Preparation Of Cobalt-based Chalcogen Composite Nanomaterials And Their Application In Cathode Of Lithium-sulfur Battery

Lithium-sulfur batteries with high energy density(2600 Wh kg-1)and high theoretical specific capacity(1672 mAh g-1)are considered the most promising candidates for a new generation of energy storage devices.because of their easy access to raw materials,low price,no pollution and other advantages,they have entered the view of researchers.However,there are a series of problems of the lithium sulfur battery,which lead to the low capacity of the battery in the research work.For example,S and discharge product Li2S have poor conductivity,the decrease of sulfur utilization is due to the fact that the intermediate lithium polysulfide(LIPSs)is easily soluble in electrolyte,the shuttling effects caused by LIPSs migration during charging and the volume change(?80%)during charging and discharging.Therefore,it is the main research work to design a kind of host material with excellent electrical conductivity,sufficient internal space which can accommodate volume expansion,trapping and catalyzing the LiPSs.Herein,metal compounds and carbon materials were used to design a variety of structures of cobalt-based oxygen-group composites,which show excellent electrochemical performance with unique structure and good conductivity.(1)Phosphorus doped carbon nitride(PCN)was synthesized by thermal polymerization and loaded on the surface of Co3O4 microspheres with cobalt nitrate hexahydrate as the cobalt source by reasonably controlling the conditions of hydrothermal reaction,then modified on the surface of Co3O4/PCN by multi-wall carbon nanotubes(MWCNT),Co3O4/PCN/MWCNT composites were prepared.Mesoporous composites can increase sulfur storage and buffer volume change caused by cycling.Co3O4 microspheres have physical confinement for LiPSs and catalyze the conversion reaction of LiPSs.PCN has a strong chemical adsorption on LiPSs.MWCNT significantly improve the conductivity of the composites.At 0.5 C,the initial discharge specific capacity is 1006.9 mAh g-1,and the capacity maintains 585.7 mAh g-1 after 500 cycles.The coulomb efficiency is basically stable above 99%,and the capacity decay rate is only 0.084%.(2)CoS nanoboxes with rough surface and sheet-like CoS stacked were synthesized by using ZIF-67 nanocubes as precursors.Amorphous nitrogen-doped carbon(NC)was modified on the shell of CoS NB to obtain the composite CoS@NC.CoS nanoboxes are uniform and their sizes are about 260 nm and the shell thicknesses are about 40 nm.The hollow structure of CoS@NC composites is beneficial to store the active substance and cushion the volume expansion caused by the discharging;the large specific surface area can facilitate penetration of electrolyte,and the good conductive layer accelerates the redox reaction and improves the sulfur utilization.The electrode S/CoS@NC shows remarkable cycling performance,an initial discharge specific capacity of 1269.6 mAh g-1 at 0.2 C,retaining 84.6%reversible specific capacity after 100 cycles;it also has excellent rate performance at 2 C(794.0 mAh g-1).More importantly,after 500 cycles at a larger current density of 1 C,the specific capacity is still 702.5 mAh g-1 with a low decay rate of 0.076%.(3)ZIF-67 nano-polyhedron was synthesized for the first time,and it was used as a template to coat by phenolic resin.After burning,the intermediate(C/Co@C)with a hollow carbon shell structure embedded of Co particles was obtained.Then the polarity of the carbon shell is improved by doping nitrogen.Finally,the cobalt particles were selenized to obtain a unique structure of C/CoSe2@NC composites.The C/CoSe2@NC composites can improve the agglomeration of cobalt selenide and the condition of large particles,and promote the catalytic conversion of CoSe2 for LiPSs.The internal cavity structure and porous conductive shell of C/CoSe2@NC inhibit the "shuttle effect" of LiPSs and improve the sulfur utilization.S@C/CoSe2@NC has excellent long-cycle performance.After 500 cycles at 1 C,the extremely low fading rate is 0.036%per cycle,and the reversible capacity is 772.3 mAh g-1.The unexceptionable discharge areal capacity is 7.1 mAh cm-2 with a larger sulfur loading of 6.14 g cm-2 at 0.2 C,which shows good commercial potential.