Synthesis Of Carbon-transition Metal Sulfide/selenide Composites And Their Applications In Lithium Ion Batteries

In order to meet the growing demand for energy,especially the booming electric vehicle market in recent years,it is very important to develop next-generation lithium-ion batteries?LIBs?with excellent performance.The anode material is an important component of LIBs,which plays an important role in the electrochemical performance of the whole battery.Conventional graphite materials for LIBs have a low specific capacity(372 mA h g^-1),and their application prospects are not optimistic.Therefore,new types of nanostructured anode materials have been exploited to promote the development of LIBs industry.Transition metal chalcogenide materials as anode materials of LIBs have attracted more and more interest because of their high theoretical specific capacity.However,LIBs prepared by transition metal chalcogenides have poor rate capability and cycle performance due to their relatively low conductivity and large volume changes.The design of high-performance transition metal chalcogenide composite nanostructured materials can effectively overcome these problems.In this paper,we have prepared novel carbon-based/transition metal chalcogenide anode materials using structural design and the introduction of carbon matrix strategy,which significantly improved the lithium storage performance of transition metal chalcogenides,showing excellent structural stability and electrical conductivity.The main content of the paper are as follows:?1?Cobalt sulfide/three-dimensional nitrogen-doped macroporous graphene?CoS/3DNMG?composites was prepared in situ by a one-step hydrothermal method using the pre-prepared 3DNMG.The results show that the composite material has a loose porous structure,and the nano-sized cobalt sulfide particles with the size from 50 to 200 nm uniformly grow in the internal structure of the 3DNMG.CoS/3DNMG composite provides an initial discharge capacity with 1834.9 mA h g^-1 at 100 mA g^-1 current density and maintains at 993.1 mA h g^-1 after 100 cycles.The coulombic efficiency maintains above 99%during the cycle experiment,showing good reversible performance.CoS/3DNMG composites can reach 669.8 mA h g^-1 after 1500 cycles at a higher current density of 1.0 A g^-1,exhibiting excellent long cycle performance.The synergy reaction between CoS and the multiple positive effects of 3DNMG greatly improve the rate performance and long-term cycle stability of LIBs.?2?The hollow spherical structure of titanium dioxide@nitrogen-doped carbon@cobalt sulfide?TiO₂@NC@CoS?nanocomposites were prepared by an effective step-by-step growth strategy.Firstly,mesoporous TiO₂ hollow nanospheres were pre-prepared by sol-gel method using polystyrene nanospheres as templates.Then nitrogen-doped carbon layer was coated on the surface of the TiO₂ hollow nanospheres?TiO₂@NC?.Finally,CoS nanoparticles were assembled on the surface of TiO₂@NC by solvothermal process to obtain hollow structured TiO₂@NC@CoS nanospheres.The synthesized nanospheres have a diameter of about 230 nm,and the CoS nanoparticles uniformly grow on the surface of TiO₂@NC@CoS.When being used as anode materials of LIBs,TiO₂@NC@CoS nanocomposites can achieve 488.2 mA h g^-1 after 100 cycles at 100 mA g^-1 with a capacity retention rate of 70.6%,which are better than that of TiO₂@NC(260.7 MA h g^-1).When current density is 1.0 A g^-1,the discharge capacity of TiO₂@NC@CoS composite can maintain at 392.7 mA h g^-1 after 500 cycles.Due to the special hybrid structure and composition property of the composite materials,TiO₂@NC@CoS hollow nanospheres exhibit good cycle stability and rate performance.?3?The MoSe₂ nanosheets grown on nitrogen-doped carbon nanotubes?MoSe₂@NCNTs?were prepared by a simple template strategy.Firstly,the complex of methyl orange and ferric chloride was used as a reactive soft template for direct the growth of polypyrrole on its surface,and thus leading to the formation of hollow nanotublar structures?PNTs?.Then,MoSe₂ nanosheets are grew on the surface of PNTs by hydrothermal treatment.Finally,the MoSe₂@NCNTs composite were obtained by the calcination step.The obtained MoSe₂@NCNTs composite inherit the one-dimensional structure of polypyrrole precursor The outer diameter of NCNTs is about 180 nm,and the thickness of MoSe₂ nanosheets wrapped the outer of NCNTs is about 10 nm.When MoSe₂@NCNTs composite was used as an anode material for LIBs,764.0 mA h g^-1 can be maintained after 200 cycles at the current density of 100 mA g^-1 and the capacity retention rate is 79.5%.The capacity as high as 524.5 mA h g^-1 can be remained after 600 cycles even at a current density of 1.0 A g^-1.The strong chemical combination between MoSe₂ nanosheets and NCNTs matrix can effectively prevent the agglomeration of the active material during cycles process,and ensure the stability of the electrode structure.The thin two-dimensional size and high specific surface area of MoSe₂ nanosheets can greatly shorten the diffusion distance of Li+to promote the reaction kinetics of the electrode,which improve the lithium storage performance of the electrode material.