Study On Preparation Of N-doped Carbon Materials And Their Applications In Li-ion And Na-ion Batteries

Over the past few decades,the world's major energy source is fossil fuels.Fossil fuels are limited and non-renewable.Fossil fuels can easily cause a series of social problems to the environment.In efforts to solve these problems,a lot of researches have focus on the clean and sustainable energy systems,including solar cells,fuel cells,and rechargeable batteries.Lithium ion batteries and sodium ion batteries have been considered as an alternative for next-generation battery systems.One possible approach to this end is to find electrode materials with excellent electrochemical performance for lithium ion batteries and sodium ion batteries.Although a wide array of materials are available for battery systems,it remains a major challenge to develop advanced electrode materials with low-cost,excellent electrical conductivity,large surface area and stable mechanical properties.Recently,graphene and carbon nanotubes have been studied as an available materials for energy storage systems,due to their low-cost and high capacity during charge and discharge processes.Graphene and their derivatives materials?the most widely used electrode materials for lithium ion batteries?have been widely used in many fields.This thesis focus on the nitrogen-doped graphene,which can improve the capacity and cycling performance for lithium ion batteries and sodium ion batteries.We find many methods to synthesis the nitrogen-doped graphene.?1?A new approach using polypyrrole as the nitrogen source has been demonstrated for the fabrication of nitrogen-doped graphene,which subsequently served as nucleation centers for the growth of metal oxides.The thin layers of the nitrogen-doped graphene are not only used as conductive pathways accelerating the electrical conductivity of metal oxides but also serve as buffer layers to improve the electrical contact with metal oxide nanostructures during the delithiation/lithiation of lithium ions.As anodes for lithium ion batteries,the nitrogen-doped graphene and their hybrids with MnO₂ nanorods exhibit exceptionally excellent capacity retention for 3000 cycles at 2500mA g^-1,and ultrafast rate capability,which pave the way for developing electrode materials for long cycle-life energy storage devices.?2?A novel concept for synthesizing nitrogen-doped carbon sheets has been revealed by using okara as the source of both carbon and nitrogen,yielding a higher content of nitrogen?about 9.89 at%?than the previous methods.Okara was treated with a carbonization process followed by exfoliation.The exfoliation process could expand the space of carbon sheets and endow the production higher surface area,which benefimants the intercalation and deintercalation of sodium ions during the cycle process.As compared to bare carbon materials,both ion and electron diffusion of the N-doped carbon-based materials are enhanced,resulting in high electronic conductivity.Performed as the anodes of sodium ions batteries,the prepared nitrogen-doped graphene exhibits exceptionally stable capacity retention and high capacities.It is believed that the nitrogen-doped graphene is so far a promising candidate for the low-cost,large-scale commercial.