First-principles Study Of High-efficiency Lithium-sulfur Battery Catalysts Based On Two-dimensional Materials

With the continuous growth of the economy and the increasingly rich lifestyle of the society,the key issues in the development of human society have gradually become energy consumption and environmental pollution.In order to alleviate the dependence on traditional fossil energy（natural gas,oil,coal,etc.）,renewable clean energy（wind energy,tidal energy and solar energy）has been gradually developed and utilized.Secondary batteries are considered as the main energy storage system to ensure the efficient use of clean energy.However,the energy and power density of traditional Lithium-ion batteries cannot meet the development requirements of the next generation of electric vehicles,so it is necessary to explore a secondary battery system with higher energy density.Due to its high theoretical energy density(2600 W h kg^-1),Lithium-sulfur（Li-S）batteries provide the possibility for large-scale application of the next generation of energy-powered vehicles.However,the application of Li-S batteries is still affected by volume expansion,short cycle life and shuttle effect in the process of charge and discharge.Sulfur cathode is an important part of Li-S battery.Rational design of composite cathode with high sulfur utilization rate and long service life is the first step to show the potential of Li-S battery.Two-dimensional（2D）materials have a wide range of applications in the cathode of Li-S batteries due to their large specific area,weak interlayer interaction and high electrocatalytic activity.However,most two-dimensional materials are difficult to have all advantages of high adsorption strength,high conductivity and high specific surface area.Therefore,it is an important task to design a Li-S battery with better electrochemical performance based on two-dimensional materials.With the continuous development of first-principles technology,theoretical simulation plays an increasingly important role in the prediction and performance characterization of battery systems.In this paper,we use the advantages of density functional theory（DFT）to conduct structural design（doping,defects）based on two-dimensional materials to screen potential cathode materials for Lithium-Sulfur batteries.The potential application value of substrate in Li-S batteries was studied from the aspects of inhibiting the shuttle effect of polysulfides and the ability to catalyze the conversion of polysulfides,which provided practical theoretical guidance for the development and research of Li-S batteries.The main contents of this paper are as follows:（1）Fe/Co-based diatomic catalysts（DACs）for Li-S batteries were systematically studied by first-principles calculations.The adsorption energy of diatomic catalyst for polysulfide is much larger than that of corresponding monoatomic catalyst and DOL/DME solvent,indicating that it has the ability to inhibit shuttle effect.In addition,DACs maintain good metal conductivity after adsorbing polysulfides,thus improving the electrochemical performance of Lithium-Sulfur batteries.In all DACs,Fe Co N₈@Gra In the process of SRR and SER,it shows bifunctional catalytic activity with low energy barrier,which can effectively accelerate the conversion of Li₂S_x,ensure high sulfur utilization and long cycle stability.These properties indicate that diatomic catalysts are expected to be excellent cathode materials for Lithium-Sulfur batteries.（2）Through first-principles calculation,the application of VS₂（V_S）with vacancy sulfur defect in Lithium-Sulfur battery was studied.The Li-S/S-V double interaction between polysulfides and V_Scan effectively anchor polysulfides,thus inhibiting the shuttle effect.Compared with the strong adsorption energy of original VS₂for polysulfides,the moderate adsorption energy of V_Sis more conducive to catalytic conversion of polysulfides.The research shows that V_Shas lower Gibbs limiting step energy（0.24 e V）,lower decomposition potential barrier of Li₂S（0.18 e V）and lithium-ion diffusion rate（0.23 e V）comparable to original VS₂in the catalytic process.In addition,V_Scan still maintain good metal conductivity after adsorbing polysulfide,ensuring good conductivity of sulfur cathode during the whole charging and discharging process.This proves that the defective design is beneficial to improve the electrochemical performance of Li-S battery.