Preparation And Property Of Mesoporous Carbon/Sulfide Composites For Lithium-Sulfur Batteries

Elemental sulfur shows almost the highest theoretical capacity among all known cathode materials for rechargeable lithium batteries.However,it exhibits poor rechargeability due to the fact that the discharge products are easily dissolved in electrolyte and diffuse to anode where they react directly with the lithium.In order to overcome the above mentioned problem,four methods were tried in this thesis:1.A solvothermal synthetic route has been successfully implemented to prepare mesoporous MnS microspheres with manganese acetate as the source of manganese,elemental sulfur as the sourse of sulfur and Poly(vinyl-pyrrolidone)(PVP)was used as lead agent and reducing agent.When charged and discharged with constant current,the discharge capacity reached as high as 1253.7 mA h g-1 at the first cycle,which is much higher than the theoretical capacity of MnS.The reason is that part of MnS is oxided to elemental sulfur.The second discharge specific capacity was 180.5 mAh/g,the charge capacity was 175.9 mAh/g.After 20 cycles,cyclic capacity was 140 mAh/g.The cell showed good reversibility and cycle stability.2.The composites of MnS@BC was synthesized by solvothermal synthetic method with manganese acetate as the source of manganese,elemental sulfur as the sourse of sulfur,and commercial bamboo charcoal as host.The composites of S-BC?S-MnS@BC were prepared by heat treatment.The results of SEM showed that MnS and elemental sulfur can be well loaded in the holes of bamboo charcoal.When S-BC was used as active substance in cathode,the specific discharge capacity was 754.8 mAh/g in the initial cycle and the utilization rate of active substance was 40.5%.After 50 cycles,the specific discharge capacity of the cell remained 107 mAh/g.When S-MnS@BC was used as active substance in cathode,the specific discharge capacity was 1346.1 mAh/g in the initial cycle and the utilization rate of active substance was 80.5%.After 50 cycles,the specific discharge capacity of the cell remained 504 mAh/g.3?The Mesoporous carbon was prepared by a template and was used as the carrier of active substance.S-MC composites was synthesized by heat treatment,then it was immersed in a PMA(1wt%)solution prepared beforehand for 12 hours.The composites was dried to obtain S-MC@PMA.The samples were characterized by X-ray powder diffractions(XRD)?Scanning electron microscopy(SEM).The results showed that these carbon matrixes had two dimension ordered structure,which was porous and hexagonal.The particle size distribution of these carbon particles was uneven.Assembling cell was charged and discharged at the current of 100mA/g.The specific discharge capacity of the S-MC composite was 928.6 mAh/g in the initial cycle,the utilization rate of active substance was 55.5%.The specific discharge capacity remained 456.7 mAh/g after 50 cycles.When S-MC@PMA was used as active substance,the specific discharge capacity was 1447.3 mAh/g in the initial cycle and the utilization rate was 88.6%.After 50 cycles,its specific capacity still maintained 618.6 mA h/g.4.The interlayer be prepared by carbonization of filter paper using sulfuric acid as carbonizer at 100?.The morphology of the interlayer were examined by SEM,it showed that the morphology of carbonized samples was the same as that of uncarbonized samples.When S-MnS@MC was used as cathode and the interlayer was added to the cell,the capacity retention of the cell increased from 25.6%to 54.1%and the charage transfer resistance decreased from 692.2? to 69.36?.The decreasing value was larger than that without interlayer.It showed that the reversibility of electrode improved.