Article pubs.acs.org/JPCB
Mass and Charge Transport in Cyclic Carbonates: Implications for Improved Lithium Ion Battery Electrolytes Matt Petrowsky, Mohd Ismail, Daniel T. Glatzhofer, and Roger Frech* Department of Chemistry and Biochemistry, University of Oklahoma, 101 Stephenson Parkway, Norman, Oklahoma 73019, United States S Supporting Information *
ABSTRACT: The compensated Arrhenius formalism (CAF) is applied to conductivity and diffusion data for a family of cyclic carbonates composed of octylene carbonate, decylene carbonate, undecylene carbonate, and dodecylene carbonate. The strong intermolecular interactions in these liquids lead to diffusion activation energies that are higher than those for typical aprotic solvents. The conductivity results show that activation energies are similar between TbaTf and LiTf cyclic carbonate electrolytes. However, the conductivities of the TbaTf solutions are higher than those for the LiTf solutions, and this is attributed to the greater number of charge carriers in the TbaTf electrolytes. These CAF results are then used to give a possible explanation of why the ionic conductivity in lithium ion battery electrolytes is often optimized by mixing a cyclic carbonate with a lower viscosity liquid. atom type, and dipole moment.1−3 Comparing cyclopentanone to 2-pentanone in Table 1 shows that the boiling points of molecules with comparable dipole moments can vary to some extent, but not nearly to the degree observed when comparing molecules with substantially different dipole moments. It has been claimed that intermolecular association occurs in the cyclic carbonates that is similar to although weaker than the association observed in hydrogen bonding liquids,8 but dielectric and spectroscopic data suggest that this is not the case. The Kirkwood g factor is often used as a measure of association in a liquid.9 Most aprotic liquids have g factors close to 1, while protic solvents such as water and the aliphatic alcohols have g factors well above unity. Furthermore, associated liquids usually have strongly temperature-dependent g factors. Temperature-independent g factors that are close to unity imply that Onsager’s model is appropriate to characterize εs of a liquid.10 Both Simeral11 and Payne12 have shown that g factors for propylene carbonate are close to unity and are temperature independent. Additionally, IR studies of propylene carbonate−carbon tetrachloride mixtures indicate no association since the carbonyl stretching mode of propylene carbonate does not shift in frequency upon dilution.11 Finally, temperature-dependent NMR results are consistent with propylene carbonate acting like a typical polar aprotic liquid.11,13 These data suggest that high dipole moment, aprotic liquids experience strong, but nonspecific, dipole−dipole intermolecular interactions.11,14 However, Fini et al. have observed that the carbonyl stretching mode in the cyclic carbonates is higher in frequency for IR data compared to Raman data, and this
1. INTRODUCTION Aprotic liquids with high dipole moments, such as propylene carbonate and dimethyl sulfoxide, have anomalous physical and transport properties compared to aprotic liquids with similar number and type of atoms that have lower dipole moments. The boiling points and viscosities of these liquids are usually very high while the self-diffusion coefficients are low. Table 1 Table 1. Boiling Points and Dipole Moments for Eight Different Aprotic Liquids solvent propylene carbonate4 ethyl methyl carbonate5 1-methyl-2pyrrolidinone4 1-butyl isocyanate6,7 γ-butyrolactone4 ethyl acetate4 cyclopentanone4 2-pentanone4 a
chemical formula
boiling point (°C)
dipole moment (D)
C4H6O3 C4H8O3
242 110
4.94 0.89
C5H9NO
202
4.09
C5H9NO C4H6O2 C4H8O2 C5H8O C5H10O
111 204 77 131 102
2.81a 4.12 1.82 2.86 2.70
The dipole moment listed is for ethyl isocyanate.
