The Preparation And Electrochemical Performance Of Sulfur-based Composites As Cathode Materials For Lithium-sulfur Battery

As a light-weight, multi-electrons reaction cathode active material, the elemental sulfur has a theoretical capacity of1675mAh/g. The theoretical energy density of lithium-sulfur battery, based on the metallic lithium as anode and elemental sulfur as cathode, can reach up to2600Wh/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 pratical 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 Li2S, 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 incorporated into conductive carbon black to prepare a series of sulfur-based composites in order to overcome the problems mentioned above. The introduction of conductive carbon black with excellent electrical conductivity, high specific surface area and good absorption capability is expected to improve the conductivity of sulfur-based composites, effectively suppress the dissolution and shuttle of intermediate products, thereby improving the utilization of sulfur active material and also improving cycle performance and rate performance of sulfur electrode.Firstly, commercial conductive carbon black Ketjenblack EC600JD was used as matrix to prepare sulfur/carbon composites by ball-milling sulfur and conductive carbon black in a certain proportion and a subsequent heat treatment. Then, polyaniline coated sulfur/carbon multi-composites(PANI@S/C) with unique core/shell structure were successfully prepared by an in-situ chemical oxidative polymerization method. It’s demonstrated that the PANI@S/C composite with43.7wt%sulfur presents the optimum electrochemical performance. The conductive polyaniline with a thickness of ca.5-10nm, is uniformly coated onto the surface of the S/C composite to form a core/shell structure, while sulfur is highly dispersed in the nanopores of conductive carbon black matrix. The initial discharge capacity can reach up to1405.5mAh/g and a discharge capacity of596mAh/g can still be retained after100cycles at1C. Even at10C rate, a maximum discharge capacity of635mAh/g could be obtained for the composite, and the capacity retention is over60%after180cycles. The synergistic effect of the conductive carbon black and polyaniline 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 excellent high rate charge/discharge performance, good cycle stability and relatively high utilization of sulfur.In order to further improve the sulfur loading in the composites, commercial conductive carbon black BP2000(CCB) was used as raw material and KOH as pore-expanding agent to prepare activated conductive carbon black(A-CCB) material with high specific surface area and large pore volume by activating at high temperature. Then, sulfur/activated conductive carbon black(S/A-CCB) composites with64wt%and73wt%sulfur, and sulfur/conductive carbon black(S/CCB) composite with65wt%sulfur were further prepared by two-step heat treatment, and the electrochemical performance for the composites were comparatively investigated. It’s demonstrated that the specific surface area and pore volume of commercial CCB were significantly improved after activation. The S/A-CCB composite with64wt%sulfur shows the optimum electrochemical performance.. The initial discharge capacity of S/A-CCB composite with64wt%sulfur is956.7mAh/g and can be maintained at531.9mAh/g after100cycles at the current density of160mA/g. The maximum discharge capacity of771.8mAh/g can still be obtained after several cycles’activation at the current density of800mA/g, indicating a satisfactory rate performance. However, the results show that the employment of CCB as conductive matrix or the increase of sulfur loading in S/A-CCB composite, under the same condition, would lead to a poorer electrochemical performance for sulfur electrode.Finally, considering that both sulfurized polyacrylonitrile and sulfur/carbon composites can be served as cathode materials for lithium-sulfur battery, to construct multi-composites, sulfur/carbon/polyacrylonitrile ternary mixtures were firstly obtained through dissolution-precipitation process by still using conductive carbon black BP2000as matrix. Then, the mixture were pre-calcinated and heat treated at different calcinating time to prepared three sulfur/carbon/sulfurized polyacrylonitrile composites(S/C/sPAN). The CVs and charge/discharge performance of the as-prapared composites were further characterized in1M LiPF6(PC+EC+DEC, v/v/v=1:4:5) carbonate-based electrolyte and1M LiTFSI+0.2M LiNO3(DOL+TEGDME, v/v=1:1)ether-based electrolyte, respectively. The results show that the three samples demonstrate different electrochemical response characterizations in the two electrolytes. By comparison, the S/C/sPAN-7composite with7h heat-treatment shows the optimum electrochemical performance in carbonate-based electrolyte. The initial discharge capacity of1103.4mAh/g can be obtained at the current density of40mA/g when calculated by the whole composite as active material, and the discharge capacity can be maintained at350mAh/g after40cycles. Moreover, the discharge capacity of243.5mAh/g can be maintained after200cycles when the current increased to100mA/g, and the capacity retention can reach up to77%from the second cycle. The composite can also show good cycle performance at300mA/g, together with ca.100%charge/discharge coulombic efficiency.In summary, in this paper, two novle sulfur/carbon/polymer multi-composites were designed and synthesized by employing commercial conductive carbon black as matrix. The relationship between the structures and electrochemical performance of the composites were then discussed. In addition, the activated conductive carbon material with high specific surface and pore volume was also prepared using conductive carbon black as raw material, and the sulfur-based composites with good electrochemical performance were obtained. Starting with stable and commercial carbon 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.