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

The resources of the earth are decreasing nowadays,as a result,the development of new generation of energy has become a contemporary inevitable requirement.As new generation of energy storage device,lithium ion battery has been concerned and studied by many researchers.However,the current cathode materials for lithium ion batteries are still restricted by the high manufacturing cost,small capacity and low energy density,which limit their further promotion and application.At the same time,the appearance of lithium sulfur battery makes up for the shortage of current lithium ion battery,and provides a much widely application potential for lithium ion battery.However,the inherent insulation property of lithium sulfur battery,shuttle effect and volume expansion effect in the charging and discharging process have caused much damage to the electrode material structure of lithium sulfur battery,resulting in serious loss of electrode material and poor cycling performance.In view of the problems,we mainly prepared morphology-controlled porous carbon,various size of sulfur materials and optimized the carbon-sulfur composite process,so as to optimize the performance of lithium sulfur batteries.The specific research contents and results are as follows:?1?With fallen leaves served as precursor,porous carbon materials with controllable morphology were successfully prepared by simple experimental steps under the activation of phosphoric acid.A series of porous carbon materials with good specific surface area and pore volume were prepared under different ratios of phosphoric acid and carbon precursor.The PC-31 and PC-61 porous carbon materials were compound with elemental sulfur and the electrode was then assembled.S@PC-31 and S@PC-61 have respectively showed specific capacity of 1050 and1270 mAh/g at 0.1C.After 300 cycles at 0.1C,71%and 78%capacity retention of S@PC-31 and S@PC-61 are obtained,which indicate a modified cycling performance.?2?A series of sulfur materials with different sizes and morphology were prepared by reaction of Na₂S₂O₃·5H₂O and H₂SO₄,that contains spherical sulfur material for 100 nm,flower-like sulfur material for 300 nm,spherical sulfur material for 1?m.Spherical sulfur material for 100 nm has a smaller size,which can effectively improve the composite effect of carbon and sulfur materials.The flower-like sulfur material for 300 nm has a higher specific surface area and can be much more fully contacted with the carbon material to improve the overall conductivity of the electrode material.Therefore,among the prepared sulfur materials,100 nm spherical sulfur material and 300 nm flower-like sulfur material were selected for further research of electrochemical property.The electrochemical properties were tested after composite with porous carbon and electrode assembly,spherical sulfur material for 100 nm composite and flower-like sulfur material for 300 nm composites have respectively showed specific capacity of 1360 and 1450 mAh/g at 0.1C.The results of electrochemical performance test showed good rate capability and cycling performance.After 300 cycles at 0.1C,81%and 83%capacity retention of spherical sulfur material for 100 nm composite and flower-like sulfur material for 300nm composite are obtained,which indicates that the cycling performance of the electrode is improved.?3?To improve the electrical conductivity of the whole electrode materials and modify the electrochemical performance of lithium sulfur batteries,elemental sulfur materials are combined with other high-electrical materials.Carbon and sulfur composite plays an important role in the related modification.The porous carbon material generally has good electrical conductivity,which was possessed by carbon materials.In addition,it also has abundant pore structure with larger specific surface area and pore volume.The micropores and mesopores in the material can fix sulfur in the charging and discharging process of lithium sulfur battery,and also provide a more convenient channel for the transmission of lithium ion,which effectively promotes the transmission rate of lithium ion.Through literature research,it is found that evident differences stand in the composite process of carbon and sulfur materials.In the meanwhile,there are few relevant research on the composite process,for that,we used the same carbon and sulfur materials to get the electrode composite materials.With PC-61 served as the matrix,elemental sulfur served as the sulfur source,the influence of different composite methods on the performance of lithium sulfur battery was studied by comparing several common composite methods,such as carbon and sulfur diffusion method,hydrothermal method and carbon tetrachloride solvent thermal method.