Research On The Carbon Nanofiber As The Anode Material Of Lithium Ion Secondary Batteries

With the development of society, the lithium ion secondary battery is demanded with higher capacity and better stability performance. The effect of anode materials on the performance of lithium ion secondary battery is important, so the research of preparing anode materials with high capacity and good stability performance have attracted a great deal of attention. Carbon nanofiber and composite materials are studied as one kind of anode materials for lithium ion secondary battery. Carbon nanofibers prepared by the electrospinning possess many advantages, such as low density, high specific surface area, good conductivity, less structure defect and so on. The performance of carbon nanofiber makes it promising for a wide range of applications.In this dissertation, electrospinning was used to synthesize the polyvinylpyrrolidone(PVP)polymer nanofibers using 8% weight ratio ethanol solution. the major parameters of electrospinning are as following: the humidity was controlled to less than 30%, the feeding rate for the precursor solution was controlled at 0.8 mL/h, the electric field for electrospinning was approximately 1.9 kV/cm, with the distance between spinneret and collector about 13 cm.Preoxidation temperature, preoxidation time, carbonization temperature and carbonization time are determined by experiments and tests. The experimental results show that carbon nanofibers with good morphology can be fabricated by preoxidizing PVP polymer fibers at 150℃for 24 hours and at 280℃second preoxidation for 2 hours, then carbonizing the pre-oxidized fiber at 800℃for 2 hours. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), fourier translation infrared spectroscopy (FT-IR), scanning electron microscope (SEM), x-ray diffraction (XRD) and Raman spectra were used to characterize the pre-oxidized PVP nanofibers and carbon nanofibers.Button cell were assembled with carbon nanofibers made in different annealing conditionsan as anode materials, metal Li as cathode. The electrochemical behavior of the system was evaluated in ethylene carbonate/dimethyl carbonate solution, using 1 M LiPF6 as supporting electrolyte.The capacity and cycle stability of the button cells were tested. The research results indicate that PVP nanofibers be pre-oxidized at 360℃for 2 hours after pre-oxidizing at 150℃for 24 hours, then be carbonized at 800℃for 2 hours, which show the best charge-discharge property, the first discharge capacity is 1254 mAh/g, and retain 65% of initial capacity at 30th cycle.