Structure Design And Electrochemical Performances Of Silicon/Carbon Nanocomposites For Lithium-ion Batteries

As a new type of chemical power source,lithium ion batteries are widely used because of high energy density,long cycle life and environmental friendliness.At present,graphite as the anode material for lithium ion batteries has a low theoretical capacity and can not meet the needs of high capacity in the future.Therefore,tin-based materials,carbon-based materials and silicon-based materials have become important research objects.Among these materials,silicon-based materials have the advantages of its safety,abundant reserves,relatively low discharge voltage?<0.5 V?and the theoretical specific capacity of 4200 mAh·g^-1,which is greater than 10 times that of graphite negative electrodes.It may replace the current commercial graphite anode material.However,silicon as a semiconducting material,lacks sufficient conductivity and happens large volume expansion?>330%?during lithiation and delithiation process,resulting in collapsing structure,losing electrical contact and forming unstable solid electrolyte interface films that leads to poor cycle performance,which limit the application of silicon in practice.Therefore,it is very important to prepare silicon-based negative electrode material with excellent electrochemical performance.This paper intends to improve the conductivity by constructing three-dimensionally connected silicon-carbon composites,inhibiting particles from agglomerating and reserveing enough buffer layers to relieve lithium intercalation,in order to increase the cycle stability and rate performance of the material.?1?Silica template layer is uniformly coated on the surface of irregularly shaped industrial large-size silicon particles by using sol-gel method,and at the same time using 3-ammonia propyl triethoxy silane?APTES?provide surface amino functional and hydrolyze to carbon chain priority to TEOS.The silica layer contain a carbon chain by adsorbing small particles of glucose.Then,it is important to form a carbon shell to obtain Si@void@C structure by calcination and selective etching by HF acid.Due to the dual role of APTES,lower density and cross-linked internal silica matrix can be used to distribute the carbonized amorphous carbon in the shell cavity to form an internally connected carbon network structure,acting as transmission channel between the silicon core and the carbon shell to short the electron channel and reduce the conductive resistance.Controllable silicon dioxide etching layer thickness?-100nm?fully buffers the swelling effect of the silicon volume of 330%.After multiple cycles of lithium removal,industrial large-size silicon is pulverized into small particles due to the volume expansion effect.Amorphous carbon in the shell cavity is dispersed on the surface of the small-particle silicon,and the negative effect of small particle agglomeration can be prevented.Therefore,the Si@void@C structure that internally connects the carbon network exhibits a very high initial coulomb efficiency?62%?as a negative electrode material for lithium ion batteries.In addition,the composites also exhibited good cycle and rate performance,with a capacity of 950.7mAh·g^?1 for 100 cycles at 0.1 C,and a capacity of 550 mAh·g^?1 for 100 cycles at 1 C.?2?The tetraethyl silicate was used to hydrolyze nano SiO₂ to form a suspension on the surface of the graphene oxide nanosheet,and then the positively charged polystyrene sphere was used as a template to attract precursor that form a hollow interconnected sandwich graphene spheres framework.In-situ aluminothermic reduction gives sandwich graphene spheres structure to encapsulate with small-size silicon.Microstructure characterization shows that the silicon particles with a diameter of about 20-30 nm that are tightly and uniformly load in the the hollow graphene spheres shell with the thickness of about 50 nm.Such sandwich structure of nano-silicon particles and graphene can well inhibit the agglomeration of silicon particles and also strengthen the electron channels.Small particles of silicon can shorten the channel and resistance of Li⁺ions,and the internal cavity and flexible graphene skeleton can reserve buffer space when insertion of lithium.The resulting composite exhibits a higher reversible specific capacity,which still maintains a high specific capacity of 1085.6 mAh·g^-1 after 500 cycles at a lower current density of 100mAh·g^-1?depth charge and discharge?.?3?The SiO₂ layer is preliminarily hydrolyzed and deposited on the surface of polystyrene sphere,and thermally decomposed and reduced by magnesium to obtain hollow silicon.A thin carbon layer is coated on the surface of silicon sphere by chemical vapor deposition,and then is compounded with amino-functionalized graphene oxide to prepare a hollow silicon/graphene composites.Due to a thin layer of carbon shell,the hollow silicon sphere particles can significantly enhance the stability of the spherical structure during the cycling process and reduce the electron transport resistance.The flexible graphene nanosheet and the semi-wrapped layer of silicon spheres are compounded into a layered compound,which the graphene maintains the entire composite structure firmly,improves the overall conductivity of the composite material,and inhibits the agglomeration effect for silicon sphere particles.In addition,the hollow silicon spheres reserve sufficient buffer space for intercalation of lithium during the cycle,and also reduce the effect of volume expansion on the structure to some extent.By optimizing the mass ratio of graphene to silicon spheres?1:1?,the resulting composite material exhibits excellent electrochemical performance and good structural stability,which still has 813.2mAh·g^-1 after 100 cycles at a current density of 0.1 C.The specific capacity is significantly higher than that of the comparative sample in which only the chemical vapor deposition of the coated carbon and the graphene coated directly on the surface of the hollow silicon spheres.