Investigation of polymers used in lithium oxygen batteries as electrolyte and cathode materials
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It has been well established that the electrolytes and cathodes have a significant effect on the electrochemical performance of lithium oxygen batteries. In this Master project, polymers were employed as electrolyte and cathode materials due to their unique superior properties. Using different methods, we synthesized suitable gel polymer electrolytes and conducting polymer catalysts for lithium oxygen batteries. Techniques such as field emission gun scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy were used to characterize the physical properties. Electrochemical analyses including the galvanostatic discharge and charge method, the cyclic voltammetry, the linear sweep voltammetry and the impedance spectra were conducted to determine the electrochemical performance for the as-prepared materials. Gel polymer electrolytes based on low molecular weight polyethylene glycol were prepared and used as electrolyte in lithium oxygen batteries. The as-prepared polymer electrolytes showed improved stability compared with liquid electrolytes and exhibited good performance in lithium oxygen batteries. Additionally, the addition of ceramic filler SiO₂ was found to reduce the stability of polymer electrolyte towards oxygen reduction reaction although higher ionic conductivity was obtained. Polyethylene glycol based gel polymer electrolyte without SiO₂ addition exhibited excellent cycling performance and it could be used for achieving long-life lithium oxygen batteries. Poly(vinylidene fluoride-co-hexafluoropropylene) based gel polymer electrolytes were prepared by solvent casting and employed as electrolytes in lithium oxygen batteries. The stability of the gelled electrolyte with tetraethylene glycol dimethyl ether has been greatly increased than the liquid one. The as-prepared polymer electrolyte was demonstrated excellent cycling performances. This thesis also investigated the effect of different plasticizers on the performance of lithium oxygen batteries. The reason could lie on the interactions among the components when the gelled structure was set. The tetraethylene glycol dimethyl ether based gel polymer electrolyte showed the best electrochemical performance and can be used for long-life lithium oxygen batteries. Polypyrrole conducting polymers with different dopants have been synthesized and applied as the cathode catalysts in lithium oxygen batteries. Polypyrrole polymers exhibited an effective catalytic activity for oxygen reduction in lithium oxygen batteries. It was discovered that dopant significantly influenced the electrochemical performance of polypyrrole. The polypyrrole doped with Cl⁻ demonstrated higher capactity and more stable cyclability than that doped with ClO₄⁻. Polypyrrole conducting polymers also exhibited higher capacity and better cycling performance than that of carbon catalyst. Conducting polymer coated carbon nanotubes were synthesized and used as catalysts in lithium oxygen batteries. It was found that both polypyrrole and poly(3,4-ethylenedioxythiophene) coated carbon nanotubes could provide high cycling performance while polypyrrole based one exhibited higher capacities. The ratio of conducting polymer coating also affected the electrochemical performance of lithium oxygen batteries. The conducting polymer coated carbon nanotubes also showed better performance than the bare carbon nanotubes.
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