High-performance Cobalt-based Carbon Composite Cathode Carrier And Electrochemical Performance For Lithium-sulfur Battery

Lithium-sulfur secondary battery with high theoretical specific energy(2600 Wh kg^-1),cheap sulfur,abundant raw materials and other advantages have become one of the most popular next-generation energy devices.After more than a decade of research,the problems of conductivity and volume expansion for the sulfur can be easily solved by using a conductive skeleton and building porous structures,however,the slow electrochemical reaction kinetics of the sulfur is still the main obstacle to the application of lithium-sulfur battery.Giving catalytic functionality to the cathode carrier is an essential strategy to address the sluggish kinetics of sulfur.In order to improve the overall electrochemical performance of the battery,current research has focused on the loading of redox-active catalysts（e.g.transition metal nanoparticles,single-atom catalysts）onto the preferred conductive carriers in order to increase the reaction rate.However,the integrated preparation of high conductivity,high adsorption and high catalytic activity cathode carriers,commonly using complex single-component preparation processes and loading composite processes,is bound to affect the development of lithium-sulfur battery applications.Based on this,this thesis uses a simple and efficient in situ one-pot method as well as commercial single-component material to construct multifunctional integrated sulfur cathode carriers,which significantly enhance the electrochemical reaction kinetics of lithium-sulfur battery,inhibit polysulfide shuttling and improve the electrochemical performance of the battery.Details of the work and conclusions are as follows.（1）Study of the preparation of cobalt-based high electrical conductivity hollow carbon nanospheres and their electrochemical performance for lithium-sulfur battery.Highly conductive hollow carbon nano-spheres were prepared by using commercial Ketjen Black（KB）as raw material and high temperature treatment at3000℃,and then cobalt phthalocyanine was loaded on the alkali-etched hollow carbon spheres to prepare a high conductivity hollow carbon nano-sphere cathode carrier loaded with cobalt phthalocyanine.It was shown that the use of an optimized potassium hydroxide treatment process to etch micro-pores on the surface of the carbon spheres facilitated the entry of sulfur into the interior of the hollow carbon spheres.The hollow carbon nanosphere structure effectively physically inhibits the entry of polymorphic sulfur ions into the electrolyte,thereby suppressing the shuttle effect.Further studies have shown that the hollow carbon nanosphere composite carriers loaded with cobalt phthalocyanine can significantly improve their ability to adsorb polysulfide ions and catalyze the kinetics of sulfur electrochemical reactions after high temperature activation.The lithium-sulfur battery assembled with cobalt-based high conductivity hollow carbon sphere carriers activated at 600°C achieved a specific capacity of 702.8 m Ah g^-1at 2C,and a capacity decay rate of only0.096%per cycle after 300 cycles at 1C,showing good overall electrochemical performance.（2）Study of in-situ preparation of nano-cobalt/graphene composite carriers and their electrochemical performance for lithium-sulfur battery.A multifunctional carrier material（Co/Co-Nx@NG）with nitrogen-doped graphene-loaded cobalt nanoparticles and Co-Nx groups was prepared by a simple and efficient in situ one-pot method using dicyandiamide,glucose and cobalt chloride hexahydrate as raw materials and heat treatment at 900℃.The two-dimensional graphene lamellar carrier structures with high electrical conductivity were synthesized under the action of a two-dimensional template guide of dicyandiamide,combined with the low-temperature catalytic graphitization of the loaded cobalt nanoparticles during the heat treatment at 900°C,which facilitated the transportation of electrons as well as ions.The formation of Co-Nx coordination groups on the graphene surface is beneficial to inhibiting the aggregation and growth of cobalt nanoparticles on the surface,and the cobalt nanoparticles and Co-Nx coordination groups exhibit efficient adsorption of polysulfide ions and inhibit the shuttle effect.Electrochemical tests show that the Co-S bond formed between the cobalt-based catalytic active center and sulfur in the multifunctional carrier weakens the bonding between S-S,reduces the activity energy of the sulfur electrochemical reaction and increases the rate of the sulfur electrochemical redox reaction,which improves the electrochemical performance of the lithium-sulfur battery.The lithium-sulfur battery with Co/Co-Nx@NG/S cathode,has initial discharge capacity as high as 1300.3 m Ah g^-1at 0.1C,and exihibits first discharge capacity 884.2 m Ah g^-1at 2C.Moreover,after705 cycles at 2C,the capacity decay rate is only 0.069%per cycle,showing excellent cycling stability.