Theoretical Study On The Catalytic Effect Of Modified 2D Transition Metal Chalcogenides In Lithium-sulfur Batteries

Lithium-sulfur batteries have the advantages of high theoretical specific capacity and energy density,low cost and environmental friendliness,and are considered to be a new generation of secondary batteries that meet the requirements of future energy storage systems.The transition metal chalcogenide catalyst materials used in the cathode and the separator can improve the electrochemical redox reaction kinetics,fundamentally inhibit the polysulfide shuttle effect,and optimize the electrochemical performance of the battery.However,the basal surface of the material that occupies most of the reaction contact sites exhibits a weaker catalytic activity than a few edge sites.Activation of the inert base surface through the modification strategy will greatly enhance the catalytic activity of metal chalcogenides.In this paper,theoretical calculations based on density functional theory are carried out to explore the catalytic mechanism of modified transition metal chalcogenides（heteroatom-doped MoS₂ and vacancy defect WSe₂）in lithium-sulfur batteries.The mechanism by which Co,P co-doped MoS₂ improves the electrochemical performance of lithium-sulfur batteries is revealed,the relationship between the vacancy concentration and the catalytic ability of WSe₂ is clarified,and the favorable Se vacancy concentration is determined.The specific research results are as follows:1)MoS₂ material has received widespread attention due to the affinity for polysulfidesand catalytic ability.However,the base surface that occupies most of the reaction interface exhibits weaker catalytic activity than the few edge sites.The Co,P co-doping strategy was used for the first time to modify MoS₂ material as cathode material in lithium-sulfur battery.Compared with undoped and Co single-doped materials,it achieves excellent discharge capacity,rate performance and cycling stability.To explore the origin of Co,P co-doping MoS₂ to improve the electrochemical performance of lithium-sulfur batteries,the first-principles research was carried out,clarifying the adsorption and conversion of polysulfides on the surface of undoped,Co single-doped,Co,P co-doped MoS₂.The calculation results indicate that the adsorption energy shows the trend of undoped<Co single doped<Co,P co-doped MoS₂.The Li-S and S-P double bonding between the co-doped surface and polysulfides make it has the greatest adsorption capacity.Additionally,the significant stabilizing effect for discharge product Li₂S reduces the energy requirement of the dissociation step and increases the overall energy release during the conversion process,providing driving forces for the polysulfide conversion.These two factors jointly enhance the catalytic activity of MoS₂,effectively inhibit the shuttle effect,and improve the electrochemical performance of the battery.It is hoped that the conclusion could promote the application of surface modified metal sulfide catalytic materials in lithium-sulfur batteries.2)WSe₂ has a unique layered structure and low hydrogen atom adsorption free energy,showing good catalytic activity for hydrogen evolution.Vacancy defect strategy is an important way to improve the performance of the base surface of two-dimensional materials.In order to explore the feasibility of WSe₂ and WSe₂ with surface Se vacancy defects as catalytic materials in lithium-sulfur batteries,this work focuses on WSe₂ and WSe₂ with vacancy concentrations of 3.125%,6.25%,9.375%and 12.5%,respectively,to investigate the adsorption capacity,catalytic conversion capacity,lithium ion migration capacity,reveal the influence of vacancy defect concentration,and clarify the favorable Se vacancy concentration.The results show that the concentration of Se vacancy has little effect on the adsorption of medium and long-chain polysulfides,while the adsorption energy of short-chain polysulfides increases significantly with the increase of the vacancy concentration.Lithium ion migration energy barrier results show that low vacancy concentration（3.125%）is not conducive to lithium ion migration,and 6.25%-12.5%high vacancy defects will promote the rapid transmission of lithium ions.The results of the change of overpotential during charging and discharging process further show that the6.25%-12.5%vacancy concentrations of WSe₂ in the case of high vacancy defects have a lower overpotential,and the surface has more excellent polysulfide catalytic conversion ability.Based on this,WSe₂ with a vacancy concentration of 6.25%-12.5%has moderate polysulfide adsorption capacity,fast lithium ion transmission,and excellent polysulfide catalytic conversion ability,and has the potential to be used in lithium-sulfur battery systems.