Design And Preparation Of Transition Metal Sulfide Composites And Their Applications In The Cathodes Of Lithium-sulfur Batteries

As modern society's energy storage requirements for portable electronic devices,emerging electric vehicles and smart grids continue to increase,it is urgent for people to find energy storage devices with high energy density,long cycle durability and high electromotive force.Lithium-sulfur?Li-S?batteries stand out among many energy storage devices due to their high theoretical capacity(1675 mA h g^-1)and energy density(2600 W h kg^-1),which have attracted widespread attention from researchers.Moreover,the active substance sulfur has the advantages of low cost,safety and environmental friendliness,which can well meet the demand of sustainable energy development.However,the existing problems such as poor conductivity of sulfur and Li₂S,volume expansion caused by lithiation reaction,and shuttle effect of polysulfides have limited the further development of Li-S batteries.In view of the above problems,designing new type of positive electrode composite materials that can effectively fix polysulfides and reduce their diffusion is an effective way to improve the electrochemical performance of Li-S batteries.Based on the advantages of transition metal sulfides with high electrical conductivity and catalytic activity,the design and preparation of cathode composites are mainly based on transition metal sulfides.The main contents are as follows:?1?The precursors Co?OH?₂/PAA-Zn NSs were synthesized using PAA-Zn nanospheres?NSs?as templates and CoCl₂·6H₂O as raw materials.Subsequently,the precursors were carbonized and the composite materials of Co and micro-mesoporous carbon?Co@MMC NSs?were synthesized.Finally,the composite materials of ultra-fine CoS₂ nanocrystals and micro-mesoporous carbon spheres,namely CoS₂@MMC NSs,were prepared by sulfuration reaction.The obtained CoS₂@MMC NSs can achieve synergistic effect of physical confinement and chemical catalysis of polysulfides through carbon spheres with micro-mesoporous structures and well-dispersed ultrafine CoS₂ nanocrystals.At the same time,the micro-mesoporous structure also provides sufficient space for sulfur loading.Based on these advantages,the initial capacity of the S/CoS₂@MMC cathode at 0.1 C can reach 1227 mA h g^-1.Moreover,after 1000 cycles at 1.0 C,it still has a capacity of 606 mA h g^-1,and the attenuation rate per turn is only 0.035%.?2?The precursors Ni?OH?₂/PAA-Mo NSs were synthesized using PAA-Mo NSs as templates and NiCl₂·6H₂O as raw materials.Subsequently,MoS₂/Ni₃S₂ yolk-shell heterostructure nanomaterials were synthesized by a simple hydrothermal reaction and annealing treatment,and used as the host materials of Li-S batteries.The combination of yolk-shell structure and heterostructure enables MoS₂/Ni₃S₂ NSs to have large sulfur storage space and abundant active sites,which can effectively capture polysulfides and dynamically promote the redox reaction.As the cathode of Li-S batteries,the initial discharge capacity of MoS₂/Ni₃S₂@S at 0.1 C is 1445.4 mA h g^-1.In addition,after 1000 cycles under 1.0 C,the capacity of 739 mA h g^-1can still be maintained,and the capacity only decreases by 29.4%,indicating that the capacity attenuation rate per cycle is 0.029%.