Investigation of organic materials for lithium batteries
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The performances of lithium-based batteries can be significantly influenced by the electrolyte and cathode materials. In this PhD project, functional organic materials were synthesised and applied as electrolyte and cathode components for lithium-ion (Li-ion) and lithium-oxygen (Li-O₂) batteries to improve overall performances. Theoretically, organic materials can be tailored with functional groups to fit various purposes, making them suitable for battery applications. Post-synthesis techniques such as field emission scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy were used to characterise the physical properties. Electrochemical analyses including galvanostatic discharge-charge method, cyclic voltammetry, linear sweep voltammetry, and impedance spectroscopy were conducted to determine the electrochemical performance of the materials. Porous polymer membranes based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) are prepared using the breath-figure method. The as-prepared PVDF-HFP porous membrane showed a highly ordered honeycomb-like structure. The highly porous structure could absorb large quantities of liquid electrolyte, resulting in a high ionic conductivity. Moreover, the non-combustible PVDF-HFP membrane could significantly enhance safety properties of Li-ion batteries compared to the one using a conventional Celgard separator. The combination of the PVDF-HFP membrane with liquid electrolyte resulted in a higher capacity and prolonged cycle life. Further investigation on coating poly(methyl methacrylate) (PMMA) on PVDF-HFP porous membranes to achieve hierarchical structures with sandwich-like morphology was carried and studied. By the combination of porous PVDF-HFP and PMMA with higher affinity towards liquid electrolyte, the ionic conductivity was further improved. As a result, the electrochemical performance of Li-ion battery was significantly enhanced. A bi-functional organic catalyst poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate) (PTMA) was synthesized by a two-step method. The unique properties of PTMA during n-doping and p-doping could facilitate the discharge and charge processes, respectively. It was discovered that the discharge capacity increased and the charge over-potential was reduced. The mechanism investigation showed that it was the functional N-O radical interacting with oxygen and Li₂O₂ to catalyse the battery reactions. As a result, the cycling life of the Li-O₂ battery was significantly prolonged. Further investigation of employing co-polymers as binder in the cathode for Li-O₂ batteries was carried out. The binder was synthesized by co-polymerizing methyl methacrylate (MMA) and 2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate (TMA) monomers. The ratio between both monomers was also studied. The results indicated that the direct contact of co-polymer binder and liquid electrolyte would transfer it into a gel polymer electrolyte membrane which enabled the catalytic N-O radical group to function as restricted redox mediator. The electrochemical performance of Li-O₂ batteries can be further enhanced by employing PTMA. An investigation of the mechanism when tetrathiafulvalene (TTF) was used in Li-O₂ batteries in the presence of LiCl was studied. It is revealed that the addition of LiCl in the electrolyte completely changed the functional mechanism of TTF in Li-O₂ batteries. Instead of functioning as a solution-based redox mediator, the combination of TTF and LiCl resulted in a deposition of an organic conductor on the surface of Li₂O₂, providing additional ways for electron transference. As a result, the cycling efficiency was improved, and with the use of porous graphene as cathode material the cycle life was prolonged.
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