Preparation And Lithium Storage Performancs Of Silicon/Copper/Carbon Composites As Anodes

Lithium-ion batteries have been widely used in portable electronic products and hybrid vehicles in the past 20 years.The lower theoretical capacity of commercial graphite anodes limits their application in outstanding performance lithium-ion batteries.Silicon,with extremely high theoretical specific capacity and low discharge voltage,is considered to be one of the candidates for the negative electrode of next-generation lithium-ion batteries.However,huge volume changes and low electrical conductivity will lead to poor cycle performance and rate performance of lithium-ion batteries.Therefore,this paper designed a double-coated structure of a metal copper shell and carbon layer to suppress the volume expansion of silicon and improve the conductivity of the electrode.(1)With copper acetate as the copper source,the synthesized copper oxalate precipitate will adhere to the surface of Si nanoparticles,obtaining the core-shell structure Si@Cu particles after hydrogen reduction.Subsequently,core-shell Si@Cu particles are coated with phenolic resin,forming Si@Cu@C composite after pyrolysis.The good mechanical properties of copper can alleviate the stress caused by the expansion of silicon.The carbon layer forms a conductive network between the Si@Cu particles,providing ion and electron transmission channels,while maintaining the stability of the SEI film.Silicon-copper forms an alloy phase at high temperature,which improves the interface adhesion performance and conductivity.At a current density of 100 mAh g-1,the specific discharge capacity reached 1724.3 mAh g-1 after 50 cycles.The specific charge capacity was still 1366.2 mAh g-1 after 100 cycles at the current density of 1 A g-1,and the capacity retention rate 83.5%.(2)Dopamine self-polymerizes on the surface of Si@Cu particles and then coats the Si@Cu particles,obtaining the Si@Cu3Si-Cu@Si composite after carbonization.The dopamine pyrolysis carbon layer still retains nitrogen atoms,which can improve the conductivity and charge transfer of carbon materials.The synergistic effect of carbon conductive network and metallic copper are conducive to the transmission rate of ion and electron.Si@Cu3Si-Cu@NC composite has excellent cycle performance and rate performance,and the specific capacity maintained 1925.1 mAh g-1 at the current density of 100 mAh g-1 after 50 cycles.Silicon-copper alloy phase(Cu3Si)is formed at the interface between silicon and copper,which can improve the interface strength of the silicon-copper and prevent the copper layer from detaching.At a high current density of 5 A g-1,the specific capacity of 1105.5 mAh g-1 can still be maintained after 100 cycles(3)Si@CuC2O4 was synthesized by precipitation method,and the modified Si@CuC2O4 and graphene oxide were electrostatically self-assembled to prepare Si@Cu@rGO composite.Copper can enhance the charge transfer kinetics of the electrode and inhibit the agglomeration of Si nanoparticles.The graphene nanocage encapsulates the Si@Cu nanoparticles inside,which limits the polarization and pulverization of silicon.The graphene sheets not only provide buffer space for the expansion of core-shell Si@Cu nanoparticles,but also provides a conductive network to enhance the electronic conductivity and flexibility of the electrode.The good synergy between the copper shell and the graphene sheet improves the electrochemical performance.The Si@Cu@rGO composite has a reversible capacity of 2095.2 mAh g-1 at 200 mA g-1 after 50 cycles.