Effect of Mg2+ on PEO morphology and conductivity

Morphology, crystallinity and ion conductivity of divalent ionic salt Mg(ClO4)2 in poly (ethylene oxide) (PEO) have been investigated and presented. The X-ray diffraction and differential scanning calorimetry (DSC) studies reveals that the PEO crystallinity is reduced with increasing Mg salt content, which becomes totally amorphous above 25 wt.% of Mg(ClO4)2 (O/Mg ratio ca. 15). Scanning electron microscopy (SEM) micrographs demonstrated improvement of surface morphology from rough to smooth with increasing salt content. The smooth morphology is attributed to the reduction of PEO crystallinity with salt. These results suggested the formation of complex between Mg2+ ionic salt and PEO, similar to that with the lithium salt. The interaction of Mg2+ ion with ether oxygen of PEO facilitated salt dissociation, as well as the disruption of crystallinity of PEO. Unlike that in lithium salt, however, formation of other crystalline complex phases is observed after extensive annealing at 120 °C. The conductivity was optimized at 15 wt.%, (O/Mg ratio=28), which degraded further with increase of salt content where severe ion pairing occurs. The pseudo activation energy derived from variable temperature conductivity measurements is about 0.68 eV, which is larger than that of the mono-valent ionic salt systems and reflecting the heavier Mg2+ ion mass transport in PEO.