Conductivities and transport properties of gelled electrolytes with and without an ionic liquid for Li and Li-ion batteries

Conductivity and transport properties have been determined for gelled polymer electrolytes of three compositions: a base PVdF-polymer gel with organic carbonate solvents as plasticizers and LiN(SO2C2F5)(2) electrolyte, a second polymer electrolyte with 5 mass % 1-ethyl-3-methylimidazolium bisperfluoroethylsulfonyl imide (EMI-BETI) added to the base polymer electrolyte, and a third PVdF polymer electrolyte using only EMI-BETI as the plasticizer. Conductivities were studied over the temperature range +25 to -40 degrees C, and for all three gels, the temperature dependence of the conductivities was found to follow the VTF equation, which is consistent with the free volume model for ion transport. For the gel containing 5 mass % EMI-BETI, transport numbers were determined from +50 to -20 degrees C and were found to decrease as the temperature decreased. Although there are no theoretical models to treat and interpret the temperature dependence of transport numbers, we found that a modified VTF equation resulted in an excellent fit to the temperature dependence of the transport number, which is another confirmation of a free volume model for transport in these gelled polymer electrolytes.