Conductivity and Viscosity of PC-DEC and PC-EC Solutions of LiBF[sub 4]

• Published in: Journal of the Electrochemical Society Vol. 151; no. 1; pp. A40 - A47
• Main Author: Ding, Michael S.
• Format: Journal Article
• Language: English
• Published: 2004
• ISSN: 0013-4651
• DOI: 10.1149/1.1630593

The conductivity kappa of propylene carbonate-diethyl carbonate (PC-DEC) and PC-EC (ethylene carbonate) solutions of LiBF(4) was experimentally determined at temperatures 0 from 60 to -80DGC and at salt molalities m from 0.2 to 2.1 mol kg(-1), and solvent compositions w from 0 to 0.7 weight fraction of DEC for the former solution and from 0.3 to 2.7 of m and 0 to 0.6 of w of EC for the latter. Viscosity eta of these solutions was also studied by measuring their glass transition temperatures T(g) in the same ranges of m and w. The T(g) was found to rise with m, and fall with w of DEC but rise with w of EC, indicating a concurrent change in the eta of the solutions. The K of the PC-DEC solution of LiBF(4) peaked in both m and w thus forming a dome in its 3D presentation in the mw coordinates, while the K of the PC-EC solution peaked only in m, resulting in an arch shaped surface. As theta lowered, these K surfaces fell in height and shifted in the direction of low eta. These observations correlated well with the changes of dielectric constant e and viscosity q of the solutions with the same set of variables. A more detailed study of the effects of DEC and EC on the kappa of PC solution of LiFB(4) demonstrated that ion association in these solutions was very weakly dependent ontheta, in agreement with an earlier finding. The kappa-T data of the PC-DEC solution was fitted with a Vogel-Fulcher-Tammann equation for an evaluation of its vanishing mobility temperature T(0) and apparent activation energy, E(a), both were shown to form simple surfaces in the mw coordinates slanting up in the direction of high eta. Furthermore, when compared to the T(g) surface of the same solution, the To surface was oriented similarly but lower in value by a few dozen degrees.