Preparation And Lithium Storage Performances Of Carbon Coated Cobalt-Based Metal Selenide Composites

Lithium ion battery as a decisive link in the new energy industry has received more and more attention,but now commercial graphite anode still has the disadvantage of low capacity.As the anode material of lithium ion battery,metal selenide has the characteristics of higher theoretical specific capacity,better electron conductivity and larger layer spacing.Due to the abundant cobalt reserves and the weak selenium bond in Co Se₂,Co Se₂ has a high conversion reaction efficiency as a transformational material,so it has been widely concerned by scholars.However,the volume of Co Se₂ changes too much during the cycle,and the conductivity and electrochemical activity of Cose₂ are poor.In this thesis,starting from Co Se₂ material,through the preparation of Co Se₂ with special nanostructure,composite alloy type material,carbon coating and other means to improve its capacity,electrochemical activity and cycling performance.The main contents of this thesis are as follows:In order to inhibit the volume change of the material,the water-phase synthesized nano-cube ZIF-67 was used as precursor,and the hollow structure was etched by dopamine hydrochloride（PDA）.After high temperature vapor phase selenization,the PDA was decomposed into N-doped carbon layer（NC）coated on the surface of cobalt diselenide to form the NC@Co Se₂ composite with hollow structure.Cobalt diselenide is located in the inner cavity of the hollow structure,and the carbon coating can inhibit its further growth and aggregation.The effect of PDA etching time on the material structure and lithium storage performance was investigated.When the etching time is 6 h,the hollow NC@Co Se₂-6 composite has the best lithium storage performance.When the current density is 0.1 and 5 A·g^-1,the specific discharge capacity is 787 and 525 m Ah·g^-1,respectively.Its capacity is 1600 m Ah·g^-1 after 700 cycles at A current density of 0.5 A·g^-1.In order to further improve the capacity and long cycle stability of Co Se₂ material under high current density,ZIF-8 was epitaxial grown on the precursor of ZIF-67,coated with polyvinylpyrrolidone（PVP）and PDA,and then NC@Co Se₂-Zn Se composite was prepared by high temperature vapor phase selenization.Coated PVP can prevent PDA from etching the material,and the coated PDA is carbonized into N-doped carbon layer to inhibit the volume expansion of the material.The effect of coating time on the structure and storage performance of PDA was investigated.When the coating time is 12 h,the NC@Co Se₂-Zn Se-12 composite has the best lithium storage performance.When the current density is 0.1 and 5 A·g^-1,the specific discharge capacity is 815 and 541 m Ah·g^-1,respectively.The capacity is 875 m Ah·g^-1after 950 cycles at A current density of 1 A·g^-1.In order to further improve the electrochemical activity of Co Se₂ material,Co Sb₂O₆ was synthesized by liquid phase at room temperature and calcined in air,and then PDA was coated with high temperature vapor phase selenium to synthesize NC@Co Se₂-Sb₂Se₃ composite.The influence of precursor synthesis technology and reaction time on the properties of lithium storage was investigated.When the reaction time is 6 h,NC@Co Se₂-Sb₂Se₃-6 composite has the best lithium storage performance,and the specific discharge capacity is 770 m Ah·g^-1 and524 m Ah·g^-1 when the current density is 0.1 and 5 A·g^-1,respectively.The capacity is 796m Ah·g^-1 after 950 cycles at A current density of 1 A·g^-1.