Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Supervisor

Xueliang Sun

Abstract

Lithium-sulfur batteries are considered as the most promising next generation high-energy batteries. Compared with other kinds of battery, Li-S batteries have ultra-high theoretical energy density, which is a good candidate for electric vehicles and hybrid electric vehicles in future. However, there are still many challenges to be addressed in Li-S batteries. Design of electrodes, selection of electrolytes, and battery assemble have direct effects on the safety, cost and electrochemical performance of Li-S batteries. Therefore, it is greatly important to develop novel electrodes to achieve high-energy for Li-S batteries. This thesis mainly focuses on the design of sulfur cathode of Li-S batteries and mainly include six parts.

The first part in the thesis investigated effects of carbon-heteroatom bonds on sulfur cathode. A series of carbon black substrates were prepared using various treatments to introduce nitrogen or oxygen surface species. The results indicated that nitrogen-doped carbon black significantly improved the electrochemical performance of sulfur cathode materials. Synchrotron-based XPS revealed that the defect sites of nitrogen-doped carbon are favorable for the discharge product deposition, leading to a high utilization and reversibility of sulfur cathodes. The studies also found that the introduction of oxygen functional groups results in deteriorated performance of Li–sulfur batteries due to the reduced conductivity and unwanted side reactions occurring between sulfur and surface oxygen species.

To further improve the performance of sulfur cathodes, titanium nitride decorated carbon materials were designed and synthesized as carbon host in the second part of the thesis. TiN nanoparticles were synthesized via atomic layer deposition, which are uniformly distributed on porous carbon with small particle size. Electrochemical results indicated that as-prepared TiN@carbon facilitates the rate performance of sulfur cathodes, which demonstrated an improved cyclic capability and stability of Li-S batteries.

The third part of the thesis is application of metal organic framework derived carbon (MOF-C) with controllable porous structure for sulfur cathodes. The in-situ ammonia treatment successfully prepared MOF-C with different porous structure. Further, NH3 treated MOF-C as carbon host for sulfur loading performing as the cathode for Li–S batteries resulted in twice higher capacity retention than that of pristine MOF-C. Further, different Li–S electrochemical iii mechanisms regarding the different porous structures of carbon were also revealed and investigated in this part.

Apart from development of different carbon materials for sulfur cathodes, coating material is another popular strategy to improve the performance of sulfur cathodes. The fourth part of the thesis introduced atomic layer deposited Al2O3 coating to prevent dissolution of polysulfide for sulfur cathodes. It was demonstrated that the Al2O3 coating significantly improved the cycling stability of Li–sulfur batteries. The underlying mechanism by synchrotron-based Xray photoelectron spectroscopy was investigated.

To further improve coating materials for sulfur cathode, molecular layer deposited alucone coating was developed for sulfur electrode in the fifth part of the thesis. The alucone coated C/S cathode displayed over two-times higher discharge capacity than the pristine one, demonstrating a greatly prolonged cycle life. Morphology of discharge product after cycling was also investigated in this part to understand the mechanism of alucone coating. Safety is a crucial concern for Li-S batteries.

The sixth part of the thesis was to develop safe and durable high temperature Li-S batteries. The commonly employed ether-based electrolyte does not enable to realize safe Li–S batteries operated at high-temperature due to the low boiling and flash temperatures of ether-based electrolyte. Traditional carbonate-based electrolyte can obtain safe physical properties at high temperature for Li ion batteries but cannot complete reversible electrochemical reaction for most Li-S batteries. By using molecular layer deposition (MLD), sulfur cathodes with alucone coating can complete the reversible electrochemical process in carbonate electrolyte and exhibit a safe and ultra-stable cycle life at high temperature. Synchrotron-based X-ray analysis was carried out to understand the mechanism of alucone coating in Li-S batteries.

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