shows the effect of the dipole moment on the boiling point by comparing molecules that either are isomers or only differ by no more than two hydrogen atoms. Propylene carbonate, 1methyl-2-pyrrolidinone, and γ-butyrolactone have substantially higher boiling points than ethyl methyl carbonate, 1-butyl isocyanate, and ethyl acetate, respectively. It must be noted that the boiling point is only a rough estimate of intermolecular interactions because it depends on molecular surface area, flexibility, and degree of branching in addition to atom number, © 2013 American Chemical Society
Received: March 4, 2013 Revised: April 16, 2013 Published: April 18, 2013 5963
dx.doi.org/10.1021/jp402220a | J. Phys. Chem. B 2013, 117, 5963−5970
The Journal of Physical Chemistry B
Article
function of the gradient field strength over the range 6 to 62 G/ cm. Plotting the logarithm of intensity versus the square of the gradient field strength produced a linear relationship whose slope was used to determine the diffusion coefficient.30 The cyclic carbonates used in this work were synthesized inhouse. 1H NMR spectra were obtained using a Varian Mercury300 NMR spectrometer. IR spectra were obtained using Shimadzu IRAffinity-1 spectrometer and dry KBr. Octane-, decane- and docecane-1,2-diols and 1-undecene (Sigma-Aldrich Chemical Co.) were used as received. Octylene, decylene, and dodecylene carbonates were prepared using the following procedure: A flask was charged with 1 equiv of the appropriate 1,2-diol, and methyl chloroformate (6 equiv) was added to the flask. A reflux condenser and drying tube were attached, and the mixture was heated with stirring (magnetic) to approximately 60 °C. The reaction was monitored using 1H NMR spectroscopy until the formation of the cyclic carbonate was completed (1 to 2 weeks). The mixture was cooled to room temperature, and excess methyl chloroformate was removed under reduced pressure. The product was filtered through a column packed with 50% acidic alumina (bottom layer) and 50% of neutral alumina (top layer). 1H NMR spectra (vide inf ra) were consistent with literature values, and IR spectroscopy did not show any significant hydroxyl absorption. The yield could be improved by washing the column with diethyl ether, evaporating the solvent, running the remaining product through the column again, and repeating the process as needed to remove any nonreacted alcohols (by 1H NMR). Undecylene carbonate was made in two steps from 1undecene. 1,2-Undecanediol was synthesized using the procedure described by Sudalai.31 1-Undecene (56.3 g, 365 mmol), sodium periodate (23.4 g, 110 mmol, ∼30 mol %), lithium bromide (6.3 g of 73 mmol, ∼20 mol %), and 240 mL of glacial acetic acid were added to a flask fitted with a reflux condenser. The mixture was heated to 95 °C with stirring (magnetic) for 5 days and allowed to cool to room temperature. The 1,2-diacetoxyundecane product was slowly extracted with ethyl acetate three times. The organic layer was washed with aqueous sodium thiosulfate, water, and aqueous sodium bicarbonate and dried overnight using sodium sulfate. The solution was filtered through Celite, and solvent was removed under reduced pressure. Potassium carbonate (75.6 g, 1.5 equiv) and 100 mL of methanol were added to the product, and the solution was heated for 24 h with stirring (magnetic). The solvent was removed under reduced pressure, the 1,2-diol product was extracted from the residue using dichloromethane, and the extracts were run through a neutral alumina column using dichloromethane as eluent. The solvent was removed under reduced pressure to give a 60% yield of 1,2-undecane diol; 1H NMR (300 MHz, CDCl3): δ 0.87 (t, 3H, J = 6.5); 1.14−1.55 (m, 16H); 3.41 (t, 1H, J = 9 Hz); 3.59−3.73 (m, 2H) The resulting diol was placed in a flask with 8 equiv of methyl chloroformate, and a reflux condenser and drying tube were attached. The mixture was heated with stirring (magnetic) to approximately 60 °C and monitored using 1H NMR spectroscopy until the formation of the cyclic carbonate was completed (1 to 2 weeks). The mixture was cooled to room temperature, and excess methyl chloroformate was removed under reduced pressure. The final product was distilled under reduced pressure (
