Synthesis Of Sulfur Iron Compounds And Their Applications In Lithium Ion Batteries

Lithium ion batteries?LIBs?,a kind of important energy storage devices,have been widely used on portable electronic products because of their high energy density,light weight,no pollution,and so on.However,the demand for improved electrochemical performance of LIBs is growing rapidly as the next generation of LIBs is expected to be used in power electric vehicles and smart grids to support the demand for low-carbon products.As an important component of LIBs,the anode material makes great contribution to the capacity and influences the whole electrochemical performance of LIBs.As a type of anode materials,iron sulfides are one of the hottest research highlights recently because of their high capacities,the low costs,rich resources,and green environmental protection.The volumes of iron sulfides anode materials can expand during Li+ insertion process,which leads to bad cycle stabilities.To solve this problem,carbon materials coating was used to ease the volume expansion of iron sulfides during charge/discharge,and FeS confined in the matrix of porous carbon?FeS/PC?,Fe₃S₄ nanoparticles dispersed in reduced graphene oxide?Fe₃S₄ NPs@rGO?,and iron sulfides wrapped in reduced graphene oxide?FexSy@rGO?composites were prepared with different methods.Compared with their respective pure phase materials,composite materials display higher capacities and better cycle stabilities.In this thesis,the specific research contents and results are as follows:?1?A novel FeS/PC composite was successfully prepared using a one-pot in-situ solid-state method.For FeS/PC composite,the surface of PC has many pores with size of around 200 nm,and FeS particles disperse in these pores or on the surface of the PC matrix.As the anode material for LIBs,FeS/PC can achieve a high initial discharge capacity of 1428.8 mAh/g,and 624.9 mAh/g capacity can be maintained after 150 cycles.From the second to 150th cycle,the Coulombic efficiency of FeS/PC composite is close to 100%,exhibiting excellent reversible performance.The porous structure of the FeS/PC composite can improve structural stability for the whole material,which enhances the electrochemical performance.?2?The Fe₃S₄ NPs@rGO composite was synthesized by employing a facile in-situ solvothermal method.The Fe₃S₄ NPs are wrapped in rGO matrix,which can not only improve the electrical conductivity of Fe₃S₄ NPs,but also can effectively buffer the volume expansion of Fe₃S₄ during charging/discharging.The Fe₃S₄ NPs@rGO composite exhibits a reversible capacity of 950 mAh/g after 100 cycles at 100 mA/g and the capacity as high as 720 mAh/g can be remained after the 800th cycle even at 1 A/g.Moreover,the capacity of Fe₃S₄ NPs@rGO can be kept at 953,812,727,667,623,520 and 462 mAh/g at 0.1,0.2,0.5,1,2,5 and 10 A/g,respectively,indicating its nice rate capabilities.The excellent electrochemical properties for Fe₃S₄ NPs@rGO can be ascribed to the collaborative effect between the Fe₃S₄ NPs and rGO matrix.?3?FexSy@rGO has been prepared via a simple in-situ hydrothermal method.As the anode material for LIBs,FexSy@rGO can deliver an initial discharge capacity of 1467 mAh/g at a current density of 100 mA/g,and a reversible capacity of 1190 mAh/g can be maintained after 100 cycles.Even at 1 A/g,a discharge capacity of 720 mAh/g can be left after 1000 cycles.In addition,FexSy@rGO also exhibits a good rate capability,and the reversible capacities of 1067,970,888,822,714,650 and 545 mAh/g can be left when the current densities change from 100 to 200,500,1000,2000,5000,10000 mA/g,respectively.The rGO matrix can enhance the electrical conductivity of FexSy@rGO to improve the electrical conductivity during charging and discharging,which results in very excellent electrochemical performance for FexSy@rGO.