomes high. This effect may ¹ The liquid may contract to some extent because of the strong result in the increased solvation of PF6 ions and, consequently, attractive forces between FETFEE molecules, and consequently the in the higher degree of ionic dissociation. The fluoromethyl group molar concentration of FETFEE increased. (CH2F-) may serve as an anion-attracting group. In contrast, the One terminal methyl group is fluorinated in ETFEE and EFEE polyfluorination can decrease the electron-pair donability of an molecules. It should be noted that the relative permittivity of ETFEE oxygen atom of the C-O-C moiety. The trifluoromethyl group (CF3-) was essentially the same as that of EFEE. The larger net dipole is a strong electron-withdrawing substituent. The anodic stability of moment of ETFEE may compensate for the lower molar concen- FETFEE was higher than that of EFEE: FETFEE > ETFEE > tration of ETFEE (6.38 mol dm¹3 for ETFEE and 7.15 mol dm¹3 for EFEE. The finding suggests that the electron-pair donability EFEE at 25°C). decreases in the inverse order: FETFEE < ETFEE < EFEE. Viscosity or viscosity coefficient is defined as the force per unit Fluorine is the most electronegative of all the elements. Most area necessary to maintain a unit velocity gradient between two conventional measures for the electronic effect of substituents are parallel planes a unit distance apart. It is the property which opposes provided as Taft constants (Q*) for substituents attached to aliphatic 8 the relative motion of adjacent layers of the liquid. It is regarded chains. The Taft constants of a trifluorometheyl group (CF3-: Taft * as internal friction and affects electrolytic conductivity. In contrast, Q = 2.61, Q(calcd.) = 2.60) and a fluorometheyl group (CH2F-: the kinematic viscosity is used in fluid mechanics and is the only Taft Q* = 1.10, Q(calcd.) = 1.17) are substantially larger than that of * parameter characteristic of the fluid itself to appear in the equations a metheyl group (CH3-: Taft Q = 0, Q(calcd.) = ¹0.07). The value of hydrodynamics. Figure 2 shows the temperature (T) dependence of 1.10 is comparable with a formyl group (-CHO: Taft Q* = 1.1, of (a) viscosities (©) and (b) kinematic viscosities (¯) of FETFEE, Q(calcd.) = 1.09). ¹1 EFEE, ETFEE, and DEE single solvents. The viscosity of FETFEE The plots of log10(©/mPa s) vs. T gave straight lines, as shown ¹1 ¹1 was also higher than those of EFEE and ETFEE: FETFEE > in Fig. 2(a). In contrast, the plots of log10(¬/mS cm ) vs. T EFEE > ETFEE > DEE. This order was essentially the same as displayed upward curvature. The convex curves show that the observed for the relative permittivity except for ETFEE. The conductivities of LiPF6 solutions in fluorinated dialkyl ethers are not viscosity and kinematic viscosity decreased exponentially with an inversely proportional to the viscosity.

777 Electrochemistry, 84(10), 776–778 (2016)

T / K T / K 340 320 300 280 340 320 300 280 0.4 2 FETFEE (a) FETFEE (b) ETFEE 0.2 ETFEE

0.2 EFEE ) EFEE -1 ν

DEE DEE / 10 s η 2 / mPa m

0 1 -6 0 1 -6 m / mPas) η 0.8 2 ( 0.8 s s / 10 10 ν -1 -0.2 0.6 (

10 -0.2 log 0.6 log -0.4 0.4 -0.4 0.4 33.23.43.6 33.23.43.6

T -1 / 10-3 K-1 T -1 / 10-3 K-1 Figure 2. (Color online) (a) Viscosities (©) and (b) kinematic viscosities (¯) of FETFEE, EFEE, ETFEE, and DEE single solvents as a function of temperature (T) from 283.15 K (10°C) to 343.15 K (70°C).

T / K

340 320 300 280 150 1 10

(a) 8 -1 0.8

) 6 -1

κ 100 / mScm / mAhg 0.6 4 / mScm κ -1 ( 0.4

10 2 50

log EC-FETFEE FETFEE 0.2 EC-ETFEE ETFEE EC-EFEE EFEE (b) Discharge capacity Discharge EC-DEE 0 1 0.9 0 3 3.2 3.4 3.6 3.8 0 1020304050

T -1 / 10-3 K-1 Cycle number

¹3 Figure 3. (Color online) (a) Electrolytic conductivities (¬) of 1 mol dm LiPF6 solutions in FETFEE, EFEE, and ETFEE as a function of temperature (T) from 268.15 K (¹5°C) to 333.15 K (60°C). (b) Variation of discharge capacities of Li « LiCoO2 coin cells with respect to the ¹3 cycle number at 25°C. Electrolyte solutions: 1 mol dm LiPF6 solutions in EC–FETFEE, EC–EFEE, and EC–DEE equimolar binary ¹3 mixtures and 0.5 mol dm LiPF6 solution in an EC–ETFEE equimolar binary mixture.

