Theoretical Study On Electrocatalysis Of Dual-atom Catalysts In Nitrogen-Doped Graphene For Lithium-Sulfur Batteries

Lithium-sulfur batteries have the advantages of high theoretical energy density,environmental protection and low cost,and are expected to become the next generation of energy storage devices.However,the low conductivity of sulfur,the dissolution of polysulfides,the poor sulfur reduction conversion and the slow oxidation reaction of Li₂S during charge-discharge processes seriously hinder its practical application.The key to solve these problems is to design new catalytic sulfur cathode materials.In this paper,the catalytic processes of four single-atom catalysts and three dual-atom catalysts for sulfur redox reaction during charge-discharge processes were studied by density functional theory,and the factors to improve the catalytic efficiency were systematically analyzed.The main work is as follows:（1）Four single-atom catalysts M were constructed in nitrogen-doped graphene,whose configuration was M@NG（M was Mn,Fe,Co or Ni）,and the electrocatalytic effect of these single-atom catalysts on lithium-sulfur batteries was studied.It was found that except Ni@NG,the other three single-atom catalysts had high adsorption energy for polysulfides adsorption,which helped to anchor soluble polysulfides and thus inhibited the shuttle effect.It was found that the surface of Mn@NG had the lowest decomposition energy barrier of Li₂S among the four single-atom catalysts,which facilitated the conversion of the final discharge product Li₂S into sulfur during charging process.In addition,Mn@NG also had the lowest Gibbs free energy for the eventual conversion of polysulfides into Li₂S during discharge.Therefore,through comparing the reaction kinetics in the charge-discharge processes,we found that among the four single-atom catalysts,single-atom catalyst Mn had the best catalytic effect on the charge-discharge processes of lithium-sulfur batteries.（2）In order to further improve the catalytic performance of single-atom catalyst Mn,based on single-atom catalyst Mn,we designed three dual-atom catalysts Mn M as catalytic materials of sulfur cathode,whose configuration was Mn M@NG（M was Fe,Co or Ni）.Compared with single-atom catalyst Mn,dual-atom catalysts Mn M had stronger adsorption effect on polysulfides and inhibited the dissolution of polysulfides more effectively.In addition,due to the strong adsorption of polysulfides on dual-atom catalysts Mn M,the Li-S bond in the lithium-sulfur groups was weakened,which significantly reduced the Gibbs free energy in the reduction of high-order lithium-sulfur groups to Li₂S and sped up the rate limiting step in the reduction reaction.At the same time,the diatomic synergy could significantly adjust the magnetic moment and charge redistribution of the substrate,and could improve the charge transfer with the polysulfide,thus ensuring high conductivity and fast electron transfer.Therefore,compared to single-atom catalysts,dual-atom catalysts had a stronger anchoring effect on polysulfides,and could improve the charge transfer between substrates and polysulfides more effectively,and had a better catalytic effect on the reduction reaction of polysulfides,which having more potential to be used for catalyzing high-performance lithium-sulfur batteries.