Controllable Preparation Of Transition Metal Sulfides And Selenides And Their Application In Anode Electrodes Of Lithium/Sodium Batteries

With the development of the economy and the science technology of the word,the resources of fossil fuels are increasing consuming,which led to the emergence of envriomental problems and energy crisis.Therefore,we need to develop high efficient and green energy conversion and storage.Lithium-ion batteries（LIBs）,owing to their light weight,high energy density and good cycle life,have become the predominant power source for portable electronic devices,electric vehicles and electrochemical energy storage.It has been reported that graphite,as the most commercial anode material for LIBs,hindered the development of high energy density LIBs due to its relatively lower specific capacity than other transition metal oxides and sulfides.Likewise,reserves of lithium resources is limited and fast consumption,and leading to rising prices and higher production costs.Efforts have been made to develop novel anode materials for high-performance LIBs,including metal oxides and metal chalcogenides.Rechargeable lithium ion battery（LIBs）and sodium ion battery（SIBs）have been proved to be the efficient energy storage devices to address consumption of nonrenewable fossil fuels problems.In fact,due to the abandon of lithium and sodium in the earth,cheap price and environmental friendly,lithium batteries and sodium batteries maybe expected to new energy storage in a large power station.However,the transition metal compounds have the advantage of high initial capacity,good electrochemical performance,and low cost,and thus become an ideal electrode anode electrode material.A novel and bottom-up approach has been applied to synthesize bi-metal Ni-Co coordination polymer@polydopamine（NiCoCP@PDA）core-shell nanocubes by polymerization of PDA layer on the surface of NiCoCP nanocubes in chapter three.After thermally induced sulfurization processes,biactive NiS₂@Co S₂ nanocubes encapsulated into N-doped carbon core-shell nanocubes have been successfully prepared.When used as anodes materials for lithium ion batteries（LIBs）and sodium ion batteries（SIBs）,the novel bicontinuous carbon wrapped NiS₂@Co S₂ nanocrystals hollow structures show excellent Li⁺/Na⁺storage capacities with high specific capacities,good rate capabilities and stable cycling stability.The enhanced electrochemical performance is attributed to the interconnected porous structures and large amount of mesoporous structures,which effectively reduce the diffusion length for lithium ions and electrons,buffer volume expansion during the Li⁺/Na⁺insertion/extraction processes and retained structural integrity due to double carbon frameworks.This method can applied in other area to synthesis sulfides and provide an easy and controllable method of compounds.Mixed transitional metal selenides ZnSe@CoSe₂ microspheres and Cu₂Se@CoSe₂ nanocubes have been synthesized via simple co-precipitation method and followed thermal selenization process in chapter four.The as-prepared ZnSe@CoSe₂ microspheres and Cu₂Se@CoSe₂ nanocubes composites inherited the original structure of ZnCoCP microspheres and CuCoCP nanocubes,respectively.When used as anode materials for sodium-ion batteries（SIBs）,the Zn Se@CoSe₂microspheres and Cu₂Se@CoSe₂ nanocubes deliver excellent sodiation/desodiation and rate performance when using 1 M NaPF₆ in ethyl methyl carbonate with 2%FEC as an electrolyte.Reversible capacities of 525 mAh g^-1 and 423 mAh g^-11 are obtained at 1 A g^-11 after 500 cycles,respectively.Furthermore,even at a high rate of 5 A g^-1,ZnSe@CoSe₂ microsphere still maintain 520 mAh g^-1 after 500 cycles,demonstrating remarkable capacity and excellent cycling performance.The hollow and porous structure of ZnSe@CoSe₂ microspheres and Cu₂Se@CoSe₂ nanocubes derived from the precursor facilitate the transport of sodium ions,contributing to high capacity,improved rate capability and excellent cycle stability.This work provides a facile method to control the structure of mixed transition metal selenides and explores their potential applications for rechargeable sodium-ion batteries. A facile and cost effective hydrothermal method is presented to prepare mesoporous hollow Zn_0.76Co_0.24S nanospheres uniformly wrapped in reduced graphene oxide（GO）sheets(Zn_0.76Co_0.24S@G)as high-performance anode materials for lithium ion batteries（LIBs）and sodium ion batteries（SIBs）in chapter five.The obtained mesoporous hollow Zn_0.76Co_0.24S nanospheres are composed of numerous nanoparticles,and the void spaces between the neighboring nanoparticles provide large contact areas with electrolyte and buffer zone to accommodate the volume variation during the cycling process.When the Zn_0.76Co_0.24S@G composites are evaluated as anode materials,it exhibited a high reversible capacity of 804 mAh g^-1 at1000 mA g^-1 and 605 mAh g^-11 at 1 A g^-1 after 500 cycles for LIBs and SIBs,respectively.The enhanced electrochemical performance of the well-designed Zn_0.76Co_0.24S@G composites can be attributed to the synergistic effect of mesoporous hollow Zn_0.76Co_0.24S nanospheres and graphene.On the top of that,this research not only contribute to elevation of capacity,but also provides an extra option for battery practice.