| Silicon-based materials are used as lithium ion battery anode materials have high capacity,large reserves,and good compatibility with Li+.However,the practical application of silicon-based materials is hindered by two critical problems:Si-based anode materials exhibit large volume expansion(?300%)and relatively low electric conductivity during lithium insertion and extraction.At present,the research focused on the use of nanotechnology,composite technology and the modification of material structure to solve problems.In this paper,the silicon/carbon composites of lithium ion batteries with special structure,excellent cyclic stability,rate performance and electrochemical performance were prepared by combining nano-silicon materials and carbon materials uses different methods.The performances for Si-based materials and graphene coated Si-based composite were calculated by first-principle.1.In this work,we prepared a mesoporous structure silicon/carbon composite(m-Si/C)for lithium-ion battery using mesoporous silica(SB'A-15)and dopamine as the raw materials.The results show that carbon content,carbonization temperature,conditional control of magnesiothermic reduction process and removal of MgO and unreduced SiO2 have effects on the properties of the final material.The SEM and TEM images show that m-Si/C composites have good mesoporous structure.Regular mesoporous carbon framework can be obtained by combined carbon with mesoporous structure when carbon-coated doping is carried out.When the current density was 0.1 A·g-1,the initial discharge specific capacity of m-Si/C electrode was 985.5 mAh·g-1,and the discharge specific capacity was maintained at 617.5 mAh·g-1 after 100 cycles,and the capacity retention rate was 62.7%,both were higher than that the SBA-15/C electrode.The carbon framework can stabilize the material structure and improve the cycle stability of the material.The oxygen deficiency generated by the magnesiothermic reduction is beneficial to the diffusion of the lithium ion and the electrons in the electrolyte,and the electric conductivity is improved.2.Mesoporous silicon@carbon nanocomposites(Si@C)with yolk-shell structure were prepared by solution deposition-organic carbon decomposition method and water-soluble template method.It can be seen from the SEM,EDS and TEM images that the carbon layer is uniformly coated on the silicon core.When the current density was 0.1 A·g-1,the first discharge specific capacity of the Si@C electrode was as high as 1978.3 mAh.g-1,the discharge specific capacity after the 100th cycle was 1214.5 mAh·g-1,and the capacity retention rate was 61.4%.At current densities of 0.1,0.2,0.5,1.0,2.0 A·g-1,the average discharge specific capacities of Si@C electrode were about 1559.4,1243.3,1076.1,934.3 and 795.8 mAh·g-1,respectively.The Si@C electrode material with yolk-shell structure has excellent cycle stability and rate performance.3.The geometry structures of Si materials and graphene-coated Si/graphene composites were optimized by using the PW91 method in the generalized gradient approximation(GGA)based on density functional theory(DFT),and the band structure and density of states of the.Si/graphene materials were analyzed.It is shown that the energy gap between conduction band and valence band can be reduced by graphene coating,the energy of electron transition from conduction band to valence band can be reduced,and the movement trend of the whole energy band moves towards the low energy direction,and electrons were more prone to transition.Thus the silicon material which belongs to the semiconductor property is coated with graphene to form the silicon/graphene composite with metal conductive property and the conductivity is improved.The results of first-principle calculation were consistent with practical experience,which shows that the computer simulation can predict the accuracy of the experiment and verify the empirical conclusion,which indicates the direction for the design of the experimental scheme. |
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