Morphology and Ionic Conductivity of Poly(dithiooxamide)-Containing Polymer Electrolytes and Their Uses in Lithium Polymer Batteries

Abstract

The relationship of morphology and ion conducting property of poly(dithiooxamide)-containing polymer electrolytes was investigated. A set of polymer electrolytes were prepared through specific coulombic interaction of terminal groups of poly(dithiooxamide) (PDTOA) and poly(ethylene oxide) (PEO). PEO is most widely used polymer which can conduct Li+ ions, and PDTOA is a type of polyimide with thermal stability, good chemical resistance, and excellent mechanical properties. PEOS/PDTOA polymer electrolytes were prepared through specific coulombic interaction of each polymer end groups. Accordingly, it was expected that the PEOS/PDTOA blends could have enhanced mechanical stabilities while preserving Li+ ion conducting. PDTOA have terminal amine groups, so the terminal hydroxyl groups of PEO were substituted to sulfonic acid groups to induce coulombic interactions. These terminal functional groups enable specific interactions between PDTOA and PEOS, yielding coulombic interaction driven polymer electrolytes. Atomic force microscopy (AFM) and dynamic light scattering (DLS) results show that PEOS/PDTOA form 100 nm sized particles. Moreover, phase separated lamellar morphology was confirmed by small angle x-ray scattering (SAXS) and transmission electron microscopy (TEM) experiments. It was found that the PEOS/PDTOA blend provided a good mechanical integrity even at elevated temperature above 100 oC while the phase-separated PEO domains allowed efficient Li+ ion transport properties. The ion-dipole interactions between Li+ ions and polymer blends were demonstrated by Fourier-transform infrared (FT-IR) spectroscopy to elucidate the effects of morphology on ionic conductivity. These results provide the information of Li+ ion preferences for each domain in PEO/PDTOA or PEOS/PDTOA blends. FT-IR results indicate Li+ ions prefer PEO domains than PDTOA domain in PEOS/PDTOA, while Li+ ions were entirely distributed in PEO/PDTOA. Therefore, it is clear that Li+ ions confined within the phase separated PEO domains of 100 nm sized particles with internal lamellar morphology of PEOS/PDTOA. The absolute conductivity values of PEOS/PDTOA electrolyte were about 1.5 times higher than that of disordered PEO/PDTOA blend. In particular, the normalized conductivity of PEOS/PDTOA electrolyte was 2-fold higher than that of disordered PEO/PDTOA electrolyte. This improvement of ionic conductivity is closely related to the creation of ion conducting pathways of lamellar morphology. I conclude this thesis by presenting the effect of morphology on ionic conductivity of microphase separated PEOS/PDTOA electrolyte.