Study On The Preparation And Modification Of Cathode Sulfur/Carbon Microsphere Composites For Lithium-sulfur Batteries

In recent years,with the growing depletion of oil and other energy,as well as the increasingly serious environmental pollution problems,people have began to look for new environment-friendly energy to replace oil.Therefore,the development of environment-friendly and high performance energy conversion and storage equipment has become particularly important.The secondary lithium-sulfur battery with the elemental sulfur as the positive electrode has a high theoretical specific capacity?1675 mAh/g?,and the elemental sulfur is low cost,resource-rich and environmentally friendly,so the secondary lithium-sulfur battery is expected to become one of the most promising application prospects of the lithium battery system.However,the poor conductivity of sulfur itself,the defects of the solubility of active substances and volume expansion restrict its practical process.To overcome the problems mentioned above,we mainly focused on the synthesis of cathode materials to start a series of studies.In this paper,the nano-carbon sphere?CS?was prepared via a simple hydrothermal method and high temperature treatment.At the same time,CS was actived by KOH to obtain the MPCS with a high specific surface area?1216.6 m2/g?and a larger pore volume?0.87 cm³/g?.The synthesized CS and MPCS materials spherical particles with a diameter of about 400 nm.As seen from the results of electrochemical test,the capacity and cycle performance of the S/CS and S/MPCS composite materials improved significantly compared to the elemental sulfur as the cathode material.At the rate of 0.1 C,the first discharge capacity of S/MPCS composites is 1086.6 mAh/g,and the S/MPCS composites have the best cyclic performance.The MPCS material exhibits an outstanding adsorptive effect on sulfur,but there are still a certain amount of elemental sulfur on the surface is in still direct contact with the electrolyte and easy to dissolve into the electrolyte,and resulting in loss of active material.So we continue to make surface coating modifications to the S/MPCS composite material.The SiO₂ layer was coated on the S/PMCS materials by the St?ber method to further inhibit the direct contact between sulfur and the electrolyte.The effect of coating amount on the electrochemical properties of the composites was studied.The results showed that SiO₂@S/MPCS-2 with SiO₂ cladding of about 10% exhibited higher reversible specific capacity and better cyclic stability,initial discharge capacity as high as 1040.7 mAh/g at 0.1 C,and its reversible capacity was 615 mAh/g after 100 cycles.The SiO₂@S/MPCS-2 composites show better magnification performance,after recovering by 0.1 C,the specific capacity is 80% of the initial capacity,and the discharge capacity is 510 mAh/g at 2 C.The SiO₂ solid shell can effective adsorption disulfide,enhanced ion transport,improve the utilization rate of the active materials.What is more,it could effectively alleviate the volume change of the charge and discharge process,thus improving the cycle performance of lithium-sulfur batteries.