Preparation And Application Of Co-based Materials For Lithium Sulfur Batteries

Energy materials are substances that humans rely on for survival.The exhaustion of fossil fuels and their use bring serious environmental pollution problems,prompting people to continuously develop new energy sources and apply advanced energy technologies.The development of energy storage systems that are economical,environmentally friendly,high energy density and long service life is an important link in promoting the development of new energy sources.Lithium-sulfur(Li-S)batteries with higher theoretical energy density are considered to be one of the most attractive competitors in future energy storage systems.At present,there are still many technical problems in the commercial development of Li-S batteries.The insulation of elemental sulfur and lithium sulfide(Li₂S)leads to extremely low sulfur utilization.Besides,the intermediate product polysulfide(Li PSs)is easily dissolved in ether electrolysis during charge and discharge.In the liquid,they migrate back and forth between the positive and negative electrodes,causing the"shuttle effect"and reducing the coulomb efficiency.Moreover,the density of sulfur and Li₂S are 2.03 and 1.66 g·cm^-3,respectively,which are quite different,causing the volume of the positive electrode changes greatly during the charge-discharge cycle.The volume expansion effect may even lead to the collapse of the positive electrode structure.These factors limit the practical application of Li-S batteries.Therefore,the design of sulfur matrix materials with high electrochemical activity and high sulfur content is of great significance to Li-S batteries.In order to better solve the above problems,we designed and prepared a new type of cobalt-based composite material through a simple method to optimize the battery structure,and explore its energy storage performance as a lithium-sulfur battery cathode material.The main research contents are as follows:(1)First,ZIF-67/G precursor was obtained by growing ZIF-67 on graphene,and then a Co Se₂-NC/G composite was synthesized as a sulfur carrier through in-situ carbonization and selenization processes.The 2-methylimidazole ligand is rich in carbon and nitrogen components that play an important role in electrochemical reactions.Co Se₂ nanoparticles,with their excellent chemical adsorption performance for Li PSs and inherent metal properties,effectively prevent their dissolution and diffusion,and accelerate the redox kinetics of their conversion.Graphene can not only be used as a conductive medium,but also can adjust the material composition,particle size and hybridization.In addition,graphene can also serve as a framework to disperse Co Se₂,which facilitates the adsorption of Co Se₂ to polysulfides and improves the redox activity of Co Se₂.The nitrogen atoms in the material can serve as additional polysulfide adsorption sites,further suppressing the"shuttle effect".The results show that the S@Co Se₂-NC/G electrode exhibits good electrochemical performance and cycle performance.At a current rate of 0.5 C,the initial discharge capacity is as high as 1004.5 m Ah·g^-1.After a long cycle of 500 cycles,it has a remaining discharge capacity of 623m Ah·g^-1 and the capacity attenuation rate of each circle is 0.0759%.(2)The ZIF-67 rhombic dodecahedron precursor of uniform size was prepared by a simple room temperature standing method,and a new type of sulfur-based nanostructured material was formed after high-temperature carbonization reduction and phosphating.In this material,Co and Co P nanoparticles are embedded in N-doped carbon nanotube nanopolyhedrons(Co/Co P-NCNHP).Co/Co P-NCNHP provides effective anchoring and catalytically active sites for Li PSs,which not only enhances the adsorption of Li PSs,but also accelerates the redox kinetics during the conversion of Li PSs.The micropores and mesopores in Co/Co P-NCNHP can effectively impregnate sulfur and hinder the diffusion of soluble Li PSs through physical constraints,which is proved by the battery cycle experiments.In addition,the conductive network interconnected by nitrogen-doped carbon nanotubes in Co/Co P-NCNHP can promote Li⁺transport,electron transport and electrolyte infiltration.Therefore,the specific capacity,rate performance and cycle stability of lithium-sulfur batteries are significantly improved.At a current density of 0.5 C,the initial discharge capacity is as high as1074.2 m Ah·g^-1.A discharge capacity of 885.4 m Ah·g^-1 can be maintained after500 cycles,and the capacity decay rate per cycle is 0.035%.It can be seen that S@Co/Co P-NCNHP positive electrode can show superior battery performance.