Study On The Adsorption And Catalytic Performance Of Transition Metal Phosphides For Lithium Polysulfide

With social progress and economic development,the demand for high-capacity electrode materials is increasing.Lithium-sulfur(Li-S)batteries are the most potential high-energy secondary batteries with high capacity(about 1675 m Ah/g),high energy density,abundant reserves,favorable prices,and environmental friendliness.However,the low conductivity of sulfur and Li₂S,the volume expansion during charging and discharging,and the shuttle effect caused by the easy dissolution of intermediate polysulfide lithium(Li₂S₈,Li₂S₆and Li₂S₄)in ether-based electrolytes have led to the positive electrode activity of lithium-sulfur batteries.The material is continuously lost,the capacity is rapidly attenuated,and the coulombic efficiency is reduced.How to effectively solve these problems is the key to the large-scale application of lithium-sulfur batteries.Transition metal phosphides(TMPs)have received widespread attention because of their multifunctional active sites,adjustable composition and structure,and unique physical and chemical properties.The synthesis process of transition metal phosphide is relatively simple and has metallic properties,which is conducive to the conduction of lithium ions and electrons.Transition metal phosphide can not only capture soluble polysulfide lithium,but also effectively catalyze the decomposition of polysulfide lithium and improve the utilization of active materials.In this paper,aiming at the conductivity,shuttle effect and volume expansion effect of lithium-sulfur battery cathodes,this paper uses the conductivity,adsorption and catalytic properties of transition metal phosphides to modify lithium-sulfur battery cathode materials.The main research contents are as follows:(1)In view of the poor conductivity of sulfur and Li₂S and the shuttle effect of polysulfide lithium,the carbon nanotube material with good conductivity is used as a substrate,and Co₂P with catalytic and adsorption properties is compounded on the carbon nanotube to improve the conductivity of the material and suppress the shuttle effect.A simple one-step hydrothermal method is used to grow Co₂P particles onto the surface of carbon nanotubes treated with acid and use them as the host material for the sulfur cathode.It was characterized by XRD,SEM,TEM,XPS and element distribution.Electrochemical tests showed that Co₂P not only has a certain ability to capture polysulfide lithium,it can also promote the conversion between lithium polysulfide and lithium sulfide,inhibit the dissolution of polysulfide lithium,and improve the cycle stability of lithium-sulfur batteries.The initial capacity of the cathode electrode reaches 1372 m Ah/g when the current density is 0.1 C,and the coulombic efficiency can reach more than 96%when the current density is 0.2 C for200 cycles.(2)Aiming at the problem of slow conversion kinetics between lithium polysulfide and lithium sulfide in lithium-sulfur battery,the hydrothermal method is used to composite Ni₂P with catalytic properties and carbon nanotubes with good conductivity to form a hinge-like CNT-Ni₂P-n composite material,as the host material of the sulfur cathode,has the characteristics of porosity and easy loading of sulfur.Electrochemical tests show that Ni₂P can not only catalyze the solid-liquid conversion process from S₈to Li₂S_x,but also the liquid-solid conversion process from Li₂S_xto Li₂S₂/Li₂S,and Ni₂P has a certain adsorption effect,so it can inhibit the shuttle effect,improved oxidation reaction kinetics.The composite cathode CNT-Ni₂P-n exhibits good cycle stability and high capacity.The CNT-Ni₂P-2/S electrode can reach a discharge specific capacity of 1347 m Ah/g at 0.1 C in the first cycle.The specific capacity of 701 m Ah/g is still maintained after 200 cycles at 0.2 C,and the coulombic efficiency reaches 97%.(3)The Co-NC@CNT composite material formed by combining ZIF-67 and CNT,and then Ni₂P particles are grown on the surface of the Co-NC@CNT composite material by a hydrothermal method to form a Co-NC@CNT@Ni₂P hollow network structure.The abundant pore structure can provide space for volume expansion during charging and discharging.Co-NC@CNT material has a large specific surface area and good electrical conductivity.ZIF-67 has good specific surface area characteristics,improves the physical adsorption of sulfur,Ni₂P nanoparticles and Co can capture lithium polysulfide and catalyze the oxidation-reduction process of lithium polysulfide,which can effectively improve the problem of slow oxidation-reduction reaction kinetics,and is an ideal matrix for preparing sulfur active materials.At a current density of 0.1 C,the first discharge capacity can reach 1420.2 m Ah/g,which has a higher specific discharge capacity.The material has higher capacity and better cycle stability at 0.2 C,and the coulombic efficiency reaches 98%.Co-NC@CNT@Ni₂P effectively improves the electrochemical performance of the battery.