Preparation And Electrochemical Performance Of Polypyrrole Coated Sulfur/Carbon Nano Cathodes

Lithium-sulfur batteries are favored by researchers for its higher energy density than lithium-ion batteries which have been commercially applied in the market.However,the practical application of the lithium-sulfur battery at the current stage has been greatly restricted because of those disadvantages in the poor reactivity of the active material sulfur,the large volume changes during the cycle and the susceptibility of the polysulfide to be easily dissolved in the electrolyte.On the one hand,for those disadvantages,this paper selects the well-selectively conductive acetylene black(AB)and multi-walled carbon nanotubes(MWCNTs)materials to improve the conductivity of the sulfur/carbon composites and enhance the material's rate performance;on the other hand,the carbon/sulfur composite material is coated with a layer of polypyrrole(PPY),so as to effectively relieve volume changes and shuttle effect during charge and discharge.In the end,the electrochemical performance of lithium-sulfur battery cathode materials was improved by improving sulfur conductivity and reducing polysulfide dissolution.The specific contents and main conclusions of the paper are as follows:(1)Preparation of acetylene black/sulfur(AB/S)composites by physical sulfur deposition,and explore the influence of different sulfur contents on the structure and electrochemical properties of anode materials.The results showed that when the sulfur content was 50%and the discharge rate was 0.2c,the specific capacity of 100 cycles was still 740 mAh/g.In order to strengthen the acetylene black sulfide adsorption of polysulfide,acid treatment of AB is treated,increasing the specific surface area at the same time,introducing carboxyl,hydroxyl in the surface oxygen groups,such as with the sulfur compound,and characterization of its electrochemical performance.The results showed that the discharge specific capacity of 100 cyclic anode materials was 783.5 mAh/g,and the specific capacity increased by 43.5 mAh/g at the ratio of 0.2c.In order to further improve the electrochemical performance of positive electrode materials in AB/S composite coated on the surface of a layer of PPY,characterization of its electrochemical properties found:under 0.2 C ratio,the cycle after 100 times a specific capacity of 884.5 mAh/g.After coating PPY layer,the electrochemical properties of composite materials have been greatly improved.(2)The more excellent performance and better relative adsorption of carbon nanotubes as the carrier,the preparation of carbon nanotubes(MWCNTs/S)/sulfur composites,sulfur content and explore different influence on the anode material structure and electrochemical performance.The results showed that when the sulfur content was 50%and the discharge rate was 0.2c,the discharge capacity of anode materials after 100 cycles was 668.3 mAh/g.After acid treatment and modification of MWCNTs/S anode material,the results showed that the specific capacity was still 715.2 mAh/g after 100 cycles at the sulfur content of 50%and the ratio of 0.2c.In order to further improve the cycling stability of anode materials,a layer of PPY is coated on the surface of MWCNTs/S cathode materials.The results show that the specific discharge capacity is still up to 986.3 mAh/g after 100 cycles at a rate of 0.2c.The electrochemical properties of composite materials have been greatly improved,mainly because of the synergistic effect of MWCNTs and PPY.Compared with acetylene black,carbon nanotubes have better conductivity and adsorption.