Metal Sulfide/Graphene Composites Materials As The Intermediate Layer In Li-S Batteries To Regulate Electrical Performance

With the development of science and technology,the popularization and development of electronic products have also led to the development of lithium-ion battery production technology.Nowadays,the development of lithium-ion batteries has reached the limit,Lithiumsulfur battery,a very popular new battery in recent years,is one of the most promising secondary batteries,which has been caught more and more scientists` attention in recent years.The lithium-sulfur batteries use the elemental sulfur as the cathoude,the metal lithium is the anode,and it has a theoretical specific energy of 2600 Wh/ kg,which is five times bigger than that of the lithium-ion battery.Moreover,sulfur is very rich in nature,so it is easy to obtain.As a concequence,the price of it is low.Also,sulfur is environmentally friendly,so Li-S battery has the potential of commercial development.Because of some defects of lithium-sulfur batteries,paticularly the "shuttle effect ",which is the most obvious one,people solve them in various ways.In this dessertation,a kind of cobalt sulfide quantum dots / nitrogen-sulfur doped graphene composite material were prepared by simple sol-gel method with glucose as carbon source,thiourea as template,N source and S source,and cobalt acetate as cobalt source.This material is coated on the sepertator of lithium-sulfur battery,which makes it have an excellent effect on inhibiting the "shuttle effect" of polysulfides,which can improve the electrochemical performance of lithium-sulfur battery significantly.Under the current density of 500 m A/g,the discharge capacity of the first cycle of the battery is up to 1250 m Ah/g,and after 100 cycles,the specific capacity remain 920 m Ah/g,which is a very high specific capacity and good cycle performance.In addition,the lithium-sulfur battery also has excellent rate performance.In the high current density of 1675 m A/g,the specific capacity can reach up to over 1000 m Ah/g.After 1000 cycles,the capacity is still up to 500 m Ah/g.In order to search the mechanism,I have set up a series of comparison groups to explain it.By comparing a series of excessive metal sulfides,it is found that cobalt sulfide has the most obvious effect on lithium-sulfur batteries among many metal sulfides.And it is found that cobalt sulfide has the highest binding energy to polysulfides,so it has the best adsorption to polysulfides.By comparing the prepared cobalt sulfide quantum dots with the cobalt sulfide particles,it is found that the performance of cobalt sulfide quantum dots is better than that of cobalt sulfide particles.This work suggests that the electrochemical performance of lithium-sulfur batteries can be significantly improved by using quantum dots cobalt sulfide / nitrogen-sulfur doped graphene composites as intermediate layer.