Fluorinated dialkyl ethers are much less polar and less viscous viscosity of FETFEE were higher than those of DEE. The kinematic than cyclic carbonates. We used EC–fluorinated dialkyl ether viscosity of ETFEE was as low as that of DEE above 50°C. equimolar binary mixtures as solvents to apply the fluorinated The fluorination of DEE improved the solubility of LiPF6. The ¹3 dialkyl ethers to lithium secondary batteries. We assembled 2025- electrolytic conductivity of 1 mol dm LiPF6/FETFEE was lower ¹3 type coin cells to evaluate the performance by a charge–discharge than that of 1 mol dm LiPF6/EFEE, but higher than that of ¹3 test. Figure 3(b) shows the evolution of discharge capacity of 1 mol dm LiPF6/ETFEE. The partial fluorination can increase Li « LiCoO2 coin cells with respect to the cycle number at 25°C. the ability of the solvent molecules to form hydrogen bonds. The Included in the figures for comparison are data for an EC–DEE fluoromethyl group may serve as an anion-attracting group. The equimolar binary mixture. The cycling efficiency stands for the use of an EC–FETFEE equimolar binary mixture suppressed the capacity ratio of discharge to charge. The coin cells were charged in discharge capacity fading at high cycle numbers. a constant current (0.2 C)–constant voltage regime (4.2 V) for 5 h and then discharged to 3.0 V at a constant current of 0.2 C. The References discharge capacity is expressed on the basis of the mass of LiCoO2 as a cathode. The discharge capacity in the EC–FETFEE system ¹1 1. L. A. Dominey, Lithium Batteries (Ed. G. Pistoia), Elsevier, Amsterdam, Chap. 4 was as high as 120 mA h g even at the 50th cycle. The discharge (1994). capacity observed in the EC–FETFEE system was higher than those 2. Y. Sasaki, G. Shimazaki, N. Nanbu, M. Takehara, and M. Ue, ECS Trans., 16,23 in the other systems over 50 cycles. The use of FETFEE as the (2009). alternative low-viscosity solvent improved the performance of the 3. N. Nambu and Y. Sasaki, Open J. Met., 5, 1 (2015). fl coin cell. The cathodic decomposition of the FETFEE-based 4. T. Hiyama, Organo uorine Compounds, Springer, Berlin, p. 13 (2000). 5. N. Nanbu, M. Takehara, S. Watanabe, M. Ue, and Y. Sasaki, Bull. Chem. Soc. solution on a lithium anode can form passive thin films containing Jpn., 80, 1302 (2007). adequate amounts of organofluorine compounds such as fluorinated 6. N. Nanbu, S. Watanabe, M. Takehara, M. Ue, and Y. Sasaki, J. Electroanal. alkoxides. Lithium ions may readily pass through the surface Chem., 625, 7 (2009). film. It is known that lithium alkoxides (ROLi) are formed on 7. T. Satoh, N. Nambu, M. Takehara, M. Ue, and Y. Sasaki, ECS Trans., 50, 127 fl (2013). electrode surfaces by cathodic decomposition of non uorinated 8. K. Uneyama, Organofluorine Chemistry, Fundamentals in Organic Fluorine 11 ethers. Chemistry, Blackwell Publishing, Ltd, Oxford, Chaps. 1 and 4 (2006). 9. E. N. da C. Andrade, Nature, 125, 309 (1930). 4. Conclusion 10. K.-C. Möller, T. Hodal, W. K. Appel, M. Winter, and J. O. Besenhard, J. Power Sources, 97–98, 595 (2001). 11. D. Aurbach and Y. Cohen, Lithium-Ion Batteries, Solid-Electrolyte Interphase We have investigated the effect of fluorination on the physical (Eds. P. B. Balbuena and Y. Wang), Imperical College Press, London, Chap. 2 and electrochemical properties of DEE. The relative permittivity and (2004) and references cited therein.

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