Preparation And Performance Of Sulfur/Carbon Composite Cathode For Lithium-sulfur Batteries

Lithium-sulfur?Li-S?batteries are a new type of secondary battery system with high theoretical specific capacity of 1675 m Ah g^-1 and high energy density of 2600Wh kg^-1.The element sulfur as the cathode has a high redox potential for abundance in nature.Metallic lithium as the anode was the element with the lowest redox potential for the periodic table.Therefore,the typical Li-S batteries assembled by matching these two electrode materials naturally become a high-energy-density secondary battery system that attracts much attention.However,it is affected by shuttle effect,volume expansion,and poor conductivity,leading to many terrible problems such as low coulomb efficiency and rapid capacity decay.The practical application of Li-S batteries was greatly limited as above mentioned problems.Carbon materials with excellent properties such as excellent electrical conductivity and large specific surface area can help alleviate the volume expansion effect of sulfur,improve the conductivity of cathodes,and have a certain adsorption and catalytic effect on polysulfides.In this paper,two different carbon materials:functionalized graphene?r GO?and Ketjen Black?KB?are studied.Through material design,r GO/S,KB/S/BCST(Ba_0.97Ca_0.03Sn_0.08Ti_0.92O_3-?)composite cathode materials were prepared.The research results show that:1)Graphene oxide?GO?was used as a precursor to preparing functionalized graphene.The functionalized graphene/nanosulfur composite material with a sulfur content of 83.6%was obtained by a one-step reduction and self-assembly method in water.The characterization results show that the nanosulfur particles have a uniform particle size and are evenly distributed over the two-dimensional sheet.The surface of the functionalized graphene contains a part of oxygen-containing functional groups.Encapsulating sulfur nanoparticles with graphene sheets can alleviate the volume expansion effect,and the oxygen-containing functional groups on the surface of functionalized graphene can chemically inhibit the shuttle of polysulfides.Therefore,after using the composite material as a cathode,metallic lithium as anode,a celgard2400 separator,and an electrolyte to assemble a coin battery,the battery exhibited satisfactory electrochemical performance.The assembled lithium-sulfur battery has an initial capacity of 746.6 m Ah g^-1 at a current density of 0.2 C.After 200 cycles,the capacity remains at 537.7 m Ah g^-1,and the capacity retention rate is as high as 72%.2)Ketjen Black?KB?with excellent conductivity and large specific surface area was used as a matrix of encapsulating sulfur to prepare KB/S composite materials.A polar material BCST with rich oxygen vacancies is introduced to improve the electrochemical performance of KB/S cathode,therein.KB can improve the conductivity of cathode,ease sulfur volume expansion,and physically limit polysulfide shuttles.The BCST is used to form stable chemical bonds with polysulfides,and rich oxygen vacancies of BCST are used to promote the conversion of polysulfides in the electrochemical process.Thus,the shuttle of polysulfides is chemically restricted.The polysulfides are limited by physical and chemical effects.Therefore,the shuttle effect was alleviated.A coin cell was assembled with the KB/S/BCST composite material as a cathode,metallic lithium as anode,a celgard2400 separator,and electrolyte.The assembled lithium-sulfur battery exhibits excellent cycle stability and rate performance.After 200 cycles at a current density of0.2 C,the reversible capacity can be maintained at about 850.6 m Ah g^-1.The rate performance tests were performed at different current densities of 0.1,0.2,0.5,1C,and 0.1 C.The results show that the KB/S/BCST composite cathode can deliver high reversible capacity of 1210.9,1070.7,981,906,and 1047.5 m Ah g^-1.