Conductivity and viscosity of PC-DEC and PC-EC solutions of LiPF6

Conductivity kappa and viscosity eta of propylene carbonate (PC)-diethyl carbonate (DEC) and PC-ethylene carbonate (EC) solutions of LiPF6 were experimentally determined, the former at salt molalities m from 0.2 to 2.4 mol kg(-1), solvent compositions w from 0 to 0.7 weight fraction of DEC and 0 to 0.6 weight fraction of EC, and temperatures theta from 60 to -80 degreesC, the latter indirectly and qualitatively through measuring the glass transition temperature T-g in the same ranges of m and w. The T-g increased with m, and decreased with w of DEC but increased with w of EC, indicating a concurrent change in the h of the solutions. The kappa of the PC-DEC solution of LiFP6 peaked in both m and w thus forming a dome in its 3D presentation in the mw coordinates, while that of the PC-EC solution peaked only in m resulting in an arch-shaped surface. As theta lowered, these kappa surfaces fell in height and shifted in the direction of low eta. These observations correlated well with the changes of dielectric constant epsilon and viscosity eta of the solutions with the same set of variables. More detailed study of the effects of DEC and EC on the kappa of PC solution of LiPF6 demonstrated the benefit of having a low-eta solvent component in an electrolyte for it to have a decent kappa at low theta and the independence on theta of ion association in these solutions in support of an earlier finding. The kappa-T data of the PC-DEC solution of LiPF6 were fit with the Vogel-Tamman-Fulcher equation for an evaluation of its apparent activation energy E-a and its vanishing mobility temperature T-0, the former shown to be a simple surface in the mw coordinates slanting up in the direction of high eta, and the latter another one slanting up in the same direction and lying beneath the surface of T-g by more than ten degrees. (C) 2003 The Electrochemical Society.