Sn,Ni-Doped CoSe₂/MoS₂ Heterostructures For Polysulfide Catalysis:A Theoretical Study

Lithium-sulfur（Li-S）batteries,with their extremely high theoretical specific capacity and high energy density,show great promise for application as next-generation energy storage systems.However,Li-S batteries encounter a series of challenges in practical applications,such as the slow oxidation reduction dynamics of sulfur and the shuttle effect caused by lithium polysulfides（LiPSs）,resulting in extremely low Coulomb efficiency and poor cycling performance of the batteries.To solve the above problems,a two-dimensional heterostructure CoSe₂/MoS₂ was designed and constructed in this paper for Li-S battery cathode addition material,and the properties of the CoSe₂/MoS₂ heterostructure and its anchoring and catalytic performance on LiPSs were investigated based on the first principle,density functional theory（DFT）calculation method.In addition,the effects of the dual effects of heterostructure engineering and atomic doping on the anchoring and catalytic LiPSs of the material are further investigated by introducing Ni and Sn atoms into the heterostructure,aiming to provide a novel idea for the rational development of high-performance Li-S battery cathodes.The specific studies are as follows:First,the two-dimensional heterostructure CoSe₂/MoS₂ was constructed and a large number of Co-S bonds were formed in the middle of the heterojunction,which is an important factor to ensure the stability of the structure.Subsequently,the nature of the heterostructure and its anchoring and catalytic properties on S₈/LiPSs were investigated based on DFT theory calculations.The electronic analysis shows that the appearance of a large number of electron-occupied states near the Fermi energy level of CoSe₂/MoS₂ indicates that it has metallic properties,which creates conditions for improving the defect of poor sulfur conductivity.Meanwhile,the adsorption energy results show that the heterostructure has a moderate binding strength to S₈/LiPSs with adsorption energies ranging from 0.52 to 2.77 e V,which ensures effective anchoring of LiPSs while maintaining the structural integrity of LiPSs.Moreover,the interaction between this structure and Li₂S₈,Li₂S₆ and Li₂S is stronger than that of the electrolyte molecules,which can prevent the dissolution of LiPSs in the electrolyte and thus mitigate the shuttle effect.In addition,the diffusion barrier of Li₂S on the CoSe₂/MoS₂ surface is low at 1.26e V,which can avoid the deposition of Li₂S in large quantities and prevent them from forming a passivation layer at the cathode,thus improving the cycling performance of the cell.Based on the above results,CoSe₂/MoS₂is expected to be an excellent cathode addition material for Li-S batteries.Second,Ni and Sn atoms were introduced into the heterostructures to investigate the effect of single-atom doping on the anchoring and catalysis of LiPSs in the heterostructures.Since the heterostructure surface contains three different Coatom substitution sites,six single-atom doped structures（denoted as P（1）Ni,P（2）Ni,P（3）Ni,P（1）Sn,P（2）Sn and P（3）Sn,respectively）were designed and constructed.Subsequently,the stability of the six doped configurations was evaluated,and it was found that the defect formation energy of all six structures was negative,indicating that all six structures were stable.In addition,the adsorption energy calculations showed that,with respect to different doping sites,the Sn and Ni doped conformations at the P（1）site had the best adsorption effect on S₈/LiPSs.This is due to the shorter adsorption distance of S₈/LiPSs on the Ni-doped conformation and the catalytic conversion of S₈ to Li₂S.process,the S in the lithium-sulfur molecule interacts more strongly with Ni（shorter Ni-S bond）and the Li-S bond is longer,resulting in a greater morphological change in the LiPSs.In addition,the introduction of Sn makes the LiPSs more distant from the surface of the substrate material,thus reducing the anchoring effect of the substrate on the LiPSs.Subsequently,by calculating the Gibbs free energy of redox of S₈ on the P（1）Ni and P（1）Sn structures,it was found that the introduction of both Ni and Sn atoms accelerated the redox reaction of S₈ to a greater extent（total value of Gibbs free energy:1.36 e V without additives,-0.50 e V for undoped heterostructures,-0.57 e V for P（1）Sn,and-1.17 e V for P（1）Ni）,which is improve the battery cycle efficiency is crucial.The above results indicate that the P（1）Ni type structure has a better overall performance and is expected to better suppress the shuttle effect and improve the battery efficiency.Finally,the effect of double Ni atom doping on the material anchoring and catalytic LiPSs was investigated based on the P（1）Ni-type structure.Based on the different doping sites,three double Ni atom doped structures were constructed,named Dual＿Ni＿Ⅰ,Dual＿Ni＿Ⅱand Dual＿Ni＿Ⅲ.Their adsorption energies and Gibbs free energies for LiPSs were calculated based on DFT.The results show that all three dual Ni-doped structures are able to reduce the activation energy required for S₈ conversion to a greater extent（free energy decreases by 2.97 e V,2.38 e V and 2.23 e V,respectively）.In addition,the Dual＿Ni＿Ⅰstructure further improves the adsorption performance of LiPSs,especially for Li₂S,which increases from 3.82 e V to 4.06 e V for single Ni atom doping,while the Dual＿Ni＿Ⅱstructure shows superior adsorption ability for long-chain LiPSs（S₈,Li₂S₈ and Li₂S₆）.By structural and electronic analysis,the distance between the double Ni atom-doped CoSe₂/MoS₂ and LiPSs is shorter,and the Ni-S bond and Li-Se bond are more strongly bound.Moreover,the diffusion potential barriers of Li₂S on the three structures are 1.32 e V,1.27 e V and 1.25 e V,respectively,which still maintain a low diffusion potential barrier,which is the key to promote the efficient conversion between LiPSs.In conclusion,the Ni atom-doped CoSe₂/MoS₂ heterostructures have a large potential as catalytic materials for Li-S battery anodes under the dual effect of heterojunction engineering and doping strategy.