Synthesis And Electrochemical Properties Of Carbon/sulfur Composite Cathode For Lithium-sulfur Batteries

With the diversification of electronic products and electric vehicles,new requirements for energy storage system are put forward by modern society,such as low price,long cycle life,high stability,both high energy density and power density,environmentally friendly,etc.Based on the above requirements,lithium-sulfur battery is widely regarded as one of the most promising next-generation lithium secondary batteries.However,several shortcomings of lithium-sulfur battery hinder the commercialization process,these shortcomings include: low conductivity of elemental sulfur and lithium sulfide,large volumetric expansion and “shuttle effect” during the charge and discharge process,low sulfur content and mass loading,the growth of lithium dendrite,etc.For resolving the above problems,carbon/sulfur composite cathode material prepared by electrodeposition method and TiO2 nanotubes/sulfur composite material are discussed from the aspects of preparation method of composite material and the special structure of carrier material.The relationship among the preparation process,microstructure and electrochemical properties were discussed.The main contents are as follow:(1)Kapok fiber carbon nanoflakes(KFCNF)were successfully prepared by high-temperature calcination method using kapok fiber as carbon source.The effects of electrodeposition and thermal diffusion methods for sulfur storage on the microstructure and electrochemical properties of the resultant carbon/sulfur composites were discussed.It is found that small sulfur particles are evenly distributed on the surface of KFCNF by electrodeposition method(KFCNFS-E).However,sulfur layer and carbon are glued together by thermal diffusion method(KFCNFS-D).The KFCNFS-E cathode delivers initial discharge capacity of 1199.7 mAh g-1 and remains at 623.0 mAh g-1 after 200 cycles at a current density of 100 mA g-1.The sulfur particles formed by the electrodeposition have higher utilization rate and stronger chemical bonding with the carbon,which can suppress the loss of sulfur.(2)The titanium dioxide nanotubes produced by different anodic oxidation time were prepared as sulfur carrier materials.Composites were prepared by thermal diffusion process and marked as TiO2/S-1h,TiO2/S-3h,Ti O2/S-5h.The effects of different anodic oxidation time for sulfur storage on the microstructure and electrochemical properties of the composites were discussed.It is found that the morphology of TiO2 nanotubes in the composites Ti O2/S-3h is neat and regular,the diameter of the nanotubes is uniform and sulfur is evenly distributed in nanotubes.The diameter of the TiO2/S-1h is irregular due to the fact that anodic oxidation time is too short and the sulfur is covered on the surface of the nanotubes.The nanotube structure of TiO2/S-5h was destroyed due to the anodic oxidation time is too long.The TiO2/S-3h delivers initial discharge capacity of 734.3 mAh g-1 with 63.9% capacity retention after 200 cycles at a current density of 100 mA g-1.Compared with the other two samples,the TiO2/S-3h shows better cycle stability and rate performance.It is because that the structure of TiO2/S-3h is intact and sulfur is uniformly stored in the nanotubes,which suppresses the volumetric strain and the dissolution of lithium polysulfide.