Preparation And Anode Properties Of Carbon-based Transition Metal Compounds Composites For Lithium Ion Batteries

Lithium ion batteries（LIBs）have been widely used in portable electronic devices since the commercialization and their current applications in the field of electric vehicles have also attracted wide interest of researchers.However,the energy density of conventional LIBs is greatly limited by the low-capacity of commercial graphite anodes,so new anode materials with high energy density,long cycle life,excellent rate performance,low cost and safety are eagerly in need.Carbon-based materials have attracted extensive attention due to their rich sources,good electrical conductivity,low cost,excellent structural stability and mechanical properties.However,carbon-based materials are always limited by the low theoretical specific capacity.Transition metal compounds are favored by researchers because of their high theoretical specific capacity and their wide range of sources,but their further development is hindered by poor conductivity and volume expansion during the cycle as electrode materials.Effective methods to improve its electrical conductivity and structural stability are usually to recombine with a conductive material or to construct a material with a special morphology and structural.In summary,several kinds of carbon and transition metal compound composites are studied in this paper by different preparation methods to obtain anode materials with excellent lithium storage performance.The main research contents and results are listed as following:（1）Fe₂O₃ quantum dots/nitrogen-doped 3D porous carbon composite（Fe₂O₃ QDs/3DNPC）was successfully prepared by simple electrostatic self-assembly and calcination method,in which chitosan was used as carbon precursor and ferric chloride as transition metal compound precursor.The composite consists of Fe₂O₃quantum dots（3～5 nm）and nitrogen-doped 3D porous carbon,in which monodispersed Fe₂O₃ QDs are uniformly embedded in 3DNPC.Fe₂O₃ quantum dots play the role to increase the interlayer spacing and improve the disorder degree of porous carbon,so that lithium ions can not only rapidly intercalate in carbon layers,but also react more rapidly with Fe₂O₃ nanoparticles.As negative electrode material for LIBs,Fe₂O₃ QDs/3DNPC composite exhibits a high capacity of 531 mAh g^-1 at 0.2A g^-1.At a current density of 2 A g^-1,the specific capacity reaches up to 280 mAh g^-1after 1000 cycles.Even at a current density of 20 A g^-1,Fe₂O₃ QDs/3DNPC exhibits a high specific capacity of 255 mAh g^-1,and its capacity of 211 mAh g^-1still remains82.7%of the intial capacity after 1000 cycles.The results clearly show that Fe₂O₃QDs/3DNPC has excellent cycle performance as an anode in lithium-ion battery.（2）A three-dimensional network structure of Fe₇S₈/carbon composite（Fe₇S₈/C）was successfully prepared by simple in-situ precipitation reaction and high-temperature carbonization process,in whichβ-cyclodextrin was used as adsorbent and carbon source and Na₂S as precipitant.The obtained Fe₇S₈/C composite material has a three-dimensional network structure,and the Fe₇S₈ nanoparticles of about 40 nm are uniformly distributed in the carbon substrate.Compared with the bulky Fe₇S₈,the nano-sized Fe₇S₈ can increase the contact and penetration between the active material and the electrolyte,and shorten the diffusion distance of lithium ions.At the same time,by taking advantage of the fixed adsorption of carbon-based material for Fe₇S₈ nanoparticles,it can not only alleviate the volume expansion problem occurring during the charging and discharging process es of Fe₇S₈,but also overcome the problem of poor conductivity.As negative electrode material for LIBs,the research results show that the electrode has good lithium storage performance.The capacity of Fe₇S₈/C is maintained at 590 mAh g^-1 after 300 cycles at 0.5 A g^-1.Even after 700 cycles at a high current density of 2 A g^-1,the composite still exhibits a high specific capacity of 307 mAh g^-1,showing good capacity,rate and cycle stability.（3）The PPy tubes adsorbed with Co²⁺were used as the interconnected chains for ZIF-67 polyhedrons to in-situ grow heterogeneous nucleation of ZIF-67 seed on the surface of PPy tubes.A new type Co₉S₈@carbon/carbon nanotubes composite（Co₉S₈@C-CNTs）was successfully prepared by one-step solid chemical vulcanization of above percursors.The carbonized polyhedrons（about 500 nm）are tightly coupled to the CNTs,which form a 3D interconnected network structure.This unique structure of Co₉S₈@C-CNTs exhibits excellent electrochemical performance as negative electrode material for LIBs.The capacity maintains at 679.6 mAh g^-1 after 150 cycles at 0.5 A g^-1.When the current density is 2 A g^-1,Co₉S₈@C-CNTs shows a high specific capacity of about 585.3 mAh g^-1 after 500 cycles.