Preparation And Enhanced Electrochemical Performances Of Hollow Structure Anode Materials For Lithium Ion Batteries

For commercial rechargeable lithium ion batteries?LIBs?,the traditional anode material graphite was plagued by its limited theoretical specific capacity of 372 mAh g^-1 and the poor rate performance induced by low Li diffusion constant(10^-11 to 10^-66 cm² s^-1),which didn't meet the high energy demands of the advanced electric and hybrid vehicles.A great deal of anode materials with enhanced storage capacity,high energy density and improved cycle characteristics have been proposed for lithium-ion batteries over the last decade;Among all of these candidates,alloy-type and conversion-type active materials are most attractive because of their high theroy capacities.However,their low electrical/lithium conductivity and large volume changes during the charged and discharged states are detrimental to cycling stability and application.To circumvent these issues,hollow structured materials have attracted considerable attention due to the large electrolyte/electrode contact area?short lithium-ion diffusion length and sufficient void space to buffer the volume changes.Aiming at the above problems,multi-shelled carbon hollow spheres,hollow hybrid spheres with few-layered MoS₂-entrapped carbon sheath,yolk-mesoporous shell Si@SiO₂nanohybrid and Si@double-shelled carbon yolk-like powders were designed and synthesized.The reserch contents could be seen as follows:1.Uniform multi-shelled?single,double and quadruple?carbon hollow spheres were successfully synthesized by using solid SiO₂ and hollow mesoporous SiO₂ templates.By adjusting the structures of the templates,the number of carbon shells can be accurately controlled.The shell thickness of the multi-shelled carbon hollow spheres is 6⁸ nm,and the quadruple-shelled carbon hollow spheres have superhigh specific surface area(1534m²g^-1)and ultralarge pore volume(3 cm³ g^-1).This architecture exhibit superior electrochemical results,and where specific capacity approaches 978 mAh g^-1 over 40cycles,meanwhile keeping good cycling performance and excellent rate performance(321mAh g^-1 at 800 mA g^-1).The superior performance in LIBs originates from large surface area?short lithium-ion diffusion lengths and sufficient void space to buffer the volume expansion.2.Hollow hybrid spheres with few-layered MoS₂-entrapped carbon sheath?HFMEC?have been synthesized by one-pot solvethermal method.In this unique structure,the ultrathin hybrid shell?¹2 nm?of HFMEC is composed of few-layered MoS₂??5?nanosheets and carbon matter.When tested as an anode material in lithium-ion batteries?LIBs?,HFMEC manifest good cycling stability(822 mAh g^-1 at 1 A g^-1 up to 200 cycles)and excellent rate capacities(562.9 mAh g^-11 at 10 A g^-1).The excellent performances are ascribed to ultrathin hybrid shell with enlarged MoS₂ inter-layered distance of 0.7 nm,the few-layered MoS₂ nanosheets and hollow structure.3.Yolk-mesoporous shell Si@SiO₂ nanohybrid?YMSSN?has been synthesized by a facile vesicle template method for the first time,encapsulating Si cores with mesoporous SiO₂ shell in aqueous media of surfactants.The unique structure is composed of Si nanoparticle as the yolk and uniform mesoporous SiO₂ as the shell.The thickness of silica shell can be readily controlled by adjusting the concentration of TEOS,and the silica mesoporous shell can facilitate the diffusion of lithium,permit more Li⁺flux across the interface.By comparing the electrochemical properties of bare Si and YMSSN with different shell thicikness,the YMSSN anodes with 10 nm thickness exhibit the best cycling stability(687 mAh g^-1 after 30 cycles)and high-rate performance.The special structure of composite was benefit to high capacity of active silicon and hollow porous silica shell accommodated volume expansion of silicon.4.Novel Si@double carbon shells yolk-like nanostructure was synthesized by hard-template method.In this special architecture,commercial Si nanoparticles are completely sealed inside the ultrathin and intact double-shelled carbon with rationally designed void spaces between the cores and shells,which can accommodate the volume fluctuation of Si cores.When tested as an anode material in lithium-ion batteries?LIBs?,the resulting Si@DC yolk-like powders exhibit outstanding cycling stability,enhanced lithium storage capacity(932.4 mAh g^-1 after 80 cycles)and high Coulombic efficiency?⁹8.6%?,which provide a novel strategy to controll the number of carbon shells in the yolk-shell Si@C structure.In summary,the thesis introduced four effective approaches,designed and prepared active materials with hollow and yolk-shell structures,greatly improved the electrochemical performancec of the carbon?silicon and molybdenum sulfide based anode materials.Furthermore,the technologies mentioned in this thesis enhanced the application potential of theses anode materials.