Preparation And Electrochemical Performances Of Selenium/Sulfur/Carbon Composite Cathode Materials For Lithium Batteries

Lithium-sulfur battery,with ultra-high theoretical energy density(2600 Wh·kg^-1),is considered to be one of the most valuable next generation high specific energy secondary battery systems.In addition,the cheap and environmentally friendly properties of sulfur make the system extremely commercial.However,the conductivity of elemental sulfur and discharge product lithium sulfide is very low,resulting in low reversible capacity of sulfur cathode;charge and discharge intermediate product lithium polysulfide is easy to dissolve in the electrolyte,through the separator and diffuse to lithium metal,resulting in“shuttle effect”,resulting in low cycle efficiency and short life of the battery.elemental selenium,which belongs to the same element group as sulfur,has higher electrical conductivity(Se:1×10^-3 S·m^-1;S:5×10^-28S·m^-1)and electrochemical activity,and it is easy to form selenium-sulfur solid solution.In this paper,aiming at the problems of sulfur cathode,selenium solid solution was combined into sulfur-based cathode materials,and the effect of selenium composite on the electrochemical properties of chain small sulfur molecules and ring eight sulfur molecules was studied,which improved the electrochemical properties of sulfur-based cathode materials.The main research contents of this paper include:（1）Preparation and electrochemical performance of selenium/sulfur/carbon composite cathode material based on small molecule chain Se_mS_n（2≤m+n≤4）.Through the thermal diffusion method,commercial selenium disulfide was pyrolyzed into Se solid solution composite chain-like small sulfur molecule S_2-4,and the chain-like small Se_mS_n（2≤m+n≤4）was successfully constructed in the UMC by using the spatial confinement effect of ultra-microporous carbon UMC,which was used as the cathode material of Li-S battery.DFT simulation shows that compared with the chain S_2-4 small molecule,the modified Se_mS_n（2≤m+n≤4）small molecule has higher conductivity,lower lithiumation energy and easier discharge lithiumation process.The experimental results show that the discharge process of the prepared UMC/Se_mS_n（2≤m+n≤4）composite cathode material is a one-step solid phase transformation reaction,which effectively inhibits the shuttle loss of active materials.Compared with UMC/S_2-4composite cathode material,UMC/Se_mS_n（2≤m+n≤4）composite cathode material has smaller charge transfer impedance and higher discharge capacity.Therefore,the capacity of UMC/Se_mS_n-40（2≤m+n≤4）composite cathode material remains 844m Ah·g^-1 after 100 cycles at 0.1C;when 500 cycles were performed at 0.5C for a long time,the capacity loss of each cycle was only about 0.07%,showing excellent cycle stability.（2）Preparation and electrochemical performance of selenium/sulfur/carbon composite cathode materials based on cyclic Se_nS_8-n macromolecules.By melting diffusion method,Se was dissolved and composited into cyclic octamer sulfur molecule S₈,and r GO/KB/Se_nS_8-n composite was successfully prepared by ball milling mixing and spray drying technology combined with ketjen black and graphene.Due to the solid solution combination of Se,Se_nS_8-n solid solution has higher intrinsic conductivity than elemental sulfur.In addition,the three-dimensional“point-plane”contact conductive framework network formed by graphene and ketjen black can not only accommodate high content of Se_nS_8-n solid solution,but also further improve the conductivity of electrode materials and inhibit the dissolution and loss of intermediate products.Therefore,when the Se/S molar ratio is 20%,the obtained r GO/KB/Se_nS_8-n-20composite cathode material exhibits excellent electrochemical performance.The first reversible capacity is 1200 m Ah·g^-1 at 0.1C,and the capacity remains 800 m Ah·g^-1after 100 cycles.