Preparation And Electrochemical Performance Of Cobalt-based Sulfide Sodium Ion Battery Anode Materials

In recent years,with the rapid development of portable electronic devices and new energy electric vehicles,there is a growing demand for high-performance secondary batteries.Lithium-ion batteries are developing very rapidly due to their advantages of good cycle stability and high energy density.However,the shortage of lithium resources has limited the long-term,large-scale application of lithium-ion batteries.Sodium ions have similar electrochemical properties to lithium ions and are naturally abundant and easy to extract,so sodium ion batteries have greater cost advantages in large-scale commercial applications.The key to developing sodium ion batteries is the development of efficient electrode materials.Cobalt-based sulphide,as a conversion-type anode material with high theoretical capacity,has been widely studied.However,it suffers from intrinsic disadvantages such as slow diffusion kinetics and large bulk effect,which severely restrict its practical application.In this thesis,cobalt-based sulphides are studied to enhance the sodium storage performance of electrode materials by constructing heterogeneous structures,compounding with carbon materials and special structural designs.The main contents are as follows:（1）ZIF-67/ZIF-8 nanocubes are used as precursors and a nitrogen-doped carbon shell is coated on the surface of the nanocubes by means of a high temperature carbonisation process using polydopamine coating.The nitrogen-doped carbon shell coating can significantly enhance the electrical conductivity of the material and also buffer the bulk effect during the charging and discharging process.The Co S₂/Zn S heterostructure is successfully prepared in the subsequent high temperature vulcanisation reaction.The synergistic effect of the heterogeneous structure can enhance the sodium storage performance of the material.The composite exhibits good cycle reversibility with a sodium storage capacity of 423.1 m Ah g^-1 for 50 cycles at a current density of 0.1 A g^-1 and 200.6 m Ah g^-1 for 1000 cycles at a current density of 1A g^-1.（2）Polydopamine coating is used to improve the cyclic performance of a material.The low boiling point of zinc atoms is leveraged by calcining ZIF-67/ZIF-8@PDA nanocubes at a higher temperature,which resulted in the volatilization of reduced state zinc atoms from the material.The pyrolysis of the organic ligand and volatilisation of the zinc atoms resulted in the creation of a hollow structure within the cubes.Then,the cobalt atoms in the material are partially replaced by antimony ions through cation exchange,and finally Co S₂/Sb₂S₃@NC/CN-2 is produced by high temperature sulphidation reaction.The unique heterogeneous structure and hollow nanostructure can effectively buffer the volume effect of the active material during the charging and discharging process,accelerate the reaction kinetics,and enhance the sodium storage capacity and multiplication performance of the material.Co S₂/Sb₂S₃@NC/CN-2 exhibits excellent sodium storage performance with a sodium storage capacity of 513.84 m Ah g^-1 after 100 cycles at 0.1 A g^-1 current density and a reversible capacity of 360.38 m Ah g^-1 at 5 A g^-1 current density.（3）The use of an external carbon source to enhance the sodium storage properties of cobalt-based sulphides often suffers from a cumbersome synthesis process.During the synthesis of the carbon matrix,the Co₉S₈/Ni S heterostructure is formed in situ,which ensures that Co₉S₈/Ni S is embedded within the carbon matrix and has a small size.The nitrogen/sulphur co-doped carbon matrix and the small size of the Co₉S₈/Ni S heterostructure enhance the electrical conductivity and structural stability of the material and facilitate the rapid transport of sodium ions and electrons.Therefore,the composite exhibited a relatively good sodium storage performance with a sodium storage capacity of 273.8 m Ah g^-1 after 50 cycles at a current density of0.2 A g^-1.