Transition Metal Single Atom Catalysts for High Performance Lithium Sulfur Batteries

Abstract

With advancements in technology and the migration of industries toward utilizing electrical power sources, such as in transportation (e.g., electric vehicles), the importance of sustainable energy storage systems is more critical than ever. Lithium-ion batteries have reached their potential limits in terms of energy density and specific capacity, leading to an increasing demand for alternative battery chemistries. Lithium-sulfur batteries (LSBs) are highly promising alternatives to conventional lithium-ion batteries due to their high theoretical energy density (2600 Wh.kg-1) and high theoretical specific capacity (1675 mAh.g-1). However, despite these theoretical advantages, lithium sulfur batteries face many challenges in real-world applications. Some common issues include the inherent low conductivity of elemental sulfur and lithium sulfide intermediates (which forms during the charge and discharge process), sluggish reactions, and the polysulfide shuttle effect. To address these problems, numerous methods have been employed to modify the sulfur cathode, lithium anode, separator, and electrolyte. Among these methods, utilizing catalysts in the sulfur cathode for chemical and physical adsorption and accelerating the kinetics of the redox reactions during charge and discharge shows high promise in mitigating the challenges of lithium sulfur batteries. In particular, Single atom catalysts and dual-atom catalysts have shown significant improvements in the performance of lithium-sulfur batteries due to their high atomic utilization and unique electronic and geometric structures. Additionally, single and dual-atom catalysts inherit the advantages of homogeneous catalysts in terms of high efficiency and selectivity, due to their easily accessible active sites and maximal active metal utilization, as well as the advantages of heterogeneous catalysts in terms of excellent stability and recyclability.1–3 In this thesis, initially I aimed to synthesize a dual-atom catalyst of Fe and Ni anchored on a hollow cubic nitrogen-doped carbon structure, to utilize this material as the sulfur cathode host in lithium-sulfur batteries. However, due to complexities of the synthesis method and inevitable challenges of synthesizing dual atom catalysts, the final material could not be achieved with the desired properties. Also, the battery performance results from this material were not satisfactory. Therefore, I changed the synthesis method to prepare Fe single-atom catalysts to facilitate single-atom achievement. For achieving better performance and according to the literature, I concluded that using carbon nanotubes (CNT) as the substrate material would be highly beneficial in terms of final performance results. After obtaining satisfactory battery performance results, I successfully prepared two other transition metal (Ni, Co) single-atom catalysts on the CNT and compared their influence on lithium sulfur batteries at high sulfur loading. I utilized characterization methods including XRD, X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) to investigate the morphology, structure, and mechanism of these catalysts. The XPS, XAS, STEM analyses of Fe/CNT and Co/CNT confirmed the presence of single atom structures in these samples. However, for Ni/CNT, the results indicated a mixture of single atoms and nanosized clusters. The electrochemical battery performance tests revealed that the best performance corresponded to Fe/CNT, with an initial high specific capacity of 910 mAh.g-1 at a high sulfur loading of mg.cm-2 and 0.5C rate. Following Fe/CNT, Ni/CNT and Co/CNT showed decreasing performance, respectively. This thesis also highlights the need for further characterization of the cathode materials after cycling to verify the persistence of single atom catalyst structures within the cathode. Such characterization will confirm whether the proposed synthesis method can reliably produce scalable quantities of materials with single atom structures that maintain their integrity during the cycling of lithium-sulfur batteries. Additionally, the synthesis method for preparing dual atom catalysts warrants further investigation. These are crucial steps to advance and complete the project, but they fall well beyond the scope of this thesis. It is worth mentioning that, despite the non-satisfactory performance of the FeNi/NC catalyst (that was prepared in the initial project) in LSBs, it showed promising performance in the hydrogen evolution reaction (HER) process in conjunction with Palladium Hydride (PdHx/FeNiNC), contributing to another parallel project. For the prospect of this project, I see a significant gap in the development of dual-atom catalysts using the second synthesis method that was proposed.