Synthesis And Electrochemical Performance Of Functionalized C/S Cathode Materials For Li-S Batteries

With the development of society and technology,people’s demand for energy is growing.Facing the demand for new energy utilization and energy storage for vehicle power batteries,as well as the increasingly high requirements for energy storage of mobile electronic products in the traditional secondary battery field,the secondary battery technology is receiving more and more attention.Although lithium-ion batteries,as the major commercial secondary batteries,have been widely used and occupy the market,it is approaching the limits of its energy density,it is undoubtedly even more urgent to find and develop a new generation of secondary battery system.Lithium-sulfur battery is considered as one of the potential new generation of secondary battery systems due to its high energy density,rich resources and cheap advantages.However,the problems in the performance,stability,safety and service life of lithium-sulfur batteries limit their industrial application prospects.These problems are mainly due to the shuttle effect caused by the dissolution of the intermediate polysulphides,and the insulation of S and the end product Li₂S,resulting in a slow conversion rate and a low sulfur utilization rate.As the source of contradiction,the cathode material of lithium sulfur batteries is an important node for the practical application of lithium sulfur batteries.Based on the performance requirements of cathode carbon materials for lithium-sulfur batteries,a N-doped-porous carbon nanocomposite material containing bimetallic alloy and nuclear-shell structure is designed and prepared.A variety of methods are used to adjust the element ratio and microstructure of composite materials.And the action mechanism between carbon materials and polysulfide are analyzed.Finally,from the perspective of the phases interface between the cathodes and electrolyte of the lithium-sulfur battery,the reaction behavior of the active material at the cathode/electrolyte phase interface is analyzed through the surface modification of the cathodes materials to further improve the rate performance of the battery.The main research contents are as follows:（1）The 3D metal organic frame precursor Co₃[Fe（CN）₆]₂were prepared by complexation reaction between Co（NO₃）₂and K₃Fe（CN）₆in aqueous solution.The results of the precursor carbonization process show that the precursor carbonization is a multi-step exothermic contraction process,and a additional carbon coating layer must be covered the precursor to obtain a proper material structure.The Co Fe CN@C-750 has a complete nuclear-shell structure and exhibits the best electrochemical properties.The average single cycle decay of300 cycles was 0.10%at 1C current density.（2）On the basis of Co Fe CN@C-750,the Co Fe alloy-porous carbon composite E-Co Fe CN@C was obtained by adjusting the coating thickness and acid etching.The Co Fe alloy particles（diameter is about 20 nm）are evenly distributed in the nuclear-shell structure（diameter is about 150nm）.It has a rich conductive surface(397 m²g^-1).And the Co Fe alloy particles distributed evenly in the structure,which provide catalytic and adsorption sites,improve the redox reaction kinetics of lithium-sulfur batteries and inhibit the shuttle effect of polysulfide.At the same time,the shell layer has a spatial domain-limiting effect,which helps in anchoring S and polysulfide within the structure and inhibiting the loss of active material.E-Co Fe CN@C has good cycle stability,the cycle decay rate is only 0.055%with 100 cycles under 0.1C,and the capacity retention rate is 85.5%with 300 cycles under 1C.Even at a high-sulfur loading of 5.1 mg cm^-2,the area-specific capacity is still 3.43 m Ah cm^-2after 100 cycles at 0.1C（the capacity retention rate is 75.0%）.（3）A new precursor was prepared by Co₃[Fe（CN）₆]₂precursor being grown on the classical organometallic frame structure,ZIF-8 crystals.Later,the precursors were carbonized,and the obtained carbonization products were used as the active material carrier for the cathodes of Li-S battery.Different from the expected structural state that Co Fe CN particles uniformly distributed on the surface of ZIF-8,because the hydrolysis of ZIF-8 structure and the generation of the precursor structure of Co₃[Fe（CN）₆]₂are simultaneous,a divergent crystal structure that with ZIF-8 primitive structure as the nucleus and growing in a prismatic outward diffusion,was formed.In the final product E-ZCF@C,the Co Fe alloy particle size becomes smaller（10nm）,that can provid more polysulfide catalytic conversion sites.Because the divergent conductive structure of E-ZCF@C is more favorable for charge transfer,compared with E-Co Fe CN@C/S,the rate performance of the composite cathode material E-ZCF@C/S was improved:The initial specific capacity of E-ZCF@C/S could reach 794 m Ah g^-1at 2C.On the other hand,due to the loss of the spatial domain-limiting effect of the nuclear-shell structure,the effect of E-ZCF@C to inhibit the polysulfide diffusion was weakened:after 300 times charge and discharge cycles at 2C,the cycle decay rate was up to 0.13%.（4）The electrochemical changes of the interface between cathode and electrolyte in Li-S battery were analyzed via surface modification of CNTs@S composites by different types of surfactant.And the cause of this phenomenon is speculated through some comparative experiments.The result shows that the effect of surfactant adsorption on the interface between CNTs@S particles and the electrolyte is the result of a combination of two opposing factors.In the different modified cathode materials,the charge transfer impedance of APG-CNTs@S decreased from 127.2Ωto about 6.3Ω.This change increases the maximum charge and discharge current density of the CNTs@S battery from1C to 7C.Even both with a high sulfur loading of 5.3 mg cm^-2and a low electrolyte/sulfur ratio of 4.1ml g^-1,the Li-S batteries with CNTs@S cathode modified by APG still deliver a high capacity of 2.38 m Ah cm^-2（66%of initial capacity）after 50 cycles.When the ferrocene group with adsorption capacity to polysulfides was grafted on surfactant,on the basis of improved charge transfer rates at the solid-liquid interface,the surfactant modified CNTs@S can further suppress the diffusion of polysulfides.