Desigh And Synthesis Of Cobalt-based Compounds And Their Application In Lithium Sulfur Batteries

As lithium-ion batteries are widely used in portable electronic products,electric vehicles and large smart power grids,traditional lithium-ion batteries can no longer meet the requirements of energy density and power density.Therefore,lithium sulfur（Li-S）batteries with the advantages of high specific capacity(1675 mAh g^-1),high energy density(2600 Wh kg^-1),low cost and friendly to environment have become a research hotspot for secondary batteries.However,the application of Li-S batteries is hindered due to the disadvantages of low conductivity of active material,lithium polysulfides（LiPSs）shuttling,and slow reaction kinetics.In order to solve these problems,researchers modified Li-S batteries mainly from three aspects of physical adsorption,chemical adsorption and catalytic conversion of materials:Various carbon materials with high conductivity and large specific surface area were used as sulfur-fixing materials,which can effectively solve the shuttle of LiPSs by physically trapping soluble LiPSs through porous structures;Polar heteroatom doping materials,polar compounds and various polymers form a strong affinity with LiPSs and perform chemical adsorption on LiPSs;Strong catalytic materials such as metal atoms and transition metal compounds are used to promote the conversion of LiPSs,thereby accelerating the reaction kinetics.However,a single modification study cannot enough to improve the shuttle effect and reaction kinetics of LiPSs simultaneously,so it is crucial to develop a research strategy that can have adsorption-catalysis synergy.At the same time,the polyethylene separator or polypropylene separator used in Li-S batteries needs to be modified because of its large pore size and no adsorption capacity of soluble LiPSs.Therefore,it is importent to find a multifunctional separator that can effectively promote the transformation of LiPSs.In order to solve the above problems,this paper starts from the modified functional coating separator and interlayer of Li-S battery,and designs cobalt-based compounds functional separators with adsorption-catalysis synergy.A polar interface and strong catalytic properties of cobalt-based compounds facilitate the adsorption and catalytic conversion of long-chain LiPSs.And the detailed analysis and evaluation of its electrochemical performance,the specific research contents are as follows:（1）CoMoO₄nanorods with uniform structure were prepared by simple hydrothermal method and thermal annealing method,and CoMoO₄-12 and CoMoO₄-24 nanorods with different sizes were obtained by adjusting the hydrothermal reaction time.When applying to the lithium-sulfur battery coating separator,the relationship between the structure size and performance was analyzed through the morphological structure characterization and electrochemical performance:The Co-O-Mopolar bond of CoMoO₄ produces a strong affinity with LiPSs,which can enhance the chemisorption of LiPSs;Compared with CoMoO₄-12,the CoMoO₄-24 nanorod with reduced particle size can increase the contact area and reaction sites with the electrolyte,effectively promote the penetration of the electrolyte and reduce the charge transfer resistance,thus realizing the rapid conversion of LiPSs during charging and discharging.Therefore,the Li-S battery assembled with CoMoO₄-24 coated separator exhibits remarkable electrochemical performance,with an initial capacity of 1018.5 mAh g^-1 at0.2 C and a reversible capacity of 745.5 mAh g^-1 after 200 cycles.（2）A single modified material cannot simultaneously solve the problems of polysulfide shuttle,poor conductivity,and slow reaction kinetics of LiPSs.Therefore,we prepared a CoN-Mo₂N nanorod heterojunction material with adsorption-catalytic capacity was prepared by nitriding CoMoO₄-24 through ammonia reduction method.For CoN-Mo₂N nanorods,the composition of CoN-Mo₂N has a significant interaction with LiPSs,and its chemical affinity can promote the adsorption of LiPSs,among which Mo₂N has stronger affinity with LiPSs;Li₂S nucleation experiments and the electrochemical performance show that the heterostructure interface formed between CoN-Mo₂N nanorods promotes the uniform nucleation and rapid growth of Li₂S,CoN promotes the oxidation reaction kinetics,and Mo₂N promotes the reduction reaction kinetics.Applying it to functional separators can achieve high rate and long-cycle performance of lithium sulfur batteries.Among them,a high initial reversible discharge capacity of 1605.8 mAh g^-1can be provided at 0.1 C.After 200 cycles at 0.2 C,the capacity decay rate per cycle is only 0.198%.（3）CoSe@NC nanofibrous membranes were prepared by electrospinning and used as an independent functional interlayer for lithium sulfur batteries.The visual adsorption experiments showed that the CoSe@NC nanofibers had a strong adsorption capacity for LiPSs,and combined with X-ray photoelectron（XPS）tests before and after adsorption,it was analyzed that the strong adsorption capacity of CoSe@NC nanofibers for LiPSs was due to CoSe electron transfer with LiPSs,and CoSe nanoparticles with strong catalytic activity can accelerate the conversion of LiPSs.Therefore,the adsorption-catalysis synergy of CoSe@NC can effectively improve the rate performance and cycling stability of Li-S batteries,and the electrochemical results show that the battery assembled with the CoSe@NC functional interlayer has high initial reversible discharge capacity of 1317 mAh g^-1 at 0.1 C rate.After 200 cycles at1.0 C,the capacity decay rate per cycle is only 0.16%.