Synthesis And Electrochemical Properties Of The Cathode Materials For Lithium-sulfur Batteries

Lithium-sulfur(Li-S) batteries with high theoretical specific capacity(1675 mAh/g)and theoretical specific energy(2600 Wh/kg) have drawn a great attention, and are expected to be one of the most potential secondary batteries in the future. However, Li-S batteries faced with many problems, including the poor electrical conductivity of sulfur,shuttle effect caused by the intermediate, and the volumetric changing of sulfur cathode during the cycle process. These problems affected the utilization of active material and the cycle life. To work out these issues, this paper mainly studied the complex method of graphene and elemental sulfur, and the carbon-sulfur ternary composites with different structures.Reduction graphene oxide was prepared with the improved Hummers method. With reduction graphene oxide as the carbon carrier, use three different methods to prepare the graphene/sulfur composite materials. Through the characterization of physical structure and electrochemical performances, it was found that the graphene/sulfur composite with chemical synthesis+secondary heat treatment exhibited the most excellent electrochemical performance. The graphene@sulfur-160 cathode with a high sulfur content of 73%displayed a discharge capacity of 953.9 mAh/g at the current density of 200 mA/g, and held a stable capacity of 591.7 mAh/g after the charge-discharge of 80 cycles. The capacity retention rate reached 64.76%.A novel acetylene black/sulfur@graphene composite with unique three-dimensional sandwich structure was successfully prepared. The low-cost acetylene black as conductive carbon skeleton was used to deposit sulfur in situ. Commercial grade of graphene coated the acetylene black/sulfur composite. Compared with the acetylene black/sulfur composite,the acetylene black/sulfur@graphene cathode exhibited a high utilization of sulfur, an enhanced cyclical stability and an excellent rate capability. The cathode materials with sandwich construction exhibited a high initial discharge capacity of 1603.5 mAh/g at the current density of 200 mA/g, corresponding to a high utilization of sulfur up to 95.73%.The rate capability was more outstanding. When the current density rose to 1600 mA/g,the capacity could be stable at 617.2 mAh/g.The acetylene black/sulfur@polypyrrole composites with core-shell structure were successfully prepared. Acetylene black as conductive carbon skeleton was used to deposit sulfur in situ. Polypyrrole was used as the coating layer for acetylene black/sulfur composite. By comparing the acetylene black/sulfur composite with acetylene black/sulfur@ polypyrrole composite, it was concluded that the ternary composite with a core-shell structure obtained an improved electrochemical performance. The acetylene black/sulfur@polypyrrole cathode obtained a higher discharge specific capacity of 688.9mAh/g at a current density of 800 mA/g after undergoing the activation. In addition, the cathode maintained a stable capacity of 538.5 mAh/g after the charge-discharge of 200,which meant that the capacity retention rate was as high as 78.2%.