Preparation And Electrochemical Properties Of Sulfur-carbon Composites As Cathode Materials

Sulfur is the light-weight element as cathode materials, which can react completely with metallic lithium to form Li2S with two-electron reaction, leading to high theoretical capacity of 1675 mAh/g and high theoretical energy density of 2600 Wh/kg in lithium/sulfur (Li/S) battery. Besides, sulfur has an advantage of natural abundance, low cost, and environmental friendliness. Therefore, sulfur would be promising cathode materials for the next generation of high energy density rechargeable batteries. However, there are some problems of sulfur cathode, which are mainly attributed to the highly insulating nature of sulfur and the high solubility of lithium polysulfides as intermediate products generated during the electrochemical process in organic electrolyte. Meanwhile, the reduced lithium polysulfides can also diffuse back to the cathode in the following oxidation process leading to the serious lithium anode corrosion. As a result, the poor cycle stability is generally observed due to the solubility of lithium polysulfides and gradual aggregation of insulated Li2S on the cathode surface. To overcome such problems, sulfur was incorporated with acetylene carbon black, meso-porous super conductive carbon black and microporous carbon spheres in this work. The effects of the porous structure of carbon materials on the electrochemical performance of sulfur cathode were investigated.Firstly, the sulfur-acetylene black composites with different sulfur content were prepared by thermal method. The electrochemical performances of the composite were investigated in different electrolytes, and the appropriate electrolyte and sulfur content in the composite were optimized. Meanwhile, the electrochemical performance of the optimized composite was investigated in detail. It is shown that sulfur can be embedded into the nanopores of acetylene black and the electrochemical cycle stability of the sulfur cathode is improved efficiently.To increase the sulfur content in the composite, the super conductive carbon black with large specific surface area was chosen as a matrix to incorporate sulfur by thermal method. It is demonstrated from pore structure analysis and electrochemical measurements that the effects of the sulfur content on the electrochemical performance of the sulfur cathode are different. In particular, when the sulfur content of more than 56 wt% or lower than 38 wt% is loaded in the composites, the electrochemical performance of the sulfur cathode is poor. It is also shown that the pore size of super conductive carbon black is about 3.7 nm, far more than the radius of sulfur atoms (S8). Therefore, the sulfur only contacted with the mesoporous wall of the super conductive carbon black can be reduced in the electrochemical process. Furthermore, the sulfur beneath is inactive and become a barrier for the further diffusion of lithium ions into/out the pores, resulting in the low utilization of the sulfur active material.Finally, the elemental sulfur was embedded into the narrow micropores (0.7 nm) of carbon sphere. Correspondingly, the electrochemical reaction of the sulfur cathode can be restricted inside the narrow micropores of carbon spheres, resulting in the good reversibility and excellent high rate discharge capability of the composite. At the low current density of 40 mA/g, the larger initial discharge capacity of 1333 mAh/g (near 80% of the theoretical capacity) and reversible capacity of 1006 mAh/g are presented with good cycle stability. The large reversible capacity of 730 mAh/g is still obtained at the high current density of 1200 mA/g. In particular, the sulfur-microporous carbon sphere composite presents the large reversible capacity of about 650 mAh/g after 500 cycles at the current density of 400 mA/g. It is concluded that the long electrochemical cycle stability and good high-rate discharge capability of sulfur cathode can be enhanced significantly by encapsulating sulfur into narrow micropores of carbon spheres.Based on the different porous structure of carbon materials and the electrochemical performance of the sulfur cathode, it is demonstrated that the porous structure of carbon materials has a great impact on the electrochemical performance of sulfur cathode. The excellent electrochemical cycle stability and high utilization of sulfur cathode can be obtained by embedded sulfur into the micropores rather than mesopores of carbon materials. Especially,the lithium polysulfides can be restrained inside the micropores of carbon spheres, due to the strong adsorption, excellent conductivity and large surface area of the microporous carbon spheres. The knowledge acquired in this work is important not only for the design of efficient new composites, but also for the development of the lithium/sulfur battery system.