Research On Cobalt-based Catalytic Material Functionalized Separator To Control Electrochemical Performance Of Lithium Sulfur Battery

With the excessive consumption and depletion of fossil fuels,the development of new energy storage technologies has received great attention.As an important energy storage device,lithium-ion batteries have been widely used in aerospace,electric vehicles and portable electronic products due to their advantages such as high energy density,low self-discharge,and stable cycle performance.However,with the gradual increase in human requirements for quality of life,lithium-ion batteries have been unable to meet the new needs of low prices,long battery life and high safety.Therefore,it is very urgent to develop new battery systems with high cost performance,high energy density,long cycle life and more safety.Lithium-sulfur batteries（Li-S）have low price,abundant reserves and environmental friendliness,and have high theoretical specific capacity(1675 mAh g^-1)and theoretical specific energy(2600 Wh g^-1),making them the most The next-generation power battery with potential.However,the Li-S battery system that has not yet been practically applied has problems such as poor conductivity of the sulfur element and the product Li2S,the shuttle effect of the intermediate lithium polysulfide,volume expansion,and safety hazards caused by the lithium dendrite of the negative electrode.The shuttle effect of lithium polysulfide is a key factor that causes the capacity of Li-S battery to decline.In order to solve the problem of the shuttle effect,the researchers respectively conducted research on the design of the positive electrode material,the protection of the negative electrode lithium,the design of the new electrolyte and the design of the new battery structure.Lithium polysulfide intermediates are easily separated from the cathode,dissolved in ether electrolyte,and then shuttle between the positive and negative electrodes,resulting in low utilization of active sulfur,designed and synthesized physical/chemical sulfur-fixing materials,electrocatalytic materials,and using them The functions of physical confinement,chemical adsorption and catalytic conversion can effectively relieve the shuttle behavior of lithium polysulfide.In particular,electrocatalytic materials with electrocatalytic effect on lithium polysulfide have both chemical adsorption capability and electrocatalytic conversion function compared with pure physical confinement and chemical adsorption,which can simultaneously achieve strong chemical adsorption of lithium polysulfide and The rapid catalytic conversion effectively improves the kinetics of the discharge reaction and prevents the diffusion and migration of the lithium polysulfide solution.In addition,the construction of a new lithium-sulfur battery system through the functionalization of electrocatalytic material separators can provide a physical barrier for the shuttle behavior of lithium polysulfide,and thus strengthen the suppression of its shuttle effect.The strategy of using electrocatalytic materials to functionalize membranes has the advantages of easier realization of large-scale production,as well as in-situ monitoring of the catalytic behavior of electrocatalytic materials in Li-S battery reactions,and clarification of the catalytic mechanism.There are three main types of electrocatalytic materials for lithium-sulfur batteries:heteroatom-doped carbon,single metal metals,and transition metal compounds.Compared with the former two,transition metal compounds have the advantages of simple synthesis technology,easy structure control,and low cost.Among them,cobalt-based electrocatalytic materials were first discovered to have excellent electrocatalytic activity for the conversion of lithium polysulfide.In view of the cobalt-based electrocatalytic material CoX and lithium polysulfide Li₂S_n^x-through the Lewis acid-base interaction to form a"Co²⁺-S_n^x-"bond,to achieve the chemical adsorption of lithium polysulfide,through cation regulation and anion regulation,and then adjust the polarity of cobalt ions And the surface electronic structure of the material finally achieves the purpose of enhancing chemical adsorption and catalytic conversion of lithium polysulfide.In this paper,aiming at the shuttle effect of Li-S battery,P-Co₃O₄/NCNT and cobalt-iron bimetallic sulfide functionalized separator were used to build a"graphene/sulfur|functionalized separator|electrolyte|lithium"battery system.The anion and cation control the lithium polysulfide adsorption performance of the cobalt-based catalytic material,as well as the electrochemical dynamics of battery polarization,electrochemical reaction rate,and lithium ion diffusion rate,thereby improving the electrochemical performance of lithium-sulfur batteries.The specific research is mainly divided into the following three parts:（1）Regulate the surface structure of Co₃O₄ through phosphorus anions,improve its catalytic conversion capacity for lithium polysulfide,and improve battery reaction kinetics and electrochemical performance.P-Co₃O₄/NCNT electrocatalytic materials were obtained by high-temperature in-situ growth of polyaniline modified CNT and Co₃O₄,and phosphating treatment.Explore the effects of carbonization temperature and phosphorus doping on material composition,morphology and surface micro-regions.P-Co₃O₄/NCNT composite material has stronger adsorption effect than metal Co/NCNT and Co₃O₄/NCNT,and has superior sulfur-fixing effect.The formation of Co-O-P bonds on the surface of the material changes the electronic structure of the Co₃O₄ surface,enhances the polarity of the cobalt ions,reduces the battery polarization,reduces the electrochemical reaction resistance,and significantly increases the lithium ion diffusion rate.At the same time,the P-Co₃O₄/NCNT functionalized separator assembled in Li-S batteries showed excellent electrochemical performance at 1 C rate.（2）By solvothermal method,using carbon paper as a carrier,prepare and obtain cobalt-iron bimetallic sulfide（CFS）and CFS/CP,explore the synthesis factors,and confirm the optimal synthesis conditions.With CoCl₂·6H₂O as the cobalt source and FeCl₂·6H₂O as the iron source,the solvent,sulfur source,cobalt-iron ratio and metal ion concentration were explored.The results show that when a mixed solvent of ethanol and water is 1:1,Co:Fe is 2:1,sulfur source is sodium thiosulfate pentahydrate,and the concentration of cobalt ion is 0.5 mM,the pyrite structure is obtained.,CFS-2 with hexagonal nanosheets.The lithium polysulfide adsorption experiment proves that,compared with other cobalt-iron materials,CFS-2 has a single phase structure,and its chemical adsorption to lithium polysulfide is the strongest.（3）By investigating the surface composition and microstructure of CFS-2/CP,compare the conversion kinetics and electrochemical performance of lithium polysulfide of CFS-2/CP and FeS2/CP functionalized separators.The results show that the introduction of cobalt ions in the cobalt-iron bimetallic sulfide can reduce the electrochemical reaction resistance and improve the conversion kinetics of lithium polysulfide.The first discharge specific capacity of CFS-2/CP at 0.2 C rate is 1126.5 mAh g^-1.After 400 cycles,the specific capacity is still as high as 629.9 mAh g^-1,and the average capacity decay rate per cycle is 0.11%,showing excellent performance.Cycle performance.