Preparation And Electrochemical Performance Of Carbon Nanofibers Membrane Modified With Transition Metal(M=Ti,Mo,Cr)Carbides

Lithium-sulfur(Li-S)batteries have become the research focus in recent years because of their high theoretical specific capacity and theoretical specific energy.However,the low sulfur loading,shuttling effect caused the dissolved polysulfides in electrolyte and poor cycling stability limit their applications.In order to solve the above problems,the research contents and result of this paper are as follows:Based on commercial carbon nanofibers(CNFs)and titanium carbide(Ti C),the Ti C/CNFs composite membrane was prepared by ultrasonic dispersion-vacuum filtration process,and the Li₂S₆ solution was used as the active material to prepare Ti C/CNFs/Li₂S₆ composite electrode to study their electrochemical performance.The results show that Ti C modified CNFs not only increased the migration rate of lithium ions,but also reduced the charge transfer resistance and electrode polarization,and improved the utilization rate of active materials compared with CNFs membrane.The electrochemical test results show that Ti C improves the redox kinetics of polysulfide in the electrochemical process of the electrode.The composite electrode(sulfur loading:4.74 mg)shows a capacity of 899 m Ah g^-1 and maintains at 689 m Ah g^-1 at 0.2 C over200 cycles.In addition,Ti C modified CNFs can effectively prevent the self-discharge behavior of the cell and improve the stability of the lithium anode by capturing polysulfidesBased on the above optimized process,molybdenum carbide modified carbon nanofibers(Mo₂C@CNFs)membrane was prepared in situ by ammonium molybdate((NH₄)₆Mo₇O₂₄·4H₂O)solution impregnation and carbon-thermal reduction.Mo₂C@CNFs/Li₂S₆composite electrode was prepared by using Li₂S₆ solution as the active material.Electrochemical deposition results show that the polysulfides deposition capacities of Mo₂C@CNFs and CNFs was 476.1 m Ah g^-1 and 198.7 m Ah g^-1,respectively.It was further demonstrated that Mo₂C could regulate and improve the transformation and deposition of polysulfide,reduce electrode polarization,and improve the electrochemical performance.When the sulfur loading is 4.74 mg,the discharge specific capacity of the composite electrode is 977 m Ah g^-1 and maintains at828 m Ah g^-1 at 0.2 C over 250 cycles.Even the sulfur loading is as high as 7.11 mg,the Mo₂C@CNFs/Li₂S₆ electrode still has a high capacity of 5.75 m Ah.Compared with CNFs/Li₂S₆ membrane electrode,Mo₂C@CNFs can effectively adsorb polysulfides,inhibit the"shuttle effect",and improve the self-discharge behavior of cell;regulate the surface morphology of lithium metal anodes and improve the electrochemical performance of high sulfur loading of electrode.Based on the above research results,the solution impregnation method of ammonium chromate((NH₄)₂Cr O₄)and carbon-thermal reduction technology were used to prepare the chromium carbide modified carbon nanofibers(Cr₃C₂@CNFs)membrane.When the sulfur loading is 4.74 mg,the discharge specific capacity of the composite electrode is 1012 m Ah g^-1 and maintains at 631 m Ah g^-1 at 0.2 C over 300cycles.Even the sulfur load is up to 7.11 mg,it still remains a capacity of 6.11 m Ah.The results of electrochemical study and the surface morphology of the electrode after cycling indicate that Cr₃C₂ can effectively improve the deposition of lithium sulfide on the electrode surface and improve the utilization rate of the active material.Simultaneously,it can effectively inhibit the"shuttle effect"of dissolution polysulfide and reduce the surface corrosion of lithium anode.