Conductivity and Viscosity of PC-DEC and PC-EC Solutions of LiBOB

Conductivity K and viscosity η of propylene carbonate (PC)-diethyl carbonate (DEC) and PC-ethylene carbonate (EC) solutions of LiPF 6 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 θ from 60 to -80°C, 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 η of the solutions. The K of the PC-DEC solution of LiFP 6 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 θ lowered, these K surfaces fell in height and shifted in the direction of low η. These observations correlated well with the changes of dielectric constant e and viscosity η of the solutions with the same set of variables. More detailed study of the effects of DEC and EC on the K of PC solution of LiPF 6 demonstrated the benefit of having a low-η solvent component in an electrolyte for it to have a decent K at low θ and the independence on θ of ion association in these solutions in support of an earlier finding. The K-T data of the PC-DEC solution of LiPF 6 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 η. and the latter another one slanting up in the same direction and lying beneath the surface of T g by more than ten degrees.