Ionic Liquid Assisted Synthesis Of Porous Carbon/Sulfur Composite Cathode Materials For Lithium-Sulfur Batteries

In recent years,with the incessant development of large energy storage systerms such as electric vehicles,hybrid electric vehicles and national defense power systems,energy storage devices with high energy density are needed to match them.The lithium–sulfur battery has been regarded as one of the most promising candidates for energy storage devices due to its high theoretical specific capacity(1675 mA h g^-1)and high energy density(2600 W h kg^-1).Moreover,the natural abundance,low cost,and non-toxicity characteristic of sulfur facilitate its application in the large-scale energy storage systems.In addition,its working voltage of lithium–sulfur battery is about 2.1 V,which can meet the needs of many applications.However,there are many problems with the lithium sulfur battery itself:Firstly,the ionic and electronic conductivity of simple sulfur and its series of discharge products are poor.Secondly,the highly soluble intermediate products of lithium polysulfides?Li₂S_n,n=4-8?leads to a strong shuttle effect.Thirdly,the cathode changes seriously in the process of continuous circulation.In order to solve the above problems,researchers have carried out a variety of research.The study of cathode materials is quite extensive,among which sulfur/carbon composites are considered as the most suitable cathode materials.In this paper,porous carbon materials were prepared by ionic liquid assisted method,and then carbon/sulfur composites were prepared by a typical melt-diffusion method.The physical and electrochemical properties of the composites were investigated and studied.The main research work includes the following two aspects:?1?Ionothermal Synthesis of Graphene-based Microporous Carbon for Lithium–Sulfur Batteries.In this chapter,using ionic liquid?[BMIm][H₂PO₄]?as stabilizer of graphene and glucose as carbon source,the obtained I-GPC has abundant micropore structure.The electrochemical properties was tested after the impregnation of sulfur into the porous carbon matrix.At the current rate of 0.1 C,the discharge capacity of the first cycle is up to 1510 mA h g^-1,and after cycle for 150 cycles at the high current rate of 1 C,the discharge capacity can still be kept at625.8 mA h g^-1.The outstanding electrochemical properties of the composites are mainly attributed to the high conductivity and abundant micropore structure of I-GPC.The preparation method of carbon provides a new way for the study of lithium-sulfur battery.?2?Ionic Liquid Assisted Synthesis of Graphene-based Hierarchical Micro/Mesoporous Carbons forHigh Performance Lithium–Sulfur Batteries.In this part,ionic liquids?[BMIm][HSO₄]?,glucose and graphite oxide were used to prepare micro/mesoporous carbon materials by a direct carbonization method.The prepared I-rGO-G has a specific surface area of 2467.11 m² g^-1 and a pore volume of 2.698 cm³ g^-1,which enabled the loading capacity of sulfur as high as 70%.The I-rGO-G has abundant micro/mesopore structure can effectively immobilize sulfur in carbon matrix,which makes S/I-rGO-G exhibit excellent electrochemical performance.The discharge capacity of the first cycle is up to 1333 mA h g^-1 at 0.1 C.The S/I-r GO-G cathode retains a reversible capacity of590 mA h g^-1 after 300 cycles when at the current rate of 0.5 C.When being tested at a high rate of 1 C,the capacity can still remain at 600 mA h g^-1 after 200 cycles,showing an excellent cycle stability and rate performance.