Design,Preparation And Lithium Storage Performances Of Silicon/Carbon Composites For Lithium Ion Batteries

In order to meet the requirements of smart robots and electric vehicles for high-performance lithium-ion batteries,it is urgent to develop high-capacity electrode materials.Silicon-based anode materials have attracted much attention due to their high theoretical capacity(4200 m Ah g^-1),abundant reserves and low lithium insertion potential.However,the huge volume expansion and low conductivity of silicon materials limit their large-scale commercial applications.Carbon-based materials have high mechanical stability and electrical conductivity,which can effectively alleviate the volume expansion of silicon and accelerate the transmission of electrons.Therefore,by combining the advantages of silicon nanoparticles and carbon-based materials,three silicon/carbon composites are designed and their lithium storage performance is studied.The main work content is as follows:1.Through simple solvent method and high temperature carbonization method,the silicon nanoparticles are wrapped in carbon nanocages derived from the metal organic framework ZIF-8 to prepare a core-shell structure carbon-coated silicon composites(Si@c-ZIF).Si@c-ZIF has a large number of micropores,and there are extra gaps between the internal silicon nanoparticles and carbon shell.As lithium-ion battery anode,Si@c-ZIF has better cycle performance and rate performance than those of silicon nanoparticles.The specific discharge capacity of Si@c-ZIF is 1985.0 m Ah g^-1 in the first cycle at 0.2 A g^-1.After 25 cycles,the specific discharge capacity of Si@c-ZIF can still reach 1389.5m Ah g^-1,while the specific capacity of silicon nanoparticles is only 84.6 m Ah g^-1.Meanwhile,the reversible specific capacities of Si@c-ZIF are 1436.0,1156.3,843.3,587.7,397.5 and 175.8 m Ah g^-1 at0.1,0.2,0.5,1.0,2.0 and 5.0 A g^-1,respectively.The excellent electrochemical performance is attributed to its core-shell structure which can provide extra space to relieve the volume expansion of silicon,and the porous carbon shell can improve the lithium ion transmission rate.2.The polyacrylonitrile fibers containing Si@ZIF-8 are obtained by the electrospinning method,and they are pre-oxidized and carbonized to synthesize silicon encapsulated into N-doped porous carbon nanofibers(Si@c-ZIF@CNFs).Silicon nanoparticles are dispersed in conductive network of N-doped porous carbon nanofibers,which have high specific surface area.Si@c-ZIF@CNFs carbon fiber film can be directly used as a self-supporting electrode for lithium-ion battery.It still has a specific capacity of518.6 m Ah g^-1 after 1000 cycles at 1 A g^-1.When the current density is adjusted to 5.0 A g^-1,its specific capacity can reach 523.9 m Ah g^-1.Si@c-ZIF@CNFs exhibits excellent cycle performance and rate performance,due to the strong conductive carbon fiber network that can effectively enhance electronic conductivity and withstand the volume expansion of silicon nanoparticles during the lithiation process.Meanwhile,the large number of internal pores in carbon nanofibers not only provide additional space for the expansion of silicon nanoparticles,but also facilitate the transmission of lithium ions.3.The N,P-doped Si/CNTs/C composites(N,P-Si/CNTs/C)are successfully synthesized by combining simple mechanical ball milling and high-temperature carbonization process.Silicon nanoparticles and carbon nanotubes are entangled together and covered by carbon layer derived from pitch.Through the thermal decomposition of Cl₆N₃P₃,the doping of N and P is realized.As lithium-ion battery anode,the N,P-Si/CNTs/C still remains 1474.8 m Ah g^-1 after 30 cycles at a current density of 0.2A g^-1.Meanwhile,its reversible specific capacities are 1636.0,1493.0,1153.6,633.3 and 255.3 m Ah g^-1at 0.1,0.2,0.5,1.0 and 2.0 A g^-1,respectively.The excellent electrochemical performance of N,P-Si/CNTs/C can be attributed to the fact that the molten pitch-derived carbon layer is coated on the surface of Si/CNTs,which can alleviate the volume expansion of silicon nanoparticles.In addition,N and P co-doping can increase the active sites and lithium storage performance of the material.