Carbon-based Materials As Anodes For High Performance Lithium Ion Batteries

Rechargeable lithium-ion batteries（LIBs） have attracted great attention in recent years due to their advantages of high energy density, high operating voltage and long cycle life, which have been applied to portable electronics, electric vehicles, implantable medical devices and others. Carbon material has been used as anode material for the commercial LIBs, however, its lower capacity and poor rate capability limit the growing demands. And compared with other electrode materials, carbon nanomaterial have garnered significant attention due to the outstanding advantages, such as excellent electrical conductivity, large surface area, good flexibility, high mechanical strength, abundant source, simple preparation and low price. Therefore, the study to improve the performance of carbon-based materials of LIBs, such as designing unique nanostructures and fabricating composites, has always been the research focus.In this thesis, we design and frabricate carbon nanostructures and composites to improve the electrochemical performance（such as specific capacity, cycling stability and rate performance） of LIBs. The composites besed on carbon nanofibers and graphene are prepared by various modifying or compositing methods. And the obained materials have been systemically studied from the aspects of the morphology, structure and electrochemical behavior as anodes of LIBs. Moreover, the influence of the materials structure and morphology on their electrochemical performance is deeply investigated. The main research contents and conclusions are listed as follows:1. Coral-like nitrogen-doped carbon nanofibers（N-doped CNFs） and undoped CNFs were directly grown onto stainless steel by a thermal CVD with N-rich imidazole and acetylene as precursor gasses. The morphology, structure, and electrochemical property of CNFs as anode material effected by N heteroatom were studied using various characterization techniques. Compared with the undoped CNFs,the representative anode exhibits a high reversible capacity of 862 mAh g^-1 at a current density of 200 mA g^-1 after 150 cycles, and 400 mAh g^-1 at a higher current density of 1000 mA g^-1 after 100 cycles. Such a superior electrochemical performance can be attributed to the unique coral-like nanostructure of CNFs as a result of nitrogen doping, and the N heteroatom makes the CNFs have more detects which would increase many electrochemical activation points benefiting for the storage of Li+.2. Carbon nanofibers（CNFs） were deposited on Cu foam by a floating catalyst method, and Mn3O4 layer was then coated onto the deposited CNFs via a hydrothermal process based on the redox reaction of carbon and potassium permanganate. The obtained architecture of Mn3O4 coated CNFs（CNFs@Mn3O4） on Cu foam was directly used as an anode of lithium ion battery without any binder or conducting additive. The anodes show high reversible capacity, good cycle stability and a superior rate capability. A reversible capacity up to 1178 mAh g^-1 was obtained after 100 cycles at a current density of 100 mA g^-1. When the current density increased up to 5000 mA g^-1, the electrode can deliver a capacity more than 300 mAh g^-1. The excellent electrochemical performance could be attributed to the unique core–shell structure of the CNFs@Mn3O4 composites, which can buffer the volume change, decrease the contact resistance, supply uniform charge distribution and stable structural support for Mn3O4, greatly shorten the ionic diffusion path and make the electron transport more efficient.3. Composites of graphene-wrapped Si nanoparticles（NPs） have been assembled by simple cold quenching in liquid nitrogen, freeze-drying and subsequent thermal reduction. The morphology, structure and electrochemical performance of composites with different graphene mass ratio would be systematically investigated. The composite with an optimized mass ratio of reduced graphene oxide to Si NPs（RGO/Si=1:1） exhibits significantly improved lithium storage performance with an excellent rate capability and a high reversible capacity of 1482 mAh g^-1 at a current density of 210 mA g^-1 after 300 cycles. The high specific charge capacities can be mainly ascribed to the facts that graphene sheets can enhance the electrical conductivity of the composite electrodes, relieve the pulverization of the Si NPs during cycling, and prevent the electrolyte from directly forming the SEI layer on the suface of Si NPs.4. A flexible, honeycomb-like film of reduced graphene（RGO）/Si nanoparticles（NPs） composite is fabricated by a facile vacuum filter assembly process and vacuum freeze-drying method. The Si NPs are enwrapped into the RGO sheets with an areal density of 0.65 mg cm-2. The ultra-thick composite film is directly employed as an anode for LIBs without using any binder and conductive additive. The flexible RGO sheet coat enables the electrode to maintain the structural integrity and provide continuous conductive paths for Si NPs. The electrode exhibits a high capacity of 2370 mA h g^-1 over 50 cycles at a current of 210 mA g^-1 and a capacity higher than 1000 mA h g^-1 at a current density of 4200 mA g^-1 after 500 cycles. Especially, the electrode exhibits the high reversible specific capacity, excellent long-time cyclic stability and rate capability, which are closely related to the porous structure, high specific surface area, superior conductivity and flexibility of graphene.