Preparation Of Cobalt-Based Carbon Nanocomposites And Their Application In Lithium/Potassium Ion Batteries

Lithium-ion batteries are widely used because of their cycle stability and adaptability.However,with the market demand for high energy density batteries,the conventional graphite anode with lower theoretical specific capacity can no longer meet the market needs.Therefore,there is an urgent need to design anode materials with high theoretical specific capacity and long cycle stability.In addition,potassium ion batteries are regarded as a kind of technology reserve for lithium ion batteries in the field of large-scale energy storage due to their advantages of low cost,safety and high voltage platform,and have received wide attention from scholars at home and abroad.Cobalt-based anode is considered as a promising anode material due to the advantages of abundant raw material sources and high theoretical specific capacity.However,the huge volume expansion effect of cobalt-based cathode restricts the electrochemical performance of the battery.In addition,the low conductivity and ionic conductivity of cobalt-based anode limit the electrochemical reaction of lithium/potassium de-embedding process.At present,researchers often use morphology modulation to improve the intrinsic defects of cobalt-based anode,and the preparation of one-dimensional（1D）carbon nanofibers（CNFs）is the most widely used means.Therefore,in this thesis,Co-based alloys are used to prepare Co-based carbon nanofiber composites by electrostatic spinning method to investigate the lithium/potassium storage properties and mechanism of the materials.The main research contents include:（1）Co Sb-CNFs carbon nanofiber composites were successfully prepared based on electrostatic spinning method and high-temperature pyrolysis technique,and their carbon nanofibers successfully encapsulated Co Sb quantum dots.The synthesized nanofiber network is both flexible and mechanically stable,and can be used as a flexible self-supporting electrode for lithium battery storage.When used as a self-supported negative electrode for Li-ion batteries,the electrode exhibited excellent cycling and multiplicative performance,including a reversible discharge specific capacity of 520 m Ah g^-1after 800 cycles at a current density of 0.5 A g^-1and a discharge specific capacity of 315.4 m Ah g^-1at a high current density of 3 A g^-1.（2）The research in this chapter is based on the modification of Co Sb-CNFs,using phosphating and hydrothermal synthesis processes to successfully prepare Co Sb with a three-dimensional network structure Co Sb/P-CNFs@r GO negative electrode material.The three-dimensional nanostructure of the material is formed by wrapping one-dimensional carbon nanofibers with two-dimensional reduced graphite oxide（r GO）.The phosphating process can dope phosphorus in carbon nanofibers,providing a large number of active site and additional ion storage sites for materials.RGO has good mechanical strength and can stabilize the matrix structure.In addition,it can be encapsulated on the surface of carbon nanofibers as a secondary trap for Co Sb particles,avoiding the loss of active substances.Thanks to the above structural advantages,Co Sb/P-CNFs@r GO The flexible self-supporting negative electrode has a discharge specific capacity of 870 m Ah g^-1after 2000 cycles at 0.5 A g^-1.As the negative electrode of potassium ion batteries,Co Sb/P-CNFs@r GO At a current density of 0.2 Ah g^-1,the reversible discharge specific capacity can be maintained at320 m Ah g^-1after 800 cycles.（3）In this chapter,Co Sb is used as the matrix,and Fe and Ni elements with high bulk density and non active Mn elements are introduced at the transition metal cation site.High entropy alloy nanofiber composites with Co_0.2Sb_0.2Fe_0.2Mn_0.2Ni_0.2are successfully prepared by electrospinning.Thanks to its stable crystal structure and synergistic effect of multiple components,the electrode exhibits excellent lithium/potassium storage performance.When used as a flexible self-supporting negative electrode for lithium-ion batteries,the material has an ultra-high discharge specific capacity of 1400m Ah g^-1after 800 cycles at a current density of 0.5 A g^-1.When used as a flexible self-supporting negative electrode for potassium electricity,the electrode maintains a discharge specific capacity of 280m A g^-1after 200 cycles at a current density of 0.2 A g^-1.This thesis provides a new idea for restraining the volume expansion of alloy anode materials and improving their capacity and structural stability.There are 41 figures,8 tables and 130 references in this thesis.