Preferential solvation and free volume as interrelated features governing ion conduction in plasticised polyether electrolytes

This work concerns the relationship between the Tg depression and the conductivity elevation produced by various dipolar diluents in LiN(CF3SO2)2 rubbery electrolytes based on high molecular weight poly(ethylene oxide) (PEO). Some of these diluents are tetraalkylsulfamides, (R1R2NSO2NR3R4), which are known to be stable toward strong reducing agents, but which lead to associated ionic species in the absence of PEO. The other diluents are propylene carbonate and γ-butyrolactone, which are known to yield high conductivities under the same conditions. For a fixed salt content (1.2 mol kg−1), plots of Tg as a function of the diluent content show two distinct regions. One in which the Tg depression does not depend markedly on the physical properties of the diluents, and one in which these properties become dominant. At low diluent contents in the first region (0–20 wt.% with respect to PEO), the conductivity elevation essentially depends on the Tg depression. In agreement with other works, this feature indicates that the lithium ions are preferentially coordinated to PEO. An analysis made in terms of the conductivity at T=Tg+70°C shows sudden changes above this range that suggest an initial regime in which the diluent molecules have a low diffusivity. This feature should make these electrolytes (particularly those plasticised by small amounts of sulfamides) less aggressive toward lithium and insertion electrodes than their gelled, diluent-rich counterparts. A second study performed on poly(propylene oxide) electrolytes shows that the microphase separation phenomenon characteristic of these electrolytes persists upon diluent addition.