"Ionic Liquid Electrolytes for Electrochemical Capacitors Influence of"
Ionic Liquid Electrolytes for Electrochemical Capacitors: Influence of Solvent Additives on PY15TFSI Viscosity, Density and Thermal Behavior Elie Paillard and Wesley A. Henderson Ionic Liquids and Electrolytes for Energy Technologies (ILEET) Laboratory, North Carolina State University, 911 Partners Way, Raleigh, NC 27695-7905, USA Ionic liquids (ILs) have attracted considerable attention in recent years as electrolytes for electrochemical capacitors. In general, the ionic conductivity of neat ILs is too low at ambient temperature and below, however, for practical capacitors performance. When mixed with an organic solvent (typically 1 M solutions in either propylene carbonate (PC) or acetonitrile (ACN)),1-5 though, the conductivities are acceptable for practical use. ILs are a "drop-in" electrolyte component since current commercial capacitors already use tetraalkylammonium salts similar Figure 1. Viscosity of PY15TFSI- solvent mixtures at in structure to the ILs. The justifications for replacing 20°C as a function of solvent mole fraction. these salts with an IL include: (1) an IL may be dissolved at much higher concentrations in an organic solvent than Acknowledgement: The authors gratefully acknowledge is possible with higher melting salts and (2) the ionic the financial support of the Army Research Office (ARO) conductivity of IL solutions is frequently higher than for under Contract W911NF-07-1-0556. similar solutions with higher melting salts.6 While some studies have been made with IL-solvent additive References mixtures7-9 limited information is available concerning the 1. A. B. McEwen, E. L. Ngo, K. LeCompte and J. L. effect of the additives on the thermal phase behavior, Goldman, J. Electrochem. Soc., 146, 1687 (1999). volatility, viscosity and density of these electrolyte 2. K. Shiono and Y. Nitta, Electrolyte and Electronic mixtures. Component Using Same, in, p. 9 pp (Sanyo Chemical In this study, we focus on the effect of 14 different Industries Ltd., Japan; Matsushita Electric Industrial solvents, comprising principally cyclic and acyclic Co., Ltd.). US patent: 5870275 (1999). carbonates and esters: ethylene carbonate (EC), PC, 3. Q. Zhu, Y. Song, X. Zhu and X. Wang, J. Electroanal. dimethyl carbonate (DMC) diethyl carbonate (DEC), γ- Chem., 601, 229 (2007). butyrolactone (GBL), γ-valerolactone (GVL), methyl 4. T. Devarajan, S. Higashiya, C. Dangler, M. Rane- acetate (MA), methyl propionate (MP), methyl butyrate Fondacaro, J. Snyder and P. Haldar, Electrochem. (MB), methyl isobutyrate (MI), ethyl acetate (EA), ethyl Commun., 11, 680 (2009). propionate (EP), ethyl butyrate (EB) and ACN, with a 5. E. Frackowiak, G. Lota and J. Pernak, Appl. Phys. focus on low solvent concentrations, as the interactions Lett., 86, 164104/1 (2005). between the ions and the solvents might be expected to 6. K. Yuyama, G. Masuda, H. Yoshida and T. Sato, J. decrease the solvent volatility relative to traditional Power Sources, 162, 1401 (2006). electrolytes. We chose N-methyl-N-pentylpyrrolidinium 7. A. Chagnes, H. Allouchi, B. Carre and D. Lemordant, bis(trifluoromethanesulfonyl)imide (PY15TFSI) as a Solid State Ionics, 176, 1419 (2005). reference IL as the pyrrolidinium cations are known to 8. A. Chagnes, M. Diaw, B. Carre, P. Willmann and D. have better cathodic stability than imidazolium cations. Lemordant, J. Power Sources, 145, 82 (2005). Moreover, the addition of PY1RTFSI ILs has been shown 9. M. Diaw, A. Chagnes, B. Carre, P. Willmann and D. to decrease the flammability of electrolytes.10 The melting Lemordant, J. Power Sources, 146, 682 (2005). point of PY15TFSI is higher than either PY13TFSI or 10. H. Nakagawa, Y. Fujino, S. Kozono, Y. Katayama, T. PY14TFSI, but this salt does not exhibit solid-solid phase Nukuda, H. Sakaebe, H. Matsumoto and K. Tatsumi, transitions at low temperature in contrast with salts based J. Power Sources, 174, 1021 (2007). upon pyrrolidinium cations with shorter alkyl chains,11,12 11. W. A. Henderson and S. Passerini, Chem. Mater., 16, thus making it more suitable for phase behavior studies. 2881 (2004). As already reported in the case of imidazolium-based 12. W. A. Henderson, V. G. Young, Jr., W. Pearson, S. ILs, the effect of organic solvent on IL viscosity is not Passerini, H. C. De Long and P. C. Trulove, J. Phys.: additive,13,14 thus allowing a rapid decrease of the Condens. Matter, 18, 10377 (2006). viscosity at low concentration for all of the solvents, 13. K. R. Seddon, A. Stark and M.-J. Torres, Pure Appl. especially at ambient temperature. In the low solvent Chem., 72, 2275 (2000). concentration range, however, the variations are linear 14. J. Wang, Y. Tian, Y. Zhao and K. Zhuo, Green Chem., (Fig. 1). The excess molar volumes (VmE), determined by 5, 618 (2003). density measurements, also reveal different behavior, with the linear solvent having a more negative VmE, whereas the cyclic solvents and ACN which have a higher dielectric constant have a lower VmE. The DSC data indicate that, for some binary mixtures, new melting peaks appear suggesting the formation of a new phase and, for EC-PY15TFSI, a complete phase diagram has been generated with a low-melting eutectic present.