Preparation And Electrochemical Performance Of Two-dimensional Silicon Materials

Silicon material has a high theoretical capacity of lithium storage,so it has attracted much attention as the anode material of lithium-ion battery.However,due to the significant volume expansion（～300%）in the process of charging and discharging,the electrochemical properties of silicon materials decay rapidly,which makes it difficult to realize commercial application.In order to solve the above problems,different oxidants were used in this paper to prepare silicon nanosheets by chemical stripping of CaSi₂.The effect of exfoliated time on the morphology and structure of silicon nanosheets was studied.In order to further improve the electrochemical performance of silicon-based materials,the sheets of MoS₂ and carbon layers were hydrothermal grown on the surface of siloxene to form silicon-based composites.Specific research results are as follows:（1）Silicon nanosheets were prepared by chemical exfoliate of CaSi₂ with I₂,bromine water and hydrochloric acid.When I₂ is used as an oxidant,it is difficult to completely peel off CaSi₂after 30 days of reaction due to its weak oxidation.By introducing ultrasonic treatment in the exfoliate process,the formation of monolayer silicene can be promoted,but the reaction of CaSi₂ and I₂ can not be accelerated.When bromine water is used as an oxidant,not only can CaSi₂ be exfoliated along the interlayer,but also the nanosheets can be broken into smaller particles.When HCl is used,the exfoliated speed is very fast,and the siloxene is formed only3 days.The material has a flat two-dimensional surface,and the transverse dimension reaches micron level.It provides a good substrate material for the subsequent coating of MoS₂ and carbon layer,which is very beneficial for the synthesis of two-dimensional silicon-based composites.（2）Si-based composites were prepared by hydrothermal growth of MoS₂ and C with siloxene as the substrate.The effects of MoS₂ content and carbon content on the morphology,structure and electrochemical properties of the binary composite Si NS@MoS₂ and ternary composite Si NS@MoS₂@C were investigated.It is found that with the increase of MoS₂content,the MoS₂ particles on the surface of Si NS gradually form a state of semi-coating,single-coating,multi-coating and dispersed MoS₂ particles.Moreover,the dispersion of Si NS also increases with the increase of MoS₂ content.Therefore,the specific capacity of Si NS@MoS₂ binary composites is gradually increased.With the increase of carbon content,MoS₂ and Si NS have better dispersion,so as to reduce the resistance of Li⁺insertion and detachment,and also weaken the influence of silicon volume expansion effect.When m（C/Si）=30,the specific capacity of Si NS@MoS₂@C composites reaches the highest,and the thickness of carbon layer is 3～10 nm.The strong interfacial coupling between SINS,MoS₂ and C can significantly improve the structural stability and electrochemical performance of Si NS@MoS₂@C.Therefore,Si NS@20MoS₂@30C has a high capacity of 1080 m Ah g^-1,much higher than other silicon-based materials and MoS₂.The composite material has excellent cycle stability,and the capacity loss per cycle is only 0.023%after 1000 long cycles.Therefore,the development potential of two-dimensional silicon-based materials as anode materials for lithium-ion batteries is very great.