Synthesis And Electrochemical Properties Of Carbon/sulfur Composites As Cathode Materials For Li-s Rechargeable Batteries

Sulfur with high theoretical capacity(1675 m Ah/g) and specific energy(2567 Wh/kg) has been widely used as one of the most attractive cathodes. Meanwhile, sulfur is low cost, abundant and environment friendliness. However, many significant drawbacks retard the industrialization and application of lithium-sulfur batteries. The low efficient of the active material and dissatisfactory cycle capability are due to the electrical insulating nature of sulfur and dissolving of polysulfide intermediates. To overcome the mentioned obstacles, we mainly worked on the study of the cathode materials and prepared a series of porous carbon/sulfur composites.A series of porous carbon materials with different structures were synthesized using eggshells as carbon source. At 800 oC, the obtained carbon material revealed the most outstanding morphology. The cathode prepared by melt-diffusion method with sulfur loading of 66% delivered a high initial discharge capacity of 1288.4 m Ah/g and remained high up to 758 m Ah/g after 80 cycles at 100 m A/g. The electrochemical performance of the cathode differed from the weight ratio of carbon/sulfur. As the sulfur loading reduced to 57%, the composite delivered a high initial discharge capacity of 1293.1 m Ah/g and remained high up to 895 m Ah/g after 80 cycles at 100 m A/g.Nitrogen-doped porous carbon materials with different structures were synthesized using polyaniline as carbon source at different temperatures. 700 oC was the best carbonization temperature for the preparation of porous carbon. The cathode prepared by melt-diffusion method containing 58% sulfur delivered a high initial discharge capacity of 1626.8 m Ah/g and remained 1086.3 m Ah/g after 50 cycles at 100 m A/g current density. The electrochemical performance of the cathode differed from the weight ratio of carbon/sulfur. As the sulfur loading increased, the capacity faded rapidly. The composite with 75% sulfur loading delivered a high initial discharge capacity of 1300 m Ah/g and only remained 787.2 m Ah/g after 50 cycles at 100 m A/g.In the process of preparing MWCNT/S composite, the treatment temperature of MWCNT was 800 oC. It was proved that using the conductive PPy to coat MWCNT/S composite could suppress the dissolution of polysulfides effectively and confined a relatively high amount of sulfur. The MWCNT/S composite coated by PPy delivered a high initial discharge capacity of 1649.7 m Ah/g and retained the reversible capacity of 1010 m Ah/g after 60 cycles at 100 m A/g.