Nano-carbon Fibers/Silicon Composites As Anode Materials For Lithium-ion Batteries

Nowadays, lithium-ion batteries (LIBs) are the most important power sources for portable electronic devices and electric vehicles. Graphitic carbon has been applied as anode materials for LIBs due to its low cost and long cycle life. However, its theoretical capacity of Li-storage is only372mAh/g, which can not satisfy the requirements of high energy density LIBs.Si-based materials with higher specific capacity has been proposed as a new anode candidate. But Si-based electrode’s application is hindered because of its large volume change during cycling, which leads to the quick capacity fading and gives rise to cracking and crumbling of the anode materials. A promising approach to overcome this deterrent is to create a composite microstructure comprising the active materials uniformly dispersed in a relative inert matrix. Pyrolyzed Bacterial cellulose (PBC) has a microfibril network structure. A unique kind of three-dimensional porous electrode structures can be fabricated with PBC.In this thesis, bacterial cellulose (BC) were used as precursor in the preparation of PBC. By adding carbon nanotubes (CNTs), silicon (Si) and silicon dioxide (SiO2), PBC/CNTs, PBC/Si and PBC/SiO2composites were obtained. The structures and electrochemical characteristics of composites were characterized by FESEM, TG, XRD and EIS. The main work and conclusions were presented as follows:1. Structures and electrochemical characteristics of PBC and PBC/CNTs.PBC maintained the highly porous network structure of BC. As the anode material of LIB, PBC presented a capacity of464.3mAh/g at the first cycle. After76cycles,60%of the initial capacity was maintained. The results indicated that PBC can be used as electrode active material in LIB. The introduction of CNTs did not damage the main structure of PBC. As the anode material for Li-ion batteries, the composite presented a large capacity of1280.6mAh/g and high coulombic efficiency of46%at the first cycle. After20cycles, it still retained a reversible capacity of433mAh/g, which is higher than the capacity of graphite.2. Structures and electrochemical characteristics of PBC/Si.Si nanoparticles were homogeneously distributed in the BC through refining process. After freeze-drying and heat treatment, PBC/Si was obtained. XRD and Raman spectra provide evidence for the presence of silicon and graphitic layer within the PBC nanofibrils. EIS indicated the PBC/Si can be used as the working electrode. In addition, the PBC/Si composite exhibited superior cycling performance. After65cycles under a current density of100mAh/g, the PBC/Si still retain a reversible capacity of1369mAh/g. That revealed that introduced PBC plays a critical role in suppressing the adverse volume effects of silicon upon cycling.3. Structures and electrochemical characteristics of PBC/SiO2.Through the in-situ hydrolysis of TEOS, BC was used to fabricated BC/SiO2composites and after freeze-drying and heat treatment, PBC/SiO2nanofibers were obtained easily. FESEM suggested that the SiO2particles grew along the BC fibers uniformly. XRD pattern of PBC/SiO2demonstrated the SiO2nanoparticles were amorphous silica. However the electrochemical characteristics of the composite showed that SiO2could not serve as the silicon source for anode materials.