Preparation And Electrochemical Properties Of Hollow Carbon@Metal Oxide Composite Microspheres

As the second decade of the 21st century comes to an end,on the one hand,the demand for fossil energy is growing,and the reserves of fossil energy are gradually depleted.On the other hand,increasingly abundant portable electronic products are becoming indispensable in people's lives,the balance between advancement of electronic product functions and the increase in energy consumption has gradually been broken.In this context,society has placed unprecedented demands on researchers to develop sustainable and renewable new clean energy and advanced high energy density energy storage materials.Lithium-sulfur batteries,with their high theoretical specific capacity and huge reserves in the earth's crust,have gradually become the focus of researchers in the field of new energy.In this paper,the preparation process and electrochemical test data of two kinds of lithium-sulfur battery cathode materials are introduced.Our main idea is to construct a carbon matrix hollow microsphere with polar metal oxide materials to coat the sulfur element in all directions and load on the microsphere.on the one hand,the synergy between them can effectively limit the damage of the elemental structure of the sulfur element during the charge and discharge process,on the other hand,the nitrogen-containing carbon network can effectively improve the conductivity and utilization of the positive electrode material.At the same time,the metal oxide supported on the composite microsphere can effectively adsorb polysulfide,limit the shuttle effect,and effectively improve the cycle life of the lithium-sulfur battery.In order to prove this point,we prepared SiO2@C-S composite microspheres and porous hollow TiO2@C@MnO2-S composite microspheres as cathode materials for lithium-sulfur batteries and tested their electrochemical properties.The main research contents are as follows:In the third chapter,we synthesized raspberry-like hollow porous SiO2@C composite microspheres by template method and Stober method,and we used this material as the host material of sulfur element to assemble SiO2@C-S lithium-sulfur battery cathode material.Hollow porous structure can effectively load a large amount of sulfur,and the sulfur content of composite microspheres is up to 75%.The nitrogen-containing carbon layer introduced by dopamine effectively provides an efficient conductive network for the electrode material and a strong shell for the sulfur element.This structure not only improves the utilization rate of the active material,but also slows down the volume expansion of the sulfur element in the charge-discharge process.To some extent,SiO2 on the shell slows down the shuttle effect of polysulfides and improves the specific capacity and cycle performance of lithium-sulfur batteries.At a current of 0.2 C(1 C=1675 mA h/g),the initial capacity of the composite microspheres was as high as 942 mA h/g.After 200 cycles of constant current charge and discharge,the capacity remained at 519.8 mA h/g.And the coulombic efficiency of the battery is always higher than 95%during the whole cycle.We believe this structure effectively limits the loss of the elemental sulfur and improves the performance of the lithium-sulfur battery.In Chapter 4,we assembled TiO2@C@MnO2 microspheres by template method and in-situ reduction method.We hope this structure can improve the specific capacity and the coulombic efficiency of the battery.Composite spheres have considerable pore volumes for the storage of sulfur and polysulfides,which can effectively increase the specific capacity and prevent the volume expansion of sulfur during charging and discharging.At the same time,the introduction of PDA will provide a nitrogen-containing graphite network with excellent conductivity,which can effectively improve the conductivity of the positive electrode material and improve the utilization of S element.Moreover,the TiO2 and MnO2 supported on the surface of the carbon layer can effectively limit the shuttle effect of the polysulfide and improve the coulombic efficiency of the battery.The long cycle performance of button batteries was tested at a current density of 0.2 C for 400 cycles.The initial specific capacity of TiO2@C@MnO2-S composite microspheres can reach 956.6 mA h/g.After about 50 cycles,the specific capacity of the composite microspheres can be reduced to 666 mA h/g and tends to be stable.After 400 cycles,the specific capacity was reduced to 482.9 mA h/g.During entire process,the specific capacity decreased by 0.12%per cycle,and the coulombic efficiency was always maintained above 98%.