Investigation And Li-storage Properties Of Semiconductor-based Anode Materials By Carbon Modification

Due to their high energy density and long lifespan,rechargeable lithium-ion batteries（LIBs）have received increasing attention for application in portable electronic devices,electric vehicles,aerospace and large scale energy storage.Anode material is considered as an important component of the lithium-ion battery,the specific capacity of commercial graphite-based anode electrode materials could not meet the ever-growing demand for high energy-density LIBs.There are increasing focus on developing anodes with high specific capacity and excellent cycle stability.Semiconductor materials such as oxides,sulfides and phosphides,are considered as most promising alternative anodes for commercial graphite in high energy density LIBs owing to their high specific capacity,multiple chemical valence states,abundant sources and environmental friendliness.Unfortunately,the low intrinsic Li-ion mobility and the relative low conductivity also greatly limit their high-rate performances in practical applications.Moreover,the strong pulverization caused by the drastic volume expansion of most semiconductor electrodes during the charge/discharge process,leads to degradation upon cycling and becomes the main obstacles for their practical applications.In view of the low electric conducitivity and large volume expansion for oxides and sulfides such as Li₄Ti₅O₁₂,TiO₂ and MoS₂,the hierarchical anode materials are surface-modified by high-conductivity carbon to achieve high rate capability and excellent cyclic stability.The main contents of this work are summarized as follows:First,Li₄Ti₅O₁₂@C nanosheets have been prepared by a facile solvothermal method and subsequent annealing process.It is found that carbon coating significantly improves the cycling stability and rate capability,delivering a high discharge capacity of 137m Ah/g at 20 C.Second,hollow flower-like Li₄Ti₅O₁₂ nanosheets are prepared via a hydrothermal process,followed by wrapping with graphene through electrostatic interactions.Graphene could provide a high-conductivity network for the rapid transport of electrons in composites.In addition,Li₄Ti₅O₁₂@graphene exhibits higher discharge capacity and excellent rate performance owing to the enhanced Li⁺diffusion reaction kinetics induced by graphene-wrapping.Third,flower-like TiO₂ wrapped with graphdiyne has been synthesized by a simple solvothermal method.The obtained TiO₂@graphdiyne delivers high reversible capacities,excellent rate capability and cycle stability because graphdiyne provides additional storage sites for lithium ions and enhances the electronic conductivity of the composites.Furthermore,the built-in interfacial electric field between graphdiyne and TiO₂ could also facilitate Li-ion diffusion at the interface and consequently improve the electrochemical performances.Fourth,hierarchical polyhedral MoS₂@C（HP-MoS₂@C）hollow cages have been successfully fabricated using facile solvothermal technique combined with self-sacrificed template method.The in-situ pyrolysis carbon can not only improve the conductivity of MoS₂but also efficiently alleviate the large volume expansion during the charge/discharge process,delivering excellent electrochemical performances especially at high current density.Moreover,HP-MoS₂@C could achieve a reversible capacity of 1074.8 m Ah/g at a current density of 500 m A/g when it is assembled as anode in pouch cells.The potential mechanism behind the enhanced performances of MoS₂@C composites are investigated by X-ray diffraction（XRD）and Kelvin probe atomic force microscopy（KPFM）.Combined with work function and first-principles calculations,the excellent performances of HP-MoS₂@C composites could be well explained based on local defect model and energy-band theory.Fifth,hollow MoS₂@C（HS-MoS₂@C）core-shell spheres are rationally designed and successfully fabricated using facile solvothermal technique based on self-sacrificed template method.The carbon shell on MoS₂offers a high-conductivity electron transport pathway,reducing the electrode polarization during the charge/discharge process.Furthermore,the carbon shell could efficiently relieve the volume change and prevent the MoS₂ nanoparticles from aggregation,sustaining voids for facile penetration of electrolyte and improving cycling stability.As a result,the electrochemical performances are enhanced.