Study On Micron-nano Structure Control Of Co₃O₄ And Its Lithium Storage Performance

With the rapid development of electric vehicles and portable electronic devices,there is increasing demand for high power density,green and safe energy equipment systems.Lithium ion batteries（LIBs）have attracted tremendous attention due to its high energy density and stable electrochemical performance,considered to be one of the most potential energy storage technologies.The theoretical capacity of commercial graphite anodes is low(～372 m Ah·g^-1),which limits the development of LIBs.Therefore,it is imperative to develop lithium ion battery anode materials that are environmentally friendly,with high capacity,excellent cycling performance and rate performance.Cobalt tetroxide（Co₃O₄）has been widely concerned because of its high theoretical specific capacity(890 m Ah·g^-1),abundance,simple preparation method and chemical stability.However,its poor conductivity and 300%volume change during lithium ion insertion and extraction process hinder the commercialization of Co₃O₄anode materials.The study found that the huge volume change during charge and discharge can be alleviated through compounding to improve its conductivity and designing micro-nano structures.Based on the above modification ideas,this paper uses solvothermal method and self-assembly chemical precipitation method to prepare micro-nano structure Co₃O₄,to study its structure-performance relationship and lithium storage performance.The main research results of the paper are as follows:（1）The micron cubic cobalt-based precursor is prepared by solvothermal method,and the following two micro-nano cubic cobalt tetroxide materials are obtained by calcining at high temperature:（a）the cobalt-based precursor is annealed in air atmosphere to prepare multi-shell hollow cobalt tetroxide（MS-Co₃O₄）materials,and the number of shells is related to the calcining temperature.Through non-equilibrium heat treatment,driven by the temperature difference in the precursor inner and outer,the layers of organic matter shrink during the process of continuous oxidation and decomposition,and the multi-shell material is obtained.Among them,MS-Co₃O₄-400has the most excellent electrochemical performance,which manifests a high capacity of 710 m Ah·g^-1at current density of 500 m A·g^-1after 350 cycles and 510 m Ah·g^-1at1000 m A·g^-1after 500 cycles.The hollow structure of multi-shell layer is conducive to the diffusion of lithium ions in theanode material and can effectively alleviate the volume expansion of the material in the process of charging and discharging.（b）Cobalt-based precursor is annealed in argon-hydrogen mixture and then in air atmosphere to prepare garnet-like micro-nano carbon coated cobalt tetroxide（CS-Co₃O₄@C）composites.In inert atmosphere,the cobalt ions in the precursor are reduced to cobalt particles at high temperature and the organic matter was carbonized and coated on the surface.The structure of material is similar to that of a pomegranate.The carbon-coating cobalt oxide nanoparticles are called"pomegranate seeds"and are combined to form square-shaped micron material.After 180 cycles at a low current density of 200 m A·g-1,the reversible capacity of 1040 m Ah·g^-1can be maintained,while the reversible capacity after 800 cycles at a current density of 1000 m A·g^-1is693 m Ah·g^-1.The carbon network in CS-Co₃O₄@C material can improve the conductivity of the composite material and enhance the reaction kinetics of the material.（2）The precursor of one-dimensional nanowire with large aspect ratio is prepared by self-assembly chemical precipitation method.Cobalt tetroxide material with oxygen vacancy（1D-Co₃O₄）is prepared by further annealing of the precursor,which maintain the structure of one-dimensional nanowire and is composed of ultrafine cobalt oxide particles of 10 nm.The increase of annealing temperature leads to the sintering and aggregation of nanoparticles.1D-Co₃O₄-350 electrode maintain the reversible capacity of 1491 m Ah·g^-1after 70 cycles at current density of 200m A·g^-1.Even at a high current density of 2000 m A·g^-1,the 1D-Co₃O₄-350 show a reversible capacity of 704 m Ah·g^-1after 1000 cycles.The one-dimensional nanostructure of the material provides the electron ion transport channel,which is beneficial to enhance the stability of the material.Oxygen vacancy can effectively improve the surface capacitive behavior of the material,at the same time,construct the internal electric field to promote the diffusion of lithium ions,enhance the reaction dynamics,and make the material have stable cycling stability and rate performance.