Lithium Sulfide/carbon Composites Were Prepared Based On PVP As Cathode Materials For Lithium-sulfur Batteries

Lithium-sulfur battery has the specific capacity of 1672 mAh/g and the theoretical specific energy of 2600 Wh/kg with sulfur as the cathode,but it still has a lot of problems,such as poor conductivity,the volume expansion of sulfur in the process of charge and discharge,the shuttle effect of polysulfide and the generation of the lithium dendrites.The specific capacity is 1166 mAh/g while the lithium sulfide as the cathode.Due to the lithium sulfide come with lithium source itself,which relieved the problems of volume expansion and lithium dendrites.It is considered to be the most promising secondary batteries in next generation.However,there are still two problems of poor conductivity and polysulfide shuttle effect when lithium sulfide as the cathode.In this paper,we studied the effects of different conditions on the electrochemical properties of Li₂S/C composites prepared by polyvinylpyrrolidone（PVP）and Li2 S as raw materials using the method of dissolution,recrystallization and high temperature carbonation in detail based on the researchs abroad.The effects of Li₂S/C composites prepared by carbonized reduced Lithium Sulfate and the adjunction of F127 on electrochemical properties of Li₂S/C composites has also been studied.This paper mainly includes the following parts:（1）Li₂S/C composites was prepared with PVP as carbon source,we studied the effects of different mass ratios,evaporation temperatures and carbonation temperatures on Li₂S/C composite`s morphology,structure and electrochemical properties.It was found that the prepared Li₂S/C composite powder particles were smaller,uniform and had the best electrochemical performance when mpvp : mLi2 S = 1 : 1,evaporation temperature was 150 ℃ and carbonization temperature was 900 ℃.The Li₂S/C cathode delivers a high initial capacity of 1024.8 mAh/g and a reversible capacity of 500.3 mAh/g after 100 cycles at 0.05 C.The capacity retention was 48.8 %.（2）The Li2SO4/PVP composites were obtained by immobilization and freeze-drying PVP and lithium sulfate,the Li₂S/C composites were prepared by carbonizing the Li2SO4/PVP at 800 ℃.In this chapter we studied the effects of different PVP contents on the battery electrochemical performance.The carbon content of Li₂S/C composites is positive correlation with the amount of PVP added.The appropriate carbon content can improve the electrical conductivity and electrochemical properties of the material,but the higher carbon content will hinder the diffusion of Li+ to a certain extent and increase its polarization in the charge and discharge process,affect the battery Discharge capacity.When the m PVP : mLi2SO4 = 2.8 : 1,the content of lithium sulfide is 81.2 %.At 0.05 C current density,the sample positive electrode exhibits high electrochemical performance,the first discharge capacity was 773.7 mAh/g.After 100 cycles,the specific capacity of discharge was 478.6 mAh/g,the capacity retention rate was 61.86 %.（3）Li2SO4/PVP/F127 composites were prepared by mashing and freeze-drying the F127,PVP and lithium sulfate,wherein the triblock polymer F127 as structure-directing agent,PVP as carbon source.Then,Li₂S/C composite was prepared by high temperature carburization of this material.The particle size of Li₂S/C composites decreases with the addition of F127,the particle size is about 200 nm and the specific surface area is 15.2 m2/g when mPVP : m Li2SO4 : mF127 = 2.8 : 1 : 0.75.The capacity of the first discharge was 988.7 mAh/g at 0.05 C current density,the capacity was 634.1 mAh/g after 100 cycles,and the capacity retention rate was 64.13 %.In summary,two kinds of Li₂S/C composites were prepared by using PVP as the carbon source with two methods.The electrochemical performance in the lithium-sulfur battery provides an important exploration experience for the research of lithium-sulfur battery cathode materials.