Preparation And Electrochemical Performance Of Lithium-sulfur Cathode Materials

Lithium-sulfur(Li-S)batteries are one of the most promising rechargeable energy storage systems,due to natural abundance,no pollution for the environment and high theoretical energy density(2600 Wh Kg-1).However,the pristine sulfur cathode have many disadvantages:(1)sulfur and the reduced products are insulating at room temperature;(2)the formation of highly soluble polysulfides,which causes the loss of material;(3)the large volumetric expansion leads to the destruction of cathode structure.All of these problems,which hinder the commercial application of Li-S batteries,have great influence on the specific capacity,cycle life and rate performance.Consequently,the key is to design composite electrode materials,which can enhance the conductivity and suppress the dissolution of polysulfides.Based on the above two key factors,the novel composite materials are prepared to enhance the performance of Li-S batteries.The main study results are listed as follows:(1)Sulfur/carbon composite cathode materials for Li-S battery were prepared by heat treatment with vapor-grown carbon fiber(VGCF),activated carbon(AC)and multi-wall carbon nanotube(MWCNT),which were applied as carrier of sublimed sulfur.Some measurements were employed to analysis the structure and electrochemical performance of the composites.The results showed that the S/CGCF composites delivered initial discharge capacity of 1204 mAh g-1 and remained 547 mAh g-1 after 100 cycles at the current density of 0.1 C.(2)The S/VGCF@SiO2 composites were prepared by coating a silica layer on the surface of S/VGCF composites via sol-gel method.The electrochemical performance of S/VGCF@SiO2 composites was studied by charge-discharge tests.The results indicated that the silica layer can inhibit the dissolution of polysulfides,while the VGCF can enhance the conductivity of cathode material.As a result,the S/VGCF@SiO2 composites remained discharge capacity of 710 mAh g-1 after 100 cycles at the current density of 0.5 C.Besides,the coulombic efficiency was about 97%in the whole charge-discharge process.(3)The mesoporous TiO2 spheres were firstly synthetized by using Titanium isopropoxide and hexadecylamine as titanium source and structure directing agent,respectively.Then the S/TiO2 composites were prepared by mixing sulfur with mesoporous TiO2 spheres through heat treatment.The S/TiO2 composites delivered the initial discharge capacity of 909 mAh g-1 and remained 705 mAh g-1 after 100 cycles at the current density of 0.5 C.The enhanced electrochemical performance of S/TiO2 composites was ascribed to the unique structure of mesoporous TiO2 spheres,in which the mesopores could absorb and inhibit the dissolution of polysulfides.Meanwhile,the big specific surface of TiO2 spheres provided huge reaction interface,resulting the improvement of the conductivity of sulfur cathode.