Study On Carbonization Of S-based Materials At Room Temperature And Its Properties

As a light-weight, multi-electrons reaction cathode active material, the elemental sulfur has a theoretical capacity of 1675 mAh/g. The theoretical energy density of lithium-sulfur battery, based on the metallic lithium as anode and elemental sulfur as cathode, can reach up to 2600 Wh/kg. Besides, the elemental sulfur has the advantages of natural abundance, low cost, safety and environmentally friendliness. Therefore, lithium-sulfur battery is considered as a promising high specific energy secondary battery system for the next generation. However, the practical application of lithium-sulfur battery is seriously restricted by the following fundamental problems:(1) electrically and ironically insulated nature of elemental sulfur;(2) solubility of polysulfides produced during the discharge process of sulfur electrode into organic electrolytes;(3) poor electrical conductivity of final reduction product Li2 S, et. al. These problems lead to serious decay of cycle life, low utilization of active material sulfur and poor rate performance for lithium-sulfur battery. In this work, Sulfur was encapsulated in carbon shell through in-situ carbide synthesis in order to overcome the problems mentioned above. The introduction of carbon shell enhanced the conductivity of the cathode material, prevented the loss of active substances, led to a good cycle stability and relatively high utilization of sulfur.(1) Sulfur dispersed solution was prepared by physical method and chemical method, it is demonstrated that the dispersion of Sulfur in polyethylene glycol aqueous is best.(2) To trap sulfur and its intermediate products, and facilitate the transportation of electrons and ions, generate a layer of carbon shell around the sulphur through insitu carbide synthesis. It is demonstrated that carbon on the affinity of sulfur is very poor, and when the water bath temperature is above 50 ℃, the reaction is difficult to control, easy to cause the loss of sulfur.(3) In order to further improve the affinity that the carbon on sulfur, make the grapheme oxide as inter mediate carrier because of its abundantly functional groups in this work. In order to increase the electrical conductivity of the cathode material, graphene quantum dots was introduced. Finally S/GO/C composite of core-shell structure with double shells was prepared by carbonization of sulfuric acid under the room temperature. It is demonstrated that the sulfur content was 77.14 wt%,the approximate size of the spherical composite was 1-4 μm. The initial discharge capacity can reach up to 1200.0 mAh/g and a discharge capacity of 400 mAh/g can still be retained after 50 cycles at 1 C. The synergistic effect of the carbon, graphene oxide and graphene quantum dots can significantly improve the electrochemical performance of the composites electrode. Meanwhile, the electronic conductivity of the electrode system could be improved and the shuttle effect of the electrode actions suppressed, thus leading to high rate charge/discharge performance, good cycle stability and relatively high utilization of sulfur.Starting with cheap sucrose and sulfur materials, we are confident that the work would provide forceful experimental and theoretical basis to design new sulfur-based materials and develop low cost and practical high specific energy lithium-sulfur battery in future.