Metal Chalcogenide Nanostructures for Energy Conversion and Storage Systems

Abstract

In this study, we explore metal chalcogenides (MCs) as a material of energy conversion and storage (ECS). The metal chalcogenides prepared by the liquid phase synthesis method were obtained in the form of powder or thin film with high crystallinity. These MCs were used as an electrode of the battery and an electrode of the water splitting reaction. Li-S and Li-Se batteries show capacity several times higher than the currently used Li-ion battery. MoS2 and MoSe2 were used as the electrode materials and showed high efficiency and cycling stability. Ethylene glycol was used to form a new carbon layer between the layered structures of MoS2 or MoSe2 to fabricate alternating layers of metal chalcogenide and carbon. Each material is utilized as a Li-S and Li-Se battery electrode and presents a new approach different from previous studies. We have demonstrated this through electrochemical and structural analysis. This structure can reduce the dissolution of intermediates into electrolytes, which have a significant impact on the performance of sulfur and selenium batteries. In the first chapter, we introduced the metal chalcogenides. We also discussed the application of metal chalcogenide in energy conversion and storage. We focused about the points of attention and limitations of current researches and discussed the issues to be improved. In the next chapter, MoS2 and carbon are synthesized alternately and used as electrode of lithium sulfur battery. MoS2 is decomposed in the charge and discharge process to form polysulfide. The phenomenon of polysulfide dissolving in the electrolyte is called shuttle effect (or shuttling), which reduces battery life and degrades performance. The carbon layer intercalated between the MoS2 layers gives a spatial restriction, which reduces the phenomenon that the polysulfide dissolves into the electrolyte. Ethylene glycol used as solvent is intercalated between the MoS2 layers during the synthesis process and forms a carbon layer by heat treatment. The change of structure according to annealing temperature was analyzed by SEM, XRD, TEM and so on. Electrochemical analysis confirmed the effect of this heat treatment. Compared to the case without heat treatment, it showed higher capacity and cycling stability. These heat treatments were performed at 500 and 800℃, and the higher the temperature, the better the performance of the battery. Furthermore, we focused on the characteristics of selenium, which has high capacity per unit volume and high electrical conductivity. MoSe2 was synthesized by solvothermal synthesis method and carbon was formed between these layers. When the polyselenide is dissolved in the electrolyte, the active material is reduced and self-discharge occurs. This reaction reduces the efficiency of the cell. The intercalated carbon layer acts like a cage to prevent polyselenide dissolution in the electrolyte. These Li-Se cells exhibit high capacity in the initial charging and discharging process. As the charging and discharging progresses, the capacity of the battery increases, which is a very special tendency. In addition, the C-rate test for confirming the fast charging and discharging performance shows a very high capacity and even at a short charging and discharging time of about 10 minutes, the performance is higher than the Li-ion battery. Lastly, the study of water splitting electrodes of In2O3 was carried out. In2O3 is a very stable material under water splitting reaction conditions. We set the appropriate energy level for effective water splitting reaction and increased the performance. The surface of In2O3 thin film was substituted with sulfur to form In2S3. UV-Vis spectroscopy confirmed that the amount of In2S3 increased with the reaction time. Through this bilayer structure, we confirmed that the reaction becomes more effective by adding a new energy level between the energy level of In2O3 and the water splitting reaction. Furthermore, CdS was sensitized to the In2O3 thin film and the electrolyte was changed to polyelectrolyte to perform hydrogen generation reaction. The product to change the path of the reaction by a change in the electrolyte can be changed from oxygen to hydrogen. CdS quantum dots can absorb more light in the visible region that is not absorbed by In2O3. This makes it possible to perform photoelectrochemical reactions more effectively.