Preparation Of Cobalt Nanoparticles And Vanadium Dioxide Nanoflowers Composites And Study Their Catalytic Properties In Room Temperature Sodium-sulfur Batteries

Present mobile devices,transportation tools,and renewable energy technologies are all dependent on newly developed low-cost,long-life,safe,high-capacity,high-power and environmentally friendly energy storage batteries.Rechargeable room-temperature sodium-sulfur?RT Na-S?batteries as one of the most attractive systems for future stationary energy storage have receive lots of attentions due to their ultrahigh theoretical capacity(1675 mAh g^-1)and specific energy(1273 Wh kg^-1).However,the research of RT Na-S batteries is still in the initial stage,which faces two very difficult problems:low reversible capacity and rapid capacity decay.The main reason is that the sluggish reaction kinetics of the nonconductive S and its discharge products?Na₂S?,resulting in low sulfur utilization rate.In addition,the shuttle effect of polysulfide intermediates between the cathode and anode another great challenge.Therefore,optimizing the nanostructure of electrodes for RT Na-S batteries are critical for improving the conductivity and activity of sulfur and further preventing the dissolution of polysulfide.In this paper,we have designed and synthesized three catalytic multifunctional materials as the host of sulfur,exhibiting excellent electrochemical performance.Among them,the good conductivity of carbon materials can promote conductivity of the cathode.Besides,the catalytic activity of nanoparticals/metal oxides in RT Na-S battery can inhibit the diffusion of sodium polysulfides and speed up the conversion reaction kinetics of Na₂S_n.The main contents of the study are shown as follows:1.Synthesis of Co@NPCNFs/S composite:A free-standing nitrogen-doped porous carbon nanofibers with embedded Co nanoparticles?Co@NPCNFs?is designed by electrospinning,loading sulfur to assemble RT Na-S batteries.This electrode exhibits multifold features:?i?the 1D free-standing carbon nanofibers have good electrical conductivity,with large surface area and high surface-to-volume ratio.They can act as a conduit to accelerate the transport of electrons and ions,which can be able to provide abundant active sites for sulfur.Moreover,the porous structure of composites can effectively accommodate volume expansion of sulfur during electrochemical reaction process.?ii?Co nanoparticles can trap polysulfide species through chemisorptions effect,remitting shuttling effect and prolonging the lifetime of the electrode.?iii?The most interesting thing is that the metallic Co also plays a magical catalyst in Na-S battery system to accelerate the reaction kinetics of long chain polysulfide to final product Na₂S.Benefiting from the unique structural and compositional merits,the prepared Co@NPCNFs/S electrode shows a remarkable capacity of 906 mAh g^-1 at 0.1 C and long cycling life up to 800 cycles with a very slowly capacity decay of 0.038%per cycle at 1C.2.Synthesis of CNF-L@Co/S composite:3D“branch-leaf”biomimetic design has been proposed for high-performance RT Na-S batteries.The leaves constructed from Co nanoparticles on carbon nanofibers?CNF?are able to fully expose the activity sites of Co,where CNF network acts as the conductive“branches”to ensure adequate electron and electrolyte supply for the Co leaves.As an effective electrocatalytic battery system,the3D“branch-leaf”conductive network with abundant active site and voids can effectively trap polysulfides and provide plentiful electron/ions pathways for electrochemical reaction.DFT calculation reveals that the Co nanoparticles can induce the formation of a unique Co-S-Na molecular layer on Co surface,which can enable fast reduction reaction of the polysulfides.Therefore,the prepared“branch-leaf”CNF-L@Co/S electrode exhibits a high initial specific capacity of 1201 mAh g^-1 at 0.1 C and superior rate performance.3.Synthesis of rGO/VO2/S composite:a three dimension?3D?hierarchical cathode substrate with VO₂ nanoflowers as catalyst in situ grown on reduced graphene oxide?rGO?for sulfur cathodes are designed and prepared.In this novel structure,the electronic conductivity of sulfur cathode can be greatly improved due to the high electrical conductivity of rGO substrate.More importantly,the catalytic effect of VO₂ accelerates the conversion of long-chain NaPSs to Na₂S₂/Na₂S,and enhances the cycle capability.As a result,the asobtained r GO/VO₂/S composites achieves an initial reversible capacity of876.4 mAh g^-1 at 0.2 C.Moreover,the capacity of 156.1 mAh g^-1 is demonstrated after long-cycling term of 1000 cycles at 2 C and high cycle stability of only 0.07%decay per cycle is exhibited.In summary,three kinds of catalytic multifunctional cathode materials were designed as the host of sulfur for RT Na-S batteries,all of which exhibiting brilliant electrochemical performance.Thus,it provides new ideas for the preparation of high capacity cathode composite materials for RT Na-S batteries.