Application Of Modified Carbon Nanotubes Microspheres In Cathode Materials Of Li-S Batteries

With the increasing environmental and energy problems and the rise of mobile electronic devices and electric vehicles,the demand for efficient and economical energy storage devices is growing.However,present commercial lithium ion batteries are difficult to meet the urgent demand for high energy density due to their low theoretical capacity and limited metal Li resource.Lithium sulfur battery has a high theoretical energy density?2600 Wh/kg?.Besides,non-metallic sulfur,which is positive electrode material,owns abundant natural resource and low cost,and the product of sulfur is environmentally friendly.Therefore,lithium sulfur battery has become a research hotspot in the field of energy storage.However,because sulfur has poor conductivity,and the intermediate product polysulfide of charging/discharging process is easily dissolved in the electrolyte,which is resulting in the"shuttle effect",the low utilization rate of active sulfur and coulomb efficiency,and the cycling stability of the battery needs to be strengthened.Therefore,it is the key to solve these problems,which lies in the design of host materials that can improve the conductivity and electrochemical stability of sulfur positive materials.In this paper,a series of carbon/sulfur composites were prepared by the construction of special structures and the optimization of surface properties,and their electrochemical properties were systematically analyzed to explore the relationship between structure and performance.The main research contents of this paper are as follows:?1?A kind of Mn₃O₄-CNTs microspheres with special structure were prepared by ultrasonic spray method and impregnation method.The character of the material was analyzed through X-ray diffraction,scanning electron microscope and transmission electron microscope.The results showed that Mn₃O₄-CNTs microspheres are wool spherical porous structures,which is formed via carbon nanotubes twining containing Mn₃O₄ nanoparticles.This special structure can provide sufficient space for sulfur storage and fixation.At the same time,Mn₃O₄ nanoparticles form strong chemical bonds with polysulfide,which can inhibit the diffusion of polysulfide into the electrolyte during charging and discharging and improve the cycling stability of the battery.The electrochemical performance test results showed that Mn₃O₄-CNTs microspheres had excellent electrochemical performance:the first charging and discharging capacity at 0.1 C was 1216 and 1227 m Ah/g,and after 200 cycles at 0.2 C,the reversible discharge capacity was retained at 1087 m Ah/g.?2?Nitrogen-doped wool spherical carbon nanotube microspheres?NCNTs?were prepared by a simple ultrasonic spray method.To prepare Nb₂O₅-NCNTs and Sb-NCNTs microspheres by impregnation with different metal salt solutions.Because Nb₂O₅ has a better modification effect than Sb,Nb₂O₅ nanoparticles can penetrate into the pores and interior of hollow nanotubes of NCNTs microspheres,while Sb particles only exist on the surface of NCNTs microspheres.The Nb₂O₅-NCNTs microspheres had a high first charge and discharge capacity?1369.5 and1335.3 m Ah/g,respectively?.The reversible capacity was 1201.7 m Ah/g after 200 cycles at 0.1 C,showing good cyclic stability.The capacity of846.2 m Ah/g is retained after 1000 cycles at 2.0 C,and the capacity attenuation rate of each cycle is only 0.016%,which is an outstanding long cycle life.This is because the spherical porous structure of Nb₂O₅-NCNTs microspheres provides sufficient space for the load of nano-sulfur particles.Besides it can effectively limit the volume expansion of elemental sulfur during the reaction process.At the same time,both nitrogen atoms and Nb₂O₅ nanoparticles can provide strong chemisorption to polysulfide,which inhibit the shuttle effect and improve the electrochemical performance of the battery.?3?Using sucrose,graphene and water-soluble carbon nanotubes as carbon sources,three-dimensional fractional nitrogen-doped porous carbon nanomaterials N-GCNTs microspheres were prepared by one-step ultrasonic spray method.They were named according to the raw material ratios of 1:1:1,1:3:1 and 1:5:1,respectively:N-GCNTs-1,N-GCNTs-2and N-GCNTs-3.As the carbon nanotubes and graphene are wound together under the action of ultrasound,the N-GCNTs-2 microspheres present a three-dimensional porous structure with abundant N atoms,while maintaining the structural characteristics of one-and two-dimensional carbon nanomaterials.In contrast,N-GCNTs-1 microspheres showed fragmentation microspheres,while N-GCNTs-3 microspheres hardly found CNTs.The excellent physical characteristics of N-GCNTs-2microspheres put up the large specific surface area and pore volume as well as the high sulfur load.In addition,the N-GCNTs-2 microspheres had a high initial discharge capacity?1465.1 m Ah/g?at 0.1C,and a reversible capacity of 1315.1 m Ah/g after 200 cycles,and the capacity of849.1 m Ah/g after 500 cycles at 2.0 C.The N-GCNTs-2 microspheres reveals outstanding long cycle performance and excellent cycle stability due to the three-dimensional layered porous structure and successful nitrogen atom doping.