Synthesis,Characteristics And Electrochemical Performance Of Co₉S₈ Composite Cathode Materials

The increase in the consumption of energy and global warming has accelerated the immense research on portable electronic devices and hybrid electric vehicles.Therefore,the requirement for the development and utilization of new energy sources is increasing exponentially.Lithium-sulfur battery is considered as a potentiale nergy storage device owing to its high theoretical energy density,high specific capacity,low cost and better safety.Nevertheless,the intrinsic insulatiing nature of elemental sulfur,deposition of stable Li₂S₂/Li₂S on electrodes,the volume expansion of sulfur during charge and discharge process,and the“shuttle effect”caused by the shuttling of intermediate polysulfides between positive and negative electrodes lead to the poor electrochemical performance and thus limit its application.In order to conquer these challenges,numerous attempts have been made.In this work,metal sulfide has been synthesized and employed as a solid additive in order to confine the polysulfide and to enhance the electronic conductivity of elemental sulfur in the cathode.Metal sulfide Co₉S₈ and nitrogen doped carbon nanotubes both can significantly adsorb and stabilize polysulfides through chemical force,promote the migration of electron and maintain the stability of structure during the continuous charge/discharge cycles.In this paper,three kinds of lithium sulfur composite electrodes have been designed and prepared to combine the advantages of metal sulfide Co₉S₈ and nitrogen doped carbon nanotubes synergistically.The morphology and electrochemical properties of the composites have been characterized.The following conclusions are drawn:?1?The Co₉S₈/S,which has three-dimensional flower porous composite structure,has been synthesized by one-step hydrothermal and vapor diffusion methods.This structure not only provides space for the infiltration of elemental sulfur,but also can shorten the transmission distance of the lithium ion and electrons so that improves the rate performance of cathode material.Therefore,compared with pure sulfur electrode,the Co₉S₈/S composites of three-dimensional flowered porous structure exhibit good electrochemical performance.?2?The uniformly dispersed microspherical Co₉S₈/S composites cathode materials were synthesized by one-step solvothermal and vapor diffusion methods.Spherical Co₉S₈powder materials can efficiently improve the density and volume ratio capacity of lithium sulfur batteries.In addition,nano-particles can effectively shorten the diffusion path of lithium ions and increase the specific surface area of materials.Thus,the discharge specific capacity and rate performance of the electrode will be improved.?3?The microspherical Co₉S₈@N-CNTs/S composite was prepared by one-step solvothermal and vapor diffusion methods.Co₉S₈@N-CNTs/S composites can combine the advantages of Co₉S₈ and N-CNTs so as to adsorb and stabilize polysulfides strongly and then improve the utilization of active substances.In addition,Co₉S₈@N-CNTs/S shows good electronic conductivity,large surface area.Morever,the hollow structure of carbon nanotubes can also provide sufficient space for carrying sulfur to alleviate the volume expansion of active substances.Consequently,Co₉S₈@N-CNTs/S composites exhibited significantly excellent specific capacity,rate capability and cycling stability.