Lithium-sulfur batteries have received extensive attention from academia and industry due to their high energy density,low cost,and non-toxicity,which are considered to be the best choice to meet the needs of the accelerated development of electric vehicles.However,at present,as a new generation of high-performance energy storage devices,Li-S batteries still face the challenges of shuttle effect caused by soluble polysulfide(Li PSs),slow chemical kinetics,and uncontrolled growth of lithium dendrite.Therefore,it is of great importance to design the host material structure and control the behavior of polysulfides by physical and chemical means for the development of high-performance lithium-sulfur batteries.Among the various electrocatalyst materials,transition metals are considered attractive electrocatalysts for lithium-sulfur batteries due to their high intrinsic conductivity and strong Lewis acid-base interactions with sulfur-containing species.Furthermore,transition metal compounds(TMCs)enable the tailoring and assembly of electronic structures to facilitate the catalytic conversion of Li PSs by tuning the transition metal d-band center.Among them,transition metal oxides(TMOs)have a stronger adsorption capacity for polysulfides than other materials,and transition metal phosphides(TMPs)play a prominent role in realizing interfacial ion and electronic coupling and regulating the catalytic conversion of Li PSs.Therefore,the preparation of nickel-cobalt alloys and polar nickel-cobalt double-metal oxide/phosphide heterostructures based on nickel-cobalt bimetallic matrix can effectively improve the electrochemical performance of lithium-sulfur batteries.Based on the above,the main research contents are as follows:(1)Given the problem that single-site catalysts are difficult to realize the whole process of polysulfide conversion,using dicyandiamide as a carbon source,with cobalt acetate and nickel acetate through a one-step calcination method to prepare nickel-cobalt bimetallic alloy/carbon nanotube composites(Ni Co/BCNTs).Ni Co/BCNTs were used as modified materials to coat common separators,and coin cells were assembled to further analyze the electrochemical performance.Bamboo-like carbon nanotubes have long electron/ion transport channels,which accelerate the diffusion rate of lithium ions and electron conduction.Meanwhile,the original nano-scale Ni Co alloy particles are rich in catalytic active sites,which can efficiently catalyze polysulfides transformation.Combined with the efficient catalysis of nanoalloys and the high electrical conductivity of carbon nanotubes,Ni Co/BCNTs improve the adsorption capacity of polysulphides,effectively avoiding the failure of catalyst surface poisoning and sustaining catalytic polysulphide conversion;while ensuring efficient catalysis,lowering the activation energy barrier of the reaction and accelerating the whole process of adsorption-transformation-diffusion reaction.The ordinary separator modified with Ni Co/BCNTs has a high capacity(0.1C,1495.6 m Ah g-1),and the capacity remains at 544.8 m Ah g-1 after 1000 ultra-long cycles at a high current density of 2 C.(2)Aiming at the problem that the accumulated solid sulfur species deposited on the catalyst surface during the long cycle leads to its deactivation,the Ni Co-LDH hollow nanocage structure was prepared by in-situ transformation using ZIF-67 as the precursor,and further heat treatment and phosphating treatment were used to obtain the Ni Co O2/Ni Co P heterostructure.The successful construction of the hollow cage-like heterostructure exposes more active sites,while the heterostructure lowers the interfacial reaction barrier and accelerates the reduction kinetics of polysulfides during discharge.The Ni Co O2/Ni Co P heterostructure combines the properties of polar Ni Co O2 materials with the advantages of Ni Co P to promote the rapid dissolution of solid sulfur species during charging,thereby playing a self-cleaning role in reducing solid accumulation on the catalyst surface during long cycling.The results show that the prepared Ni Co O2/Ni Co P modified PP separator can achieve 1000 ultra-long cycles at a current density of 2 C with a capacity decay rate of 0.043%per cycle and a high coulombic efficiency(above 98%).The cells maintain high coulombic efficiencies even at high areal sulfur loadings of 4 mg cm-2. |