Co-based Transition Metal Oxides As Anode Materials For Lithium-ion Batteries

Lithium-ion batteries（LIBs）were widely used in electronic mobile devices and electric vehicles due to their high energy density,high specific capacity and long cycle life.The commercial graphite anode material has a low theoretical capacity of only 372mAh g^-1,which further limited its application in LIBs due to poor conductivity,low lithium ion diffusion coefficient.In this thesis,CoFe₂O₄ and NiFe₂O₄ nanoporous spheres,hierarchical Co₃O₄（h-Co₃O₄）,Ag-doped Co₃O₄（Ag-Co₃O₄）and hollow Co₃S₄/MoS₂ composite materials were prepared by solvothermal and dealloying methods and as anode materials for LIBs.The electrochemical performance of various materials were improved by means of morphology control,heteroatom doping and coating,and the effects of different modification behaviors on their properties were studied.The main contents of the thesis are as follows:1.CoFe₂O₄ and NiFe₂O₄ nanoporous spheres with a diameter of about 500 nm were prepared by solvothermal method.The structure and morphology of the samples were characterized by XRD,TEM,SEM,EDS,Mappings,BET,HRTEM and XPS.Electrochemical performance tests showed that the capacity of the CoFe₂O₄ and NiFe₂O₄ nanoporous spheres could be maintained at 300 mAh g^-1 and 180 mAh g^-1 after1000 cycles at a current density of 1000 mA g^-1,respectively.Even at a current density of 3000 mA g^-1,their capacity still reached at 255 mAh g^-1 and 173 mAh g^-1 after 1000cycles,respectively.We explained why the capacity of CoFe₂O₄ is higher than that of NiFe₂O₄ through analyzing the nanoporous spherical morphology of CoFe₂O₄ and NiFe₂O₄.It was found that the excellent electrochemical performance was mainly attributed to the nanoporous structure when used as anode for LIBs.The porous nanostructure can buffer the volume expansion during charge/discharge process,shorten the path of the Li⁺diffusion,increase the area of contaction between the electrolyte and electrode material,and provide more Li⁺reactive sites.2.Co-based metal-organic framework（Co-MOF）was used as a precursor for further synthesis of hierarchical Co₃O₄（h-Co₃O₄）by co-heated with sulfur powder in air.The formed outer layer was about 200 nm and the spacing between layers was about²nm.The morphology and structure of h-Co₃O₄ were characterized by XRD,TEM and SEM.When used as a anode for LIBs,h-Co₃O₄ nanoparticles exhibited stable cycle performance and excellent rate capability.At a current density of 5000 mA g^-1,the h-Co₃O₄ electrode maintained a capacity of 210 mAh g^-1 after 5000 cycles.At a high current density of 10000 mA g^-1,the capacity can still reach 177 mAh g^-1 after 5000cycles.The long cycle life and excellent rate capability resulted from its hierarchical structure.The amorphous outer layer remained the internal Co₃O₄ crystal stable during charge/discharge process.The cavity also provided a buffer for the volume change which caused by the Li⁺insertion/extraction process,and kept the structure stable during charging/discharging process at high current density.3.Ag doped Co₃O₄ nanosheets were synthesized by de-alloying method.The thickness of the nanosheet was about 0.5 nm.The structure and morphology of the materials were characterized by XRD,TEM and SEM.Ag-Co₃O₄ nanosheets exhibit excellent electrochemical performance as a anode for LIBs.At a current density of1000 mA g^-1,the capacities of first discharge/charge were 925 and 609 mAh g^-1,respectively,The discharge capacity maintained at⁵00 mAh g^-1 after 1000 cycles,.Even at the higher current densities of 2000 and 3000 mA g^-1,the capacity could be remained at 209 and 155 mAh g^-1 after 1000 cycles,respectively.It was found that the Ag doping can greatly enhances the conductivity of Co₃O₄ and reduced the internal resistance of the battery,which was beneficial to improve the long cycle performance.4.Hollow Co₃S₄ nanocubes were prepared by Zeolitic imidazolate frameworks-67（ZIF-67）as a precursor reacting with sulfur power.Then,MoS₂ coated on the surface of Co₃S₄ to obtain Co₃S₄/MoS₂ composite.The structure and morphology of the Co₃S₄/MoS₂ nanocomposites were characterized by XRD,TEM,SEM and XPS.Co₃S₄nanocubes have poor electrochemical performance.At a current density of 1000 mA g^-1,the capacity of Co₃S₄ only remained at 90 mAh g^-1 after 120 cycles.However,the Co₃S₄/MoS₂ composites exhibited significantly improved capacity and excellent cyclic stability.At a current density of 1000 mA g^-1,the capacity of Co₃S₄/MoS₂ can be maintained at 640 mAh g^-1 after 120 cycles,while the capacity could also keep at 365mAh g^-1 after 1000 cycles.When the current density was increased to 2000 mA g^-1,the capacity of Co₃S₄/MoS₂ was stable at 171 mAh g^-1 after 1000 cycles.The coating of MoS₂ inhibited the volume effect of Co₃S₄ nanocubes,and maintained the material structure stability during charge/discharge processes.Both the capacity and cycle stability of Co₃S₄/MoS₂ composite significantly improved comparing with the pristine Co₃S₄ materials.