Design And Synthesis Of Mesoporous Hollow Carbon Spheres/Molybdenum-based Composites And Their Application In Lithium-sulfur Battery

Lithium-sulfur（Li-S）batteries have prominent advantages such as high theoretical energy density,rich sulfur resources,and environmental friendliness.They are considered to be one of the most competitive next-generation secondary batteries.However,the practical application of Li-S batteries still faces many technical challenges,such as poor conductivity of sulfur and its discharge products,severe volume expansion during charging and discharging,shuttle effect caused by polysulfides soluble in electrolytes and slow reaction kinetics,etc.These problems have led to low discharge specific capacity,severe capacity degradation,and difficulty in achieving rapid charging and discharging,which have hindered the commercialization of Li-S batteries.In this paper,from the idea of structural design of sulfur host materials,mesoporous hollow carbon spheres with different structures were synthesized to improve the poor sulfur conductivity and volume expansion during charging and discharging.Furthermore,based on the strategy of optimizing the adsorption and catalytic properties,different molybdenum-based compounds were introduced into the mesoporous hollow carbon spheres to construct a multifunctional sulfur host in order to improve the comprehensive performance of Li-S batteries.Results of the study are as follows:（1）To improve the problems of poor sulfur conductivity and volume expansion,mesoporous hollow carbon spheres（MHCS）were prepared by a simple one-pot self-assembly method,carbonization,and etching.Three compounds（resorcinol,formaldehyde,and tetrapropyl orthosilicate）,were utilized as raw materials with ammonia as catalyst.The structural parameters（specific surface area,pore size,pore volume,and cavity）of MHCS were controlled by adjusting the concentration of reactants.The structural features of three typical MHCS were analyzed in detail,their formation reasons were explained,and they were used as sulfur hosts to evaluate their electrochemical properties.Among them,the optimized MHCS featured with high specific surface area(1547.9 m² g^-1),large pore volume(2.04 cm³ g^-1),moderate pore size（～6 nm）,and moderate internal cavity（r₂/r₁=0.62）,which can meet the requirements of high sulfur loading,close carbon/sulfur contact,sufficient buffer space,and fast electron/ion transport paths.As a result,the Li-S batteries based on this material achieved a reversible discharge specific capacity of 483 mAh g^-1 after 500 cycles at 0.5 C.The optimized MHCS@S cathode with a sulfur loading of 90.02 wt%and an electrolyte/sulfur ratio（E/S）of10μL mg^-1 displayed good electrochemical performance(484 mAh g^-1 reversible specific capacity after 500 cycles at 0.5 C with 0.068%decay rate).In addition,the raw materials required for MHCS involved only three small molecule compounds,which were cheap and easy to obtain,the synthesis steps were simple and easy to perform without the need for expensive equipment.（2）To address the shuttle effect and slow reaction kinetics,mesoporous hollow carbon spheres loaded with MoO₂ nanosheet composites（MHCS@MoO₂）were designed and synthesized,their morphology and structure were characterized in detail,and the mechanism of the adsorption-catalysis of polysulfides by MoO₂ was investigated.The advantages of MHCS were retained in this material,the mesoporous shell and large internal cavity could provide sufficient sulfur-carrying space and buffer the volume expansion of sulfur;the high specific surface area and porous structure were favorable for the penetration of electrolyte and electron/ion transport.The MoO₂ nanosheets loaded on MHCS surfaces can not only anchor polysulfides by forming Mo-S bonds,but also accelerate the conversion of polysulfides and slow down their dissolution.As a result,the MHCS@MoO₂-S cathode exhibited excellent cycling stability,maintaining a reversible capacity of 771 mAh g^-1 after 500 cycles at 0.5 C,with a capacity decay of only 0.045%.In particular,a retention capacity of 3.32 mAh cm^-2 was achieved after 500 cycles at 0.1 C under a high sulfur loading of 5.0 mg cm^-2.This study provides a new idea for the design of a multifunctional sulfur host.（3）To further enhance the electrochemical performance of Li-S batteries at high current densities,the Mo₂C/C nanosheets grown in situ on mesoporous hollow carbon spheres composites（MHCS@Mo₂C/C）were designed and synthesised.Mo₂C/C nanosheets with heterogeneous structures can shorten electron/ion transport pathways.The ultra-small Mo₂C nanocrystals expose more active sites,enhancing the chemisorption and catalytic conversion efficiency of polysulfides.As a result,the MHCS@Mo₂C/C-S cathode exhibited a reversible capacity of 874 mAh g^-1 after 500 cycles at 0.5 C,with a capacity decay of only 0.045%.At a high sulfur surface loading of 4.5 mg cm^-2,it still exhibited a reversible capacity of 542 mAh g^-1 after 500 cycles at 0.5 C.This study further confirms the importance of combining rational structural design with efficient catalysts to construct multifunctional sulfur host.（4）To further demonstrate the universality of combining a rational structural design with an effective catalyst to construct multifunctional sulfur hosts,mesoporous hollow carbon sphere modified with ultrathin MoS₂ nanosheets composites（MHCS@uMoS₂）were designed and synthesized.Ultrathin and 1T metal-phase-containing MoS₂ nanosheets（uMoS₂）enhanced the electrical conductivity of the material while exposing a richer variety of active sites,which improved the facilitation of the kinetics of the oxygen reduction reaction of polysulfides and sufficiently suppressed the shuttle effect,thus enhancing the rate performance and the cycling stability of the high-specific-capacity Li-S batteries.As a result,the MHCS@uMoS₂-S cathode exhibited a reversible capacity of 940 mAh g^-1 after 500 cycles at 0.5 C with a capacity decay of only 0.037%.Under high sulfur loading of 5.6 mg cm^-2 and electrolyte-less(E/S=10μL mg^-1)conditions,it still exhibited a reversible capacity of 675 mAh g^-1 after 500 cycles at 0.5C.This study provides a reference for the development of sulfur host materials for high-capacity,long-life,and practical Li-S batteries.