Hierarchical Hollow Nanospheres Composed Of Mo-based Nanoparticles Grown On Carbon Nanosheets For High-performance Lithium-sulfur Batteries

Batteries with high energy density and long cycle stability have received great attentions due to the rapid development of electric vehicles,portable electronics and renewable energy such as solar energy and wind energy.Li-S battery has been considered as one of the most promising candidates for next-generation electrochemical energy storage devices owing to the high theoretical energy density(2600 Wh kg^-1).However,severe barriers have impeded the practical application of Li-S battery,including the poor conductivity of sulfur,large volume variation during cycling and the severe shuttle effects caused by soluble lithium polysulfide(Li PS),which result in fast capacity decay and poor coulombic efficiency.To improve the electrochemical performance of Li-S batteries,we synthesize hollow nanospheres composed of Mo-based nanoparticles grown on carbon nanosheets(Mo_xZ_y/C HS,Z=C,N,O,and x,y=1,2,3)as suflur host and separator modification.The effect of Mo_xZ_y/C HS with different non-metal anions on the electrochemical performance of Li-S batteries and intrinsic mechanism are discussed.The main research contents and conclusions are summarized as follows:1.Preparation of Mo_xZ_y/C HS as sulfur host for Li-S batteries and their electrochemical performance.Hollow nanospheres composed of Mo_xZ_y nanoparticles grown on carbon nanosheets(Mo_xZ_y/C HS)are synthesized,and the S-Mo_xZ_y/C HS cathodes are prepared via an impregnation procedure.The S mass loading within Mo₂C/C HS,Mo₃N₂/C HS and Mo O₂/C Hs are 62.9 wt%,59.8 wt%and 61.1 wt%,respectively.Mo_xZ_y nanoparticles can adsorb and catalyze soluble Li PS,and the two-dimensional carbon sheets provide fast electron transport.The initial capacities of S-Mo₂C/C HS,S-Mo₃N₂/C HS and S-Mo O₂/C HS cathode are measured to be 1064.3,973.9 and 1016.1 m Ah g^-1 at 0.1 C(1 C=1675 m Ah g^-1),and the capacity retention rates of the three cathodes are 67.2%,59.7%and 62.6%at 1.0 C after 300cycles,respectively.The difference in electrochemical performance of such three cathodes are mainly sttributed to various adsorption and catalytic activity of Mo_xZ_y/C HS with different non-metal anions for Li PS.(1)The Mo₂C/C HS has the strongest adsorption ability for Li PS,while the Mo O₂/C HS has a slightly weaker adsorption ability,and the Mo₃N₂/C HS has the weakest adsorption ability.(2)The Mo₂C/C HS has higher catalytic conversion activity for Li PS than those of the Mo₃N₂/C HS and Mo O₂/C HS.The adsorption ability of Mo₃N₂/C HS for Li PS is relatively weak.The Mo O₂/C HS has low catalytic conversion activity for Li PS.The Mo₂C/C HS has both strongest adsorption ability and highest catalytic activity for Li PS among these three host materials.Therefore,the Mo₂C/C HS as sulfur host exihibit excellent electrochemical performance.2.Preparation of Mo_xZ_y/C HS@CNTs modified separators and their electrochemical performance of Li-S batteries.Mo_xZ_y/C HS@CNTs modified separators were prepared via the vaccum filtration of the dispersion solution of Mo_xZ_y/C HS and carbon nanotubes(CNTs)in ethanol.The Li-S batteries are assembled with S-CNT cathodes,modified separators and lithium metal anodes.The capacity retention rates of Li-S batteries assembled with Mo₂C/C HS@CNTs,Mo₃N₂/C HS@CNTs and Mo O₂/C HS@CNTs modified separators are 86.2%,83.7%and 78.9%at 0.5 C after 250 cycles respectively.And the discharge specific capacity retention at the high-voltage platform for batteries assembled with three types of separators are 80.0%,71.1%and 61.7%at 1.0 C after 500 cycles respectively.The different electrochemical performance with Mo_xZ_y/C HS@CNTs modified separators are attributed to different adsorption ability and catalytic activity for Li PS and the difference in the deposition and conversion for Li₂S.(1)The capacity of Li₂S precipitation on CP-Mo₃N₂/C HS(214.88m Ah g^-1)is slightly higher than those on CP-Mo₂C/C HS(198.03 m Ah g^-1)and CP-Mo O₂/C HS(191.37 m Ah g^-1).(2)Li-S battery used the Mo₂C/C HS@CNTs modified separator has lowest overpotentials during charging process.The Mo₂C/C HS can enhance the deposition and growth of Li₂S on their surface,and efficiently catalyze the conversion of Li₂S to Li PS.Therefore,the batteries assembled with Mo₂C/C HS@CNTs modified separators exhibit good electrochemical performance.