Synthesis And Surface-modification Of Cobalt-Based Oxide Anode Materials

It is urgent to develop new electrode materials in order to meet the requirements of modern economy and society for higher specific capacity of lithium-ion batteries.Cobalt-based oxides,as lithium ion battery anode materials,have much higher specific capacity than that of commercial graphite.However,it is severe that the volume expansion of the cobalt-based oxides during charging and discharging.It can lead the electrode materials to pulverize and agglomerate,reduce the active site and even detach the materials from the current collector,which increases the impedance of the battery.Therefore,much attention has been paid by researchers to synthesize cobalt-based oxide anode materials with high specific capacity and good cycling stability.The main work of this thesis is to synthesize cobalt-based oxides with different morphologies and modify the surface with TiO₂,SiO₂and carbon to improve the structural stability and electrochemical performance of the composites.The specific research contents are as follows:Firstly,wreath-like Co₃O₄ powders were synthesized by solvothermal method,and then the TiO₂ nano-layers were deposited on the surface of wreath-like Co₃O₄ precursor via the low-temperature hydrolysis.As a result,the TiO₂ nano-layers effectively improve the cyclic stability and rate performance of the wreath-like Co₃O₄ anode materials.The improved electrochemical performance should be attributed to the sandwich-like structure formed by the coating of TiO₂ nano-layers,which help to alleviate the volume change of Co₃O₄ materials during the Li-ion insertion/extraction processes and effectively improve the overall structural stability.At the same time,the TiO₂-Co₃O₄ heterojunction structure which is formed by coating TiO₂ constitutes a built-in electric field,which promotes the transmission of electrons and lithium ions at the interface.Secondly,SiO₂ layers were deposited on the surface of wreath-like Co₃O₄ precursor via the low-temperature hydrolysis.The results show that the SiO₂ layers improve the structural stability of wreath-like Co₃O₄ and relieve the corrosion of the electrode material by hydrofluoric acid in the electrolyte.The SiO₂ layers also improve the cyclic stability of the composite and facilitates the diffusion kinetics of lithium ions.Finally,cobalt-based materials with hollow cuboid structure were synthesized by solvothermal method,and then the surface was coated with PDA and annealed in Ar atmosphere to prepare CoO@C composites.The electrochemical test results show that the carbonized PDA can enhance the cyclic performance and rate performance of the CoO anode material,protect the electrode material structure,alleviate the volume expansion of the anode material and improve the electrochemical cyclic stability of the material.