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

The effect of stretching of LiI-(PEO)(20) polymer electrolyte film on its i onic conductivity was studied. Neither change of polymer electrolyte resist ance nor visible sample dimensions were observed on stretching the film, ha ving an initial cross-section of about 0.3 x 8 mm(2) under the loads of 0.3 -1.0 kg. At 1.25 kg, the sample began to flow. Flow, accompanied by the reo rganization of the polymer electrolyte chains, resulted in about a five-fol d increase in ionic conductivity. At the moment of applying extending axial load to a film, a resistance-decrease transient appeared. The relaxation o f the polymer electrolyte seemed to be load-independent. According to SEM m icrographs, the pristine sample is characterized by clear-grain crystalline structure, whereas the stretched sample shows fibrous structure in the axi al (force) direction. DSC thermograms of stretched polymer electrolyte show two endothermic transitions. The first one, at about 60 degrees C, is shar p and similar to that of the pristine sample; an additional broad endotherm is seen at 130 degrees C, indicating the formation of the second crystalli ne phase, which is absent in the pristine sample. These results support the theory that the main ion transport occurs along the helical axis. (C) 2000 Elsevier Science Ltd. All rights reserved.