Preparation And Properties Of Silicon-Carbon Composites Materials For High Capacity Lithium-Ion Batteries

Using the nano-silicon as silicon source, in combination of excellent conductivity and volume stability of carbon materials, the compositing materials of Si/C were prepared and different carbon materials were studied and the property of anode materials formed after the silicon was composited.Expanded graphene oxide was prepared by Hummers method, and the graphene needed was pyrolysised and reducted by high temperature, then it was composted with nano-silicon. Characterization of microstructure and micromorphology was conducted to the prepared anode active materials by variety testing methods such as SEM, TEM, XRD, Raman. The cycle performance of lithium ion batteries under different condition of mass ratio between carbon and graphene was studied, and the study result shows that batteries using 65% nano-silicon particles and 35% graphene as anode materials has the best cycle performance:the discharge capacity for the first time is 1219 mAh/g. After 100 times of cycle, the reversible capacity is 850 mAh/g, and capacity retention is 69.7%. Even under large electrical flow cycle of 1000 mA/g, the reversible capacity of the electrode is still above 850 mAh/g. Electrochemical tests shows that the internal resistance of nano-silicon/graphene composites is especially low, electrochemical performance is far better than that of nanostructured Si. And this comes from the synergistic effect silicon and graphene. The graphene layers can separate silicon particles and prevent their composition failure. In addition, flexible graphene layers can effectively absorb stress and reduce the volume effect of silicon. With their three dimensional conductive network structure, graphene layers can improve the electrical contact of nano silicon particles profoundly to maintain the structure stability of anode materials in the process of charging and discharging.On the condition of nano-silicon being the resuorce, PEG being the carbon precursor and F127 being the template reagent, the core-shell structure of carbon/silicon nanoparticles composite materials is stablished and molded and then prepared by high temperature pyrolysis. Its cycle performance is superior to the pyrolysis cycle carbon material and pure silicon material. Pyrolytic carbon has excellent conductivity and flexibility, and it is generated in situ and coated on the surface of silicon particles in good combination,which could form a conductive network effectively, buffer silicon composite volume change in the process of insertion and extraction of lithium and improve its cycle performance. Under the condition of 100 mA/g current, the reversible capacity of core-shell structure of carbon/silicon nanoparticles composite material can still remain at 1100 mAh/g after 100 times of cycle with the coulombic efficiency high up to be 99.2%. Under the condition of 1200 mA/g current, the reversible capacity of core-shell structure of carbon/silicon nanoparticles composite material can still remain at 343 mAh/g after 100 times of cycle with the coulombic efficiency high up to be 99.4%. It has showed a very excellent cycle stability.