Stretching-induced conductivity enhancement of LiI-(PEO)-polymer electrolyte

The effect of stretching of LiI-(PEO)₂₀ polymer electrolyte film on its ionic conductivity was studied. Neither change of polymer electrolyte resistance nor visible sample dimensions were observed on stretching the film, having an initial cross-section of about 0.3×8 mm² under the loads of 0.3-1.0 kg. At 1.25 kg, the sample began to flow. Flow, accompanied by the reorganization of the polymer electrolyte chains, resulted in about a five-fold increase in ionic conductivity. At the moment of applying extending axial load to a film, a resistance-decrease transient appeared. The relaxation of the polymer electrolyte seemed to be load-independent. According to SEM micrographs, the pristine sample is characterized by clear-grain crystalline structure, whereas the stretched sample shows fibrous structure in the axial (force) direction. DSC thermograms of stretched polymer electrolyte show two endothermic transitions. The first one, at about 60 °C, is sharp and similar to that of the pristine sample; an additional broad endotherm is seen at 130 °C, indicating the formation of the second crystalline phase, which is absent in the pristine sample. These results support the theory that the main ion transport occurs along the helical axis.