Study Of Preparation And Electrochemical Performance Of Sulfur/starch-based Carbon Microsphere Composites As Cathode Material For Lithium-sulfur Battery

Lithium-sulfur batteries are highly valued by researchers because of their high theoretical capacity(1675mAh/g)and high energy density(2600Wh/kg),and the sulfur in nature are very rich,so lithium-sulfur batteries are considered one of the most promising next-generation cathode materials for lithium-ion batteries.However,lithium-sulfur batteries also face many challenges.Firstly,the formation of polysulfide in the charge and discharge process dissolves in the electrolyte and it seriously affects the utilization of sulfur.Secondly,the insulation of sulfur and intermediate polysulfides leads to a large internal resistance of the lithium-sulfur battery and it also cause a rate decrease in the charge and discharge.In addition,the volume of Sulfur swells severely during the cycling,which results in structural damage to the positive electrode material.Therefore,further research on lithium-sulfur batteries is needed to overcome these problems.In this paper,starch-based porous carbon microspheres were prepared by high-temperature carbonization and potassium hydroxide activation using edible corn starch as raw material.Then the sulfur/starch-based porous carbon composite was prepared by hot-melting with sulfur in a tube furnace.The effect of sulfur content on the electrochemical performance of sulfur/starch-based porous carbon composites was studied.And The influence of polyaniline as a coating on the electrochemical performance of sulfur/porous carbon composites was further investigated.Firstly,a uniformly dispersed starch-based carbon sphere was prepared under the condition of carbonization at a temperature of 600 °C,and pores were then activated by KOH 800 °C to prepare a uniformly dispersed starch-based porous carbon sphere.The study of microstructure of the sample have shown that the original morphology of corn starch particles was maintained of starch-based carbon spheres and starch-based porous carbon spheres with an average particle size of about 2-10 microns.Electron micrographs showed that the activation of pores did not affect the original morphology of the starch-based carbon spheres,and the pore formation was uniform.The starch-based porous carbon spheres have an average pore size of 1.95 nm.The prepared starch-based porous carbon spheres were then compounded with sulfur at different temperatures to prepare sulfur/porous carbon composites with different sulfur contents S/ACS(155°C),S/ACS(250°C),S/ACS(300°C),S/ACS(330°C).The S/ACS(300°C)showed the best rate performance among these three samples,and the sulfur content was 60.9% wt.The specific capacities of S/ACS(300°C)at 100,200,and 500 mA/g charge/discharge rates were 478,332,and 229 mAh/g,respectively.When the current density returned to 100 mA/g,the specific capacity increased to 354mAh/g.The results show that the S/ACS(300°C)composite showing good electrochemical performance,because it has moderate sulphur content,with no residual sulfur on the surface,and the sulphur in the pore is effectively used.Secondly,Polyaniline was used as a coating on S/ACS composites,the electrochemical properties of the coated composites are significantly improved when compared with untreated S/ACS(300°C)composites and blank carbon spheres.The reversible capacities at 320,480,800,1600 and 3200 mA/g were 687,582,504,393 and 350 mAh/g,respectively.When the current density returns to 320 mA/g,the specific capacity can reach 514 mAh/g.After 100 cycles at a current density of 800 mA /g,the discharge capacity can be maintained at 448 mAh/g,which showed an excellent cyclic stability and rate performance.Finally,we tried to study the effect of aniline on the electrochemical performance of S/ACS composites by changing the amount of aniline coating.The results showed that when the amount of aniline coating was increased from 1g to 4g,the sulfur content of the prepared S/ACS/PANI-C composite material decreased gradually,and the aniline carbon coating gradually became thicker.When the aniline coating amount was 1g,When aniline/S/ACS complex = 1:1,It showed the best electrochemical properties of the prepared S/ACS/PANI-C composites,which indicates that the polyaniline coating plays a key role in the electrochemical performance of S/ACS/PANI-C composites.