Application Research Of Cobalt-based Metal-organic Framework Derived Composite Materials In Lithium-ion Battery Anode

Electrochemical energy storage and conversion technology is recognized as the most practical choice to alleviate the current increasingly serious energy crisis and environmental damage due to its high energy efficiency and low environmental impact.Among t hem,lithium-ion batteries have become the most important energy storage system today because of their high weight and volumetric capacity,low self-discharge effect and environmental friendliness.At present,the theoretical capacity of graphite anodes for large-scale lithium-ion batteries is relatively low,and the rate performance is poor at large current densities,which greatly limits the future development of lithium-ion batteries.Therefore,there is an urgent need to develop new type anode materials.Among a series of anode materials,transition metal compounds（such as Co₃O₄,Fe₃O₄,Co₃S₄,Ni₃S₄,Fe S₂,etc.）are considered as potential alternatives because of their much higher theoretical capacity than graphite anodes,simple synthesis process and environmental friendliness.However,when they are used as the anode of lithium-ion batteries,due to their inherent low electronic conductivity and large volume expansion effect,which likely to cause poor rate performance and irreversible decline in capacity.These shortcomings can be effectively improved by combining transition metal compounds with carbon materials.Because carbon materials can not only provide a fast electron transport path,thereby overcoming the inherent low conductivity of transition metal compounds and realizing fast charging/discharging,but also can alleviate the volume expansion of electrode materials and improve cycle performance.As a new type of precursor,metal organic framework（MOF）has many applications in the synthesis of nanocomposite materials for electrochemical energy storage and conversion.The abundant metal ions in MOF can be converted into transition metal compounds under proper calcination conditions.At the same time,carbon materials derived from organic ligands in MOF also have significant effects,such as ordered porous structure,higher surface area,and heteroatom doping inherited from MOF.Therefore,composite materials derived from metal-organic framework materials are a very promising new electrode material,and it is of great significance to explore their applications in lithium-ion batteries.This research is based on metal-organic framework materials.By combining metal-organic framework-derived materials with carbon materials,new composite materials wit h special structures and compositions are obtained,and a series of tests are conducted to explore their electrochemical performance as anode materials for lithium-ion batteries.The main work content is as follows:1)A nanocomposite consists of N-doped carbon nanofibers and Co-containing MOFs（ZIF-67）derived ultrasmall Co₃O₄ nanoparticles（NCNF/Z-Co₃O₄NPs）is prepared via introducing electrospun and coordination-assisted assembly strategies.In this design,Co²⁺in the electrospun PAN-Co（Ac）₂ nanofibers coordinate with 2-Me IM to form ZIF-67nanocrystals and and further convert into ultrasmall Co₃O₄ NPs dispersed in the carbon fibers matrix.The carbon nanofibers serve as a highly conductive framework interconnect and confine the Co₃O₄ NPs,thereby improving the electrical conductivity and buffering the volume expansion of Co₃O₄.Due to these advantages,NCNF/Z-Co₃O₄ NPs anode shows enhanced lithium storage properties(1189 m Ah g^-1 at 0.1A g^-1,407 m Ah g^-1 at 1 A g^-1 after850 cycles and 329 m Ah g^-1 at 8 A g^-1).2)The unique double-shell Co₃S₄@Ni S nanoboxs covered by N and S co-doped RGO nanosheets（denoted as DS-Co₃S₄@Ni S@RGO）are for the first time synthesized through a facile metal organic framework based templating strategy.As expected,the DS-Co₃S₄@Ni S@RGO composite exhibited enhanced cycling stability and excellent rate capability when evaluated as anode materials for LIBs.Such excellent electrochemical performance of DS-Co₃S₄@Ni S@RGO could be ascribed to the uniquely double-shell nanostructure and the interconnected RGO conductive network.The double-shell structure can provide enough void space to alleviate the volume expansion and guarantee the mechanical integrity of the composite,as well as enlarge the surface area to provide more electrochemical reaction active sites and shorten Li⁺diffusion path.The interconnected N and S co-doped RGO nanosheets can increase the conductivity of the whole electrode,enrich the number of electrochemically active sites,and provide fast electron pathways.For LIBs,it displayed an ultrastable long-term cyclic performance with a capacity of 385 m Ah g^-1 at 1A g^-1 after 600 cycles and outstanding rate capability of 371 m Ah g^-1 at a current density of 8 A g-1.