Preparation Of Transition Metal Chalcogenide Materials And Its Application In Lithium-sulfur Batteries

Lithium-sulfur（Li-S）batteries have been regarded as one of the most promising candidates of next-generation high-energy batteries due to the high theoretical eenrgy density of 2600 Wh kg^-1 or 2800 Wh L^-1 and low cost of sulfur(≈0.05$kg^-1).However,sulfur cathodes suffer from some drawbacks including the low electronic conductivity,volume expansion,sluggish redox kinetics and shuttle effect.These issues lead to the low utilization of sulfur active materials and rapid capacity decay,which greatly hinders the development of Li-S batteries.In order to deal with these issues,various transition metal chalcogenide/porous carbon composites have been prepared and applied as the host materials of sulfur for Li-S batteries.On one hand,the good conductivity and porous structure of carbon mateirals are conducive to improve the dispersibility of sulfur and the conductivity of the cathodes.On the other hand,the strong chemical adsorption of sulfur species by these transition metal sulfides and selenides can effectively suppress the shuttle effect and promote the conversion between vairous sulfur species,thereby comprehensively enhancing the electrochemical performance of Li-S batteries.Firstly,the hydrothermal-sulfuration method was used to a hybrid with NiCo₂S₄nanosheets decorated on carbon nanoboxes,denoted as NiCo₂S₄/CNB.Our results demonstrate that CNB has a thin porous shell and a huge cavity for effective accommodation of sulfur and physical restriction of soluble polysulfides.At the same time,the surface-grown NiCo₂S₄nanosheets show strong chemical confinement to polysulfides and have a good catalytic effect on the conversion of polysulfides.Thanks to the dual regulation of polysulfides by NiCo₂S₄/CNB host,the prepared S/NiCo₂S₄/CNB cathode shows outstanding rate capability and cycle performance.With a sulfur loading of 4.8 mg cm^-2 and the electrolyte/sulfur（E/S）ratio of 15μL mg _S^-1,the cathode shows a low capacity decay rate of 0.17%at 0.2 C within 150 cycles.Even with a low E/S ratio of 3μL mg_S^-1,the cathode delivers a high capacity of 637m Ah g^-1 at 1 C and a low capacity decay rate of 0.15%within 200 cycles.Secondly,metal-organic frameworks（Zn-ZIF）were obtained with 2-methylimidazole and zinc nitrate as the raw materials,and Zn Se@PC nanoparticles encapsulated in nitrogen-doped porous carbon frameworks（Zn Se@PC）are subsequently obtained via a selenylation treatment.The abundant pores of PC are favorable for loading sulfur and the doped N elements can confine soluble polysulfildes via a Lewis acid-base interaction.Zn Se@PC nanoparticles shows strong chemical interaction with polysulfides,and promote both the reduction of polysulfides and the oxidation of Li₂S.The results of electrochemical tests demonstrate that the initial capacity is 1152 m Ah g^-1 at 0.1 C for S/Zn Se@PC cathode,which is much higher than that of S/PC cathode(998 m Ah g^-1).S/Zn Se@PC cathode also exhibits good cycle stability over 600 cycles at 1 C with a low capacity decay rate of 0.073%per cycle.Even a sulfur loading of～5.18 mg cm^-2 and the E/S ratio of～6μL mg_S^-1,S/Zn Se@PC cathode can offer an initial capaicty of 812 m Ah g^-1 at 0.1 C and good cycle stability.Thirdly,binary Co/Zn-ZIF precuresor was obtained with 2-methylimidazole,zinc nitrate and cobalt nitrate as the raw materials,and Zn Se/Co Se@PC composite was subsequently obtained via a selenylation treatment and applied as the sulfur host.The introduction Co Se component can further enhance the chemical confinement of polysulfides and the catalytic effect on sulfur redox reactions due to the synergistic effect between Co Se@PC and Zn Se@PC.The results demonstrate that the binary Zn Se/Co Se@PC host shows superior electrochemical performance in Li-S batteries compared with single selenide host.At 0.1 C,a high discharge capacity of 1217.6 m Ah g^-1 is realized for S/Zn Se/Co Se@PC cathode.Besides,the cathode shows excellent cycle performance at 1 C with a low capacity decay rate of 0.083%per cycle over 900cycles.Even a sulfur loading of～5.5 mg cm^-2 and the E/S ratio of～6μL mg_S^-1,S/Zn Se@PC cathode can offer an initial capaicty of 866.5 m Ah g^-1 at 0.1 C.This work indicates the potential application of binary metal selenides in Li-S batteries.In summary,in this thesis,composite host materials of sulfur based on transition metal sulfides/selenides and porous carbon are designed and prepared to address the shuttle effect and slow kinetics of sulfur cathodes.Benefited from the excellent conductivity and porous structure of carbon materials,and the chemical adsorption and catalytic effect of metal chalcogenides on polysulfides,the electrochemical performances of sulfur cathodes are comprehensively improved.The concept of integrating physical confinement,chemical adsorption,and catalytic effect can provide some references for the development of cathode materials for Li-S batteries.