Investigation of organic materials for lithium batteries

Publication Type:
Thesis
Issue Date:
2017
Metrics:
Full metadata record
Files in This Item:
Filename Description Size
01front.pdf342.4 kB
Adobe PDF
02whole.pdf13.58 MB
Adobe PDF
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.
Please use this identifier to cite or link to this item: