Design And Catalytic Performance Of Molybdenum-based Catalysts For Lithium Sulfur Batteries

Lithium-sulfur（Li-S）batteries hold great potential as a next-generation energy-storage system due to the high theoretical energy density（2600 Wh/kg）,low cost,and high abundance of sulfur resources.However,the shuttling effect of lithium polysulfide（Li PS）and sluggish sulfur conversion kinetics inevitably result in low sulfur utilization and poor cycle life of lithium-sulfur batteries,which seriously restrict their practical application.By designing appropriate catalytic materials,the shuttle effect can be suppressed and the sulfur conversion kinetics can be improved to achieve a high specific capacity Li-S battery.Among the candidate catalysts,some Mo-based catalysts show similar catalytic activity to the noble metal Pt.Moreover,these Mo-based catalysts have the advantages of low cost and rich chemical valence,which has attracted extensive attention in the field of electrocatalytic sulfur conversion.In this direction,molybdenum-based materials are used as the electrocatalyst for the catalytic conversion of Li PS in this paper.The morphology,electronic structure,and surface state of Mo-based electrocatalysts are adjusted through micro/nanostructure design,low dimensional carbon reinforced composite,heterostructure construction,and anion vacancy defect activated heterostructure,so as to optimize their catalytic performance,regulate the deposition behavior of Li₂S,enhance the kinetics of Li PS conversion,and thus improve the utilization of sulfur.In addition,the enhancement mechanism of electrochemical performance was clarified through experiments and theoretical calculations,which provides guidance and inspiration for the development of high-performance Li-S batteries.A self-template method was proposed to prepare Mo-based hierarchical porous micro flower-like catalysts.By using ZnMoO₄ as the precursor,abundant nanopores were in situ formed on Mo₂C micron"petals"by volatilization of zinc during a chemical vapor deposition（CVD）process.This strategy can adjust the micromorphology of molybdenum-based catalysts on the nanometer scale and enhance the effective specific surface area of the catalyst.The electrochemical test results show that the reversible specific capacity of HPMF-Mo₂C at 0.2 C reaches1291 m Ah/g,and maintains 960 m Ah/g even at a high rate of 2 C,much higher than that of commercial molybdenum carbide particles（375 m Ah/g at 2 C）.To further improve the utilization rate of sulfur,a new strategy for improving the deposition behavior of Li₂S over low dimensional carbon composite Mo-based catalysts was proposed.An ordered multi-dimensional array structure of carbon nanotubes（CNTs）/Mo_xC nanorods was designed.The original discrete Mo_xC nanorod arrays could be connected by introducing Ni-catalyzed CNTs in situ to form a three-dimensional spatial cross-linked network structure.When used as the sulfur host of Li-S battery,the ordered multidimensional array structure can not only provide ample sulfur loading sites/space but also induce 3D uniform deposition of lithium sulfide,reducing the formation of"dead sulfur"during charging and discharging,avoiding the passivation of active sites.The as-assembled cell achieves a high reversible capacity of 1334 m Ah/g at 0.2 C,corresponding to 79.6 wt.%sulfur utilization.To improve the utilization rate of sulfur under lean electrolyte conditions,a strategy of enhancing Li⁺diffusion by holey graphene-supported Mo₂C nanodot composite membrane was proposed,and the mechanism of Li⁺diffusion affecting the electrochemical performance was clarified,and the influence mechanism of Li⁺diffusion on electrochemical performance was clarified.The holey graphene can facilitate the transfer of Li⁺and the penetration of electrolyte,thus enhancing the utilization of sulfur under lean electrolyte conditions.The Li⁺diffusion coefficient of the Li-S battery can be nearly doubled,and the rate performance and specific capacity can be increased by 20～30%.The as-fabricated cell achieves a high reversible capacity of 1375 m Ah/g at 0.2 C,corresponding to 82.1 wt.%sulfur utilization.Furthermore,a high capacity of 1102 m Ah/g is still attained with high sulfur loading of 6 mg/cm² under lean electrolyte operation（E/S=5.6μL/mg）.Adsorption is the premise of catalysis.Amorphous coupling heterogeneous interface method was proposed to enhance the intrinsic adsorption capacity of Mo-based catalysts.Carbon nanosheets supported crystalline Mo₂C-amorphous Mo O₃heterostructure is meticulously designed by oxygen plasma assisted method.DFT calculation and experimental results show that amorphization not only increases the active site of Mo O₃ but also improves its intrinsic adsorption activity,therefore,amorphous Mo O₃ has a stronger anchoring ability toward Li PS,and Mo₂C has a stronger catalytic conversion ability.Through the synergistic effect of two components in the crystalline-amorphous heterostructure,the conversion of Li PS is effectively regulated.The cell with Mo₂C-Mo O₃（10）separator achieves an impressive reversible capacity of 1500 m Ah/g,corresponding to 89.6 wt.%sulfur utilization.Based on the above research,a vacancy defect-activated heterogeneous interface strategy is proposed to enhance the adsorption and catalytic capability of Mo-based nanomaterials toward Li PS,and further improve the utilization rate of sulfur.Mo S₂-Ni S₂ micro flowers with rich S vacancy defects and heterogeneous interfaces were grown in situ on a 3D conductive cotton-based carbon tube.The synergistic enhancement mechanism of anion vacancy and heterogeneous interface on electrochemical performance was also revealed.Experiments and theoretical calculations show that the abundant S vacancy defects significantly increase the adsorption/catalytic active sites on the Mo S₂ base surface.Heterostructure construction can provide additional adsorption/catalytic active sites.In addition,the built-in electric field formed at the interface effectively promotes charge transfer,thus enhancing the conversion kinetics of Li PS.The as-assembled Li-S battery attains a high reversible capacity of 1528 m Ah/g at 0.2 C,corresponding to a high sulfur utilization of 91.2 wt.%.