Design And Preparation Of Composite Transition Metal-based Functional Materials And Their Lithium Storage Performance

Due to the advantages of high energy density,long cycle life and no pollution to the environment,lithium ion batteries are gradually regarded as one of the most promising energy storage conversion technologies.Graphite is widely used in the negative electrode of lithium ion batteries because of its good stability,but the theoretical specific capacity(372 mAh·g^-1)is low,and it has been unable to meet the needs of some areas of high energy density.Therefore,it is very important to develop a lithium ion battery anode material with high specific capacity,environmental friendliness and low cost.Among them,the transition metal oxide has attracted wide attention of researchers due to its high reversible capacity and abundant sources.However,transition metal oxides have defects such as poor conductivity,poor cycle stability,and high irreversible capacity loss for the first time.Therefore,we need to improve the above defects through methods such as shape control,introduction of carbon materials and construction of composite materials.Metal organic framework compounds（MOFs）have the characteristics of high specific surface area and multi-pore size distribution,and are considered to be ideal templates for carbon materials,metal oxides and their composite materials.Based on this,this paper introduces MOFs as a template,and uses the carbon skeleton and metal oxide obtained after heat treatment to construct three transition metal matrix composite materials,and then explores its structure and electrochemical properties.1.A novel Co₃V₂O₈@HCB composite,as hybrid anode material for lithium-ion batteries,is designed and fabricated.ZIF-67 was prepared by liquid method,and then a holey carbon box（HCB）was obtained after heat treatment and acid pickling.The HCB and the prepared one-dimensional rod-shaped Co₃V₂O₈ were self-assembled to construct Co₃V₂O₈@HCB composite material through a facile hydrothermal process.Different from the conventional way of coating carbon outside the metal oxide particles,1D bimetal oxide Co₃V₂O₈ nanorods with high Li⁺accommodation capacity,are embedded on ZIF-67 derived 3D holey carbon boxes with large effective surface area and plentiful porous.With the combination and modification,the exquisitely designed structure can not only facilitate the migration of lithium ions,but also buffer the volume change during the charge and discharge process,ensuring the hybrid anode superior rate performance and excellent cycle stability.When used as the negative electrode of a lithium ion battery,the composite material presents a discharge specific capacity of about1000 mAh·g^-1 after 500 cycles at a current density of 1 A·g^-1,showing excellent reversible capacity retention.Moreover,it delivers a reversible specific capacity of 648 mAh·g^-1 at a high rate current of 5 A·g^-1,indicating excellent rate performance.2.A ternary Cu₂O/Co₃O₄/C composite material,as hybrid anode material for lithium-ion batteries,is designed and fabricated.Cu₂O was prepared by liquid method,and then through introducing ZIF-67/ZIF-8 dual MOFs to coat Cu₂O,and then after multiple steps of sintering,after composition and morphology characterization,it was determined that the target product Cu₂O/Co₃O₄/C composite material was successfully prepared.The composite material uses cubic Cu₂O as a matrix and cooperates with Co₃O₄.In addition,the carbon material enhances the conductivity of the overall material,and the ternary combination can buffer the volume change during charging and discharging,and ensure the excellent electrochemical performance of the hybrid anode.After electrochemical testing,the Cu₂O/Co₃O₄/C composite material is basically stable and maintains a high reversible discharge capacity of 950 mAh·g^-1after circulating 100 cycles at a current density of 500 mA·g^-1,without significant attenuation,showing excellent Cycling stability;after recovering to 100 mA·g^-1 after a large current charge and discharge,there is still a high discharge capacity of 1071 mAh·g^-1,showing good rate performance.3.A（Co,Mn）（Co,Mn）₂O₄ compound transition metal oxide,as anode material for lithium-ion batteries,is designed and fabricated.The first method is to use carbon balls as a template to react with KMnO₄ to obtain the C@MnO_x intermediate,and then introduce ZIF-67to coat the intermediate,which can be obtained by sintering in an air atmosphere to obtain A-（Co,Mn）（Co,Mn）₂O₄ sample.The second method is to mix Co and Mn raw materials in a solvent and perform a simple hydrothermal method to obtain a precursor,followed by sintering in an air atmosphere to obtain a sample of B-（Co,Mn）（Co,Mn）₂O₄.Different preparation methods can obtain materials with the same composition and different morphology and structure.The composition and morphology of the obtained samples were characterized by XRD and SEM,and then the difference between the two samples was evaluated by comparing the electrochemical properties of the two materials.After electrochemical testing,the two samples were subjected to 50 cycles at a current density of 500 mA·g^-1.A-（Co,Mn）（Co,Mn）₂O₄ has a high reversible discharge capacity of 830 mAh·g^-1,and B-（Co,Mn）（Co,Mn）₂O₄has only 420 mAh·g^-1 discharge capacity.These differences may be attributed to the difference in the morphology of the two samples.The morphological structure of the nanospheres makes A-（Co,Mn）（Co,Mn）₂O₄ can alleviate the volume change during charge and discharge to a greater extent,and the crystallinity is also better,which makes the migration of lithium ions smoother and guarantees ensures electrochemical performance.Through the improvement of the preparation method,the materials with the same composition can have completely different structures,and also have different electrochemical properties.