Synthesis And Properties Of Nano-structured Carbon Spherical Cathode Materials For Lithium-sulfur Batteries

Lithium-sulfur battery has become one of the most promising electrochemical-energy storage systems owing to their high theoretical capacity.Although sulfur has various advantages,such as environmental friendliness,natural abundance and low cost,some serious issue of sulfur like electrical insulating nature,volume change and the loss of polysulfide still needs to be solved.We herein designed nano-structured carbon spherical cathode materials of different structures based on chemical and physical confinement of sulfur,aiming to overcome these issues.The theisincludes the following two parts:(1)Graphene oxide wrapped,multi-cavity,walnut-like carbon spheres.We synthesized graphene oxide wrapped,walnut-like carbon spheres for cathode of Li-S battery.The multi-cavity,walnut-like carbon spheres were obtained through a templated sol-gel process.Sulfur and polysulfides(the discharge intermediate at the cathode)are effectively immobilized in the GO-MCWCS cathode through a hierarchical mode of confinement.The functional groups on GO also help to retain polysulfides by chemical binding.Attributed to the rational design of the GO-MCWCS cathode,our Li-S battery displayed a good rate capability(905 mAh g-1 and 680 mAh g-1 at 1C and 2C,respectively)and an excellent cycle performance(85 % capacity retention and nearly 100 % coulombic efficiency after 200 cycles at 0.5C).(2)Nitrogen-droped nano-structured porous carbon sphere.The nitrogen-doped mesporous carbon sphere(N-MCS)was synthesized by using polyphenoxazine as both carbon source and nitrogen source.Sulfur was impregnated into N-MCSs by melt impregnation method.The N-MCS/S composites exhibit good performance during electrochemical testing.The N-MCS/S composites exhibit an initial capacity of 1072 mAh g-1 and attains 840 mAh g-1 after 120 cycles(85 % capacity retention,a decay of only 0.0425 % per cycle).Owing to the doping of nitrogen,the conductivity of the N-MCS was improved,and the specific capacity still remains at 470 mAh g-1 at a current rate of 5C.The good electrochemical performance is due to the nitrogen doping structure and the main reasons are as follows: firstly,nitrogen doping greatly enhanced the binding energy between polysulfides and carbon;secondly,the N-MCS has a large specific surface area,which greatly increased the contact area of carbon and sulfur;thirdly,the nitrogen doping also improved the electrical conductivity of MCS,so that N-MCS exhibits great electrochemical performance.We designed two different kinds of carbon nanospheres based on the basic idea of physical confinement and chemical adsorption.Both of them showed good electrochemical performance,which provides a new idea for the development of cathode materials for lithium-sulfur battery.