Fabrication Of Si/C Nanocomposites With Enhanced Lithium Storage Capability

Lithium-ion batteries (LIBs) are considered as the most promising secondary battery because of their high energy density, high working voltage, long cycle life and environmental friendliness. LIBs have been widely used in portable electronic devices including mobile communications, laptop computers etc. They are also important power sources for future electric vehicles and promising energy storage. Exploring LIBs with high-energy capacity, long cycle life and high safety is one of the most important research topics in energy material and device fields.The commercial graphite anode is limited with respect to its safety and capacity (372mAh/g). Development of high capacity anodes is therefore urgent. Silicon is considered to be the most potential of the large capacity of lithium ion battery anode materials, because silicon alloys with lithium up to Li15Si4(3579mAh/g) has been reached at room temperature. However, the main disadvantage that restricts the electrode application of silicon materials is their large volume changes (～300%) during Li+insertion/extraction. This inevitable shortcoming results in pulverization of the electrode, peeling off the current connection network and rapid capacity decline upon cycling. In this thesis, we designed and fabricated a series of nanostructured Si/C composites to release the volume change, and improve the cycle performance and rate capability of the Si anodes. The structure, morphology, electrochemical performance as well as the cost-efficiency fabrication methods of the materials have been systemically investigated. The main contents and results are listed as follows:1. Fullerene-like carbon nanoparticles were prepared by depositing soot of burning castor oil. The influence of different collected distances on graphitization of carbon nanoparticles and its electrochemical performance were also discussed. It was found that carbon nanoparticle with collected distance of2cm exhibits the best electrochemical performance, its first discharge and charge capacities are1148and611mAh/g, respectively. The discharge capacity remains at471mAh/g after100cycles which is much higher than that in graphite. Carbon nanofibers (CNFs) were fabricated by electrospinning the mixed solution containing fullerene-like carbon nanoparticles/Polyvinylpyrrolidone (PVP) and thermal treatment method. The results showed that CNFs is composed of carbon nanoparticles and diameter is about200nm. It can be direct used as electrode just pressed on the current collector. The electrochemical tests revealed that CNFs enhanced first Coulombic efficiency compared with the carbon nanoparticles and exhibited good cycle performance.2. C/Si core-shell nanofibers with network structures have been fabricated by combining electrospinning and plasma enhanced chemical vapor deposition (PECVD) techniques. The resultant nanofibers were directly used as anodes for lithium ion batteries without adding any binders or conducting additives. High reversible specific capacity of1164mAh/g was retained after50discharge/charge cycles at a constant current density of100mA/g with70%capacity retention. The electrode exhibits good rate performance and markedly improved cycling performance compared to pristine Si film, which can be attributed to its special structure, and the carbon core provides facile strain relaxation to accommodate the large Si volume expansion and shrinkage during Li ion insertion and extraction.3. In order to further enhance electrochemical properties of C/Si core-shell nanofibers, a thin metal Ti layer deposited on the surface of C/Si nanofibers by electron beam evaporation method. The scanning electron microscopy, transmission electron microscopy and micro-Raman spectroscopy results demonstrated that the morphologies and structures of C/Si nanofibers did not change after Ti layer deposited and it is around5nm thick. Fewer nanometer Ti coating could significantly improve the capacity, cycling stability and rate capability of C/Si core-shell electrode under the higher current density. In the first cycle, the discharge capacity of C/Si/Ti multi-layer nanofibers electrode is2499mAh/g at a current density of400mA/g, corresponding to a first Coulombic efficiency of80.9%. High reversible specific capacity of1542mAh/g was retained after50discharge/charge cycles with61.7%capacity retention. Moreover, C/Si/Ti multi-layer nanofibers electrode exhibited excellent rate capability and maintain a capacity of1000mAh/g even at3.2A/g. The Ti surface coating can enhance conducticity and released the strain of electrode during the repeated lithiation/delithiation processes due to high mechanical strength of Ti.4. The hybrid Si/C nanofibers were synthesized from a stable suspension of Si and fullerene-like carbon nanoparticles solution via a single-nozzle electrospinning and subsequent anneal process. Si and C nanoparticles are connected together to form porous fiber structures and the diameter of nanofiber is about500nm. The TEM images of the samples confirmed that Si nanoparticle surrounded by several carbon nanoparticles that could ease the strain caused by volume change and the tangled fiber structure can prevent electrode crack and pulverization, impoved the electrical conductivity of electrode. The electrochemical tests exhibited that hybrid Si/C nanofibers have higher first Coulombic efficiency and cycle performance much better than pure Si nanoparticles.