Study Of Carbon-based Anode Materials For Lithium/Sodium Ion Batteries

Currently lithium ion batteries (LIBs) represent a key class of battery architecture for electric vehicles due to their high energy density, long cycle lifetime and pollution-free characteristics. However, as the large scale applications of LIBs become widespread, the cost of lithium and limitation of lithium terrestrial reserves have been increasingly concerned. Because of the wide abundance of sodium resources, sodium ion batteries (SIBs) would be a prior choice for large-scale applications such as grid energy storage, in which cost rather than energy density is critical. Since the energy density and cycle lifetime of batteries are mainly determined by their electrode materials, the development of high performance electrode materials have attracted intensive research interests. Carbon materials are considered promising anode materials. And the structure and morphology of carbon materials are the major factors that affect their electrochemical performance.In this thesis, carbon materials with different structures and morphologies were prepared and their electrochemical performance as anode materials in LIBs and SIBs were investigated. Through controling the preparation processes, carbon materials with desired structure and morphology were fabricated and the relationship between their performance and structure was studied. By optimizing the preparation processes, high performance carbon anode materials would be obtained. The main contents of this thesis are summarized as followed:1. Carbon microspheres (CSs) were prepared by a microwave-assisted method. The effect of heat treatment temperature (HTT) in inert gas on the stucture and electrochemical performance of CSs was studied. The results reveal that CSs heat treated at low temperature have a large (002) interplanar spacing, while heat treatment at higher temperature results in an increase in their surface area. CSs with a large interplanar spacing and H-containing enjoys a high reversible capacity in LIBs. With a good cycling stability, CSs treated at 700℃ show the best performance. In SIBs, CSs treated at 500℃ achieve a maximum capacity due to their large (002) interplanar spacing. Their reversible capacity is 202 mA h g-1 in the first cycle and 183 mA h g-1 after 50 cycles.2. Nitrogen doped carbon microspheres (NCSs) were fabricated through the heat treatment of CSs in an ammonia atmosphere. The relationship between the HTT and nitrogen doping, porosity and electrochemical performance of NCSs were studied. It is found that the nitrogen content in NCSs does not change obviously at different HTT, but the nitrogen components of different bonding types are highly related to the HTT. As the HTT increases, both the percentage of graphitic N and specific surface area increase remarkably. NCSs treated at 900℃ exhibit an optimized electrochemical performance in LIBs due to the enhanced surface reactivity resulted from an increase in both the percentage of graphitic N and surface area. And a capacity of 660 mA h g"1 can be obtained after 50 cycles. Under certain conditions, nitrogen doping can improve the sodium storage capacity of carbon materials, but the mechanism remains unclear.3. Carbon nanofibers (CFs) were prepared via an electrospinning technique. The influence of HTT on the electrochemical performance of CFs was studied. The results indicate that CFs treated at 600℃ show a turbostratic structure, which favors the storage of lithium/sodium, and thus exhibits optimized electrochemical performance. CFs treated at 600℃ deliver reversible capacities of 754 and 233 mA h g-1 during the first cycle and 476 and 217 mA h g-1 after 50 cycles in LIBs and SIBs, respectively, showing excellent electrochemical performance.4. Graphene-carbon nanotubes (CNTs) composites were synthesized by a microwave-assisted method. The influence of composite components on its structure, morphology and electrochemical performance was studied. The results demonstrate that the composite exhibits an improved performance due to the synergistic effect of graphene and CNTs. When graphene oxide and CNTs in the precursor solution have a weight ratio of 8:2, the composite displays an optimized performance due to the enhanced electron transfer and ion transportation. When used in LIBs and SIBs, the composite shows a good cycling performance and achieves a maximum initial reversible capacity of 607 and 379 mAh g-1, respectively.