The Synthesis Of Transitional Metal-Carbon Based Composites And Their Application In Li-Ion Batteries

The electrochemical performance of Lithium-ion batteries(LIBs)depends largely on the electrode materials.Among them,the anode materials with high specific capacity and superior cycle stability are one of the key materials to determine the lithium storage performance of LIBs.At present,the commercial anode materials for LIBs are mainly graphite materials with high conductivity and good stability.However,the low theoretical specific capacity of graphite anode has seriously hindered its application in the field of high specific energy batteries.Transition metal based materials,such as transition metal oxides and sulfides,have rich redox sites and high theoretical specific capacity,but the low conductivity and serious volume expansion effect result in poor rate performance and cycle stability.In this paper,through the composite of transition metal based materials and carbon based materials,special nanostructures are constructed to give full play to the synergistic effect between transition metal based materials and carbon based materials,and effectively improve the electrochemical performance of transition metal based materials.The main research contents are as follows.(1)The hollow carbon nanocages(CNC)were used as nanoreactors to confine the growth of Zeolitic Imidazolate Frameworks ZIF-67 and form yolk-shell structured ZIF-67@CNC as precursors.After two step calcination,the precursors were converted into two kinds of composite materials Co S₂@NC@CNC and Co₃O₄@NC@CNC with yolk-shell structure.As the anode material of LIBs,compared with the Co S₂@NC and Co₃O₄@NC materials directly derived from ZIF-67 by high temperature calcination,the yolk-shell structured Co S₂@NC@CNC and Co₃O₄@NC@CNC exhibited more excellent electrochemical performance,which could retain the discharging specific capacities of 506 m Ah/g and 772 m Ah/g at the high current density of 2.0 A/g after 100 cycles,respectively.The synergistic effect of porous carbon based matrtials and active matrtials(Co S₂ and Co₃O₄)shortened the diffusion distance of Li⁺,improved the conductivity of electron/ions,and accelerated the transport of electron and Li⁺.The confined growth reduced the size of active materials,resulting in higher specific surface area and more active reaction sites.The yolk-shell effectively alleviated the volume expansion effect of active materials.(2)Nickel cobalt based compound nanosheets(Ni Co₂-Pre)were directionally grown on the surface of hollow polypyrrole nanotubes(ppy)to form polypyrrole nanotube composites loaded with Ni Co₂-Pre nanosheets(ppy@Ni Co₂-Pre).Then,the as-prepared ppy@Ni Co₂-Pre nanotubes were mixed with graphene oxides(GO)to form a three dimensional cross-linked structured ppy@Ni Co₂-Pre@GO hydrogels by hydrothermal assembly method.As a precursor,the hydrogels were converted into three dimensional porous graphene aerogel composites(NC@NCO@3DG)by high temperature calcination strategy.One dimensional nitrogen doped carbon nanotubes interpenetrated and connected two dimensional graphene nanosheets to form the three dimensional framework of aerogels,while the active materials Ni Co₂O₄(NCO)were uniformly dispersed on the surface of nitrogen doped carbon nanotubes(NC).As the anode materials of LIBs,NC@NCO@3DG materials exhibited excellent capacities and electrochemical stability,which could maintain the discharge specific capacity of565.5 m Ah/g at the high current density of 2.0 A/g after 500 cycles.Moreover,we also synthesized the similar structured NC@MoS₂@3DG composites through the same synthesis method.As the anode materials of LIBs,NC@MoS₂@3DG materials greatly improved the electrochemical performance of MoS₂,which could maintain the discharge specific capacity of 433 m Ah/g at the high current density of 2.0 A/g after 700 cycles.