Study On Cobalt-nickel Bimetallic Sulfide Modified Lithium-Sulfur Battery Separator

With the development of society,the demand for energy is getting higher and higher.What followed was the severe consumption of fossil energy such as coal,oil and natural gas,which further led to global warming,increasingly harsh environments and energy shortages.Therefore,it is imperative to seek a green,renewable new energy source to replace the traditional non-renewable energy.At this time,lithium-sulfur batteries with high theoretical specific capacity,high energy density,long cycle life,environmental friendliness and low cost were born.However,the"shuttle effect"in lithium-sulfur batteries has limited its development.Polar transition metal materials such as metal sulfides,metal oxides and metal phosphides are hotspots for the development of lithium-sulfur batteries,which can effectively suppress the"shuttle effect".Among them,metal sulfide not only has better electrical conductivity,but also has strong polar covalent bond,and accelerates the catalytic charge-discharge reaction,which can effectively alleviate the dissolution and diffusion of polysulfide.In this paper,we focus on the most important challenge of the lithium-sulfur battery("shuttle effect")and synthesized the composite material based on cobalt-nickel bimetallic sulfide by hydrothermal method,and prepared the modified membrane.We explored the synthesis conditions of cobalt-nickel bimetallic sulfide,the preparation of composite materials and the mechanism for improving the electrochemical performance of lithium-sulfur batteries.The specific research contents are as follows:(1)NiCl₂·6H₂O,CoCl₂·6H₂O were used as the cobalt source and nickel source respectively,and the GO dispersion prepared by Hummers method was used as raw material to prepare3D-graphene composites with bimetal loading by hydrothermal method.The effects of different concentration of GO dispersions on the morphology of three-dimensional graphene skeleton were investigated.We studied the effects of different sulfur sources and hydrothermal solvents on the morphology,size and distribution of cobalt-nickel bimetallic sulfides.XRD,SEM and BET characterization test results showed that the optimum concentration of GO dispersion was 4mg ml^-1,and the graphene aerogel formed by hydrothermal has the most excellent three-dimensional pore structure,high three-dimensional spatial dispersion and high specific surface area,which provides more growth sites for cobalt-nickel bimetallic sulfides he spatial dispersion.In addition,when the sulfur source is Na₂S₂O₃·5H₂O and the hydrothermal solvent is a mixed solution of ethylene glycol/water(vol 3:1),Cobalt-nickel bimetallic sulfides in the form of spherical-like particles are formed on the graphene nanosheets,and the size of the particles is the smallest,concentrated between 200-400 nm,and the distribution is the most uniform.Therefore,the optimal synthesis conditions for cobalt-nickel bimetallic sulfides are.Therefore,the optimal synthesis conditions for cobalt-nickel bimetallic sulfides are ethylene glycol/water mixed solvent(vol 3:1)as a hydrothermal solvent,using a concentration of 4 mg ml^-1GO dispersion,and Na₂S₂O₃·5H₂O as a sulfur source.(2)Cobalt-nickel bimetallic sulfide/graphene aerogel composites were prepared under optimal synthesis conditions.The structure,morphology and composition of the composites were characterized and analyzed by XPS,EDS and TEM.The composites were coated on the separators for modification research and finally,assembled into button cells and carried out electrochemical tests.The results show that the electrochemical performance after modification with the CNS₂/GH functionalized material has been significantly improved.When the current density is 0.2 C,the CNS₂/GH separator has a high initial discharge specific capacity(1289.1m Ah g^-1).After 100 cycles,the CNS₂/GH separator discharge specific capacity decreases to780.4 m Ah g^-1.The raw separator has a first discharge specific capacity of only 1043 m Ah g^-1,and 670.4 m Ah g^-1 after 100 cycles.In addition,at 1 C current density,after 500 cycles,the CNS₂/GH separator discharge specific capacity can still reach 652.2 m Ah g^-1,while the raw separator only reaches to 493.1 m Ah g^-1.It suggests that the application of CNS₂/GH modified separator,the electrochemical performance is improved.The three-dimensional graphene skeleton provides a good conductive path for ion and electrons,which benefits to increase the diffusion rate of lithium ions.The oxygen-containing functional group and the cobalt-nickel bimetallic sulfide have a certain chemisorption effect on the polysulfide of the intermediate product,thereby inhibiting the diffusion of polysulfide.(3)Using NiCl₂·6H₂O,CoCl₂·6H₂O and Na₂S₂O₃·5H₂O as raw materials,cobalt-nickel bimetallic sulfide is grown on carbon paper by one-step hydrothermal method.By controlling the proportion of hydrothermal solvent and the concentration of cobalt salt added,etc.we prepare a composite interlayer under optimum conditions,which is used as a part of the separator in a lithium-sulfur battery.Characterization and analysis were carried out by means of XRD,SEM,EDS,XPS,TEM and UV/Vis.The results show that the size of bimetallic sulfide particles grown on carbon paper is the smallest(100-300 nm)and the particle size is the most uniform and evenly distributed using ethylene/water(vol 3:1)mixed solvent.When the amount of cobalt salt is 1 mmol,the metal sulfide particles on the carbon paper are most uniformly loaded and the loading is most suitable.Finally,the adsorption experiments show that the adsorption effect is best when the ratio of Co to Ni is 2:1,which is further verified by UV test.The electrochemical performance test results show that CNS₂/CP-2 has a high initial discharge specific capacity(1167.2 m Ah g^-1)when the current density is 0.2 C.After 100 cycles,compared with CP(58.24%),CNS₂/CP-2 has a higher capacity retention rate(64.36%).The addition of carbon paper not only compensates for the defects of poor conductivity of metal sulfides,but also provides more specific surface area for the growth of metal sulfides.The double metal sulfur bond can chemically bond more polysulfides,which effectively inhibits the diffusion of polysulfides,and can also catalyze the redox conversion between polysulfides leading to improved electrochemical performance.