Preparation And Electrochemical Properties Of Yolk-shell Structure Silicon/Carbon Composite Anode Materials

The theoretical capacity of silicon-based anode materials in lithium-ion batteries is extremely high,so it is ideal for new anode materials.However,the capacity attenuation because of the huge volume expansion of silicon hinders the commercial application of silicon anode materials.This problem can be overcome by a reasonable structural design.The yolk-shell structure silicon/carbon composite can reserve space for expand of silicon to improve its electrochemical performance.In this thesis,plentiful yolk-shell structure silicon/carbon composites are prepared using different precursor carbon sources.The specific research contents are as follows:(1)The yolk-shell structure silicon/carbon composite(Si@void@C)and nitrogen-doped silicon/carbon composite(Si@void@NC)are synthesized by template method using the glucose and dopamine as carbon sources,respectively.The cavity structure inside the two materials provides a buffer for the volume expansion of silicon.In contrast,Si@void@NC has more active sites,faster electron transport speed,and a more complete carbon shell due to the introduction of nitrogen,therefore the electrochemical performance is more stable.In addition,the volume expansion of silicon can be matched by adjusting the size of the cavity between the yolk and shell.In this experiment,when the reserved space is 300%of the volume of silicon,Si@void@C and Si@void@NC show the best electrochemical performance.The initial reversible capacity of Si@void@C and Si@void@NC are 2006.4 mAh g-1 and 1570.9 mAh g-1 at the current density of 100 mA g-1,respectively,after 60 cycles,the reversible capacity maintain at 812.5 mAh g-1 and 1235.8 mAh g-1.(2)Actually,the volume expansion of silicon is not isotropic,although the amorphous carbon shell has a certain protective effect on silicon,it cannot withstand the stress caused by the uneven expansion of silicon.In order to solve this problem,the carbon source can be replaced with the "rigid and soft"graphene,so the yolk-shell structure silicon/graphene cage composite(Si@void@GC)is prepared by template method.The graphene cage is formed using a templating agent,which can self-regulate the space according to the expansion of the silicon.At the same time,the pleated graphene sheets form a cross-linked secondary structure and thus has excellent electrochemical performance.The initial reversible capacity is 2537.7 mAh g-1 at 100 mA g-1,after 60 cycles,the reversible capacity is 2017 mAh g-1 and the capacity retention rate is 80%.The reversible capacity is 1469.8 mAh g-1 after 200 cycles at 1 A g’1,the coulombic efficiency is 98.13%.(3)Although reduced graphene oxide imparts elasticity to the carbon shell,many irreversible side reactions are caused by residual functional groups and defects on the surface of reduced graphene oxide,resulting in low initial coulombic efficiency.The yolk-shell structure silicon/graphene-carbon composite(Si@void@C/rGO)having a double carbon system is prepared by hydrothermal carbonization and dehydration condensation of two carbon sources to reduce the amount of graphene and decrease side reactions.The graphene and amorphous carbon in the material not only construct a shell with a cavity,but also form an elastic cross-linked network,which provides support for structural stability and cycle stability.When the input ratio of nano-silicon to graphene oxide is 10:1,the electrochemical performance of the material is optimal.The initial discharge capacity is 2819.6 mAh g-1 at 100 mA g-1 with coulombic efficiency is 81.1%,and the reversible capacity is 2112.4 mAh g-1 after 60 cycles.