Preparation And Electrochemical Performance Of 3D Porous Nitrogen-sulfur Co-doped Carbon-based Cathode Catalysts For Lithium-air Batteries

With the excessive consumption of non-renewable fossil fuels and the increasingly serious environmental burden,people are urgently looking for a green and sustainable energy to replace the traditional non-renewable energy such as oil and coal.Rechargeable li-air batteries have an ultra-high theoretical energy density(11400Wh kg^-1),which is comparable to that of fossil energy sources such as gasoline(13000Wh kg^-1)and 5-10 times higher than that of conventional lithium-ion batteries.Therefore,lithium-air battery is regarded as one of the most promising electrochemical energy storage devices.Unfortunately,the development of Li–O₂ batteries is still in its early stages and many key technical issues,such as low round-trip efficiency,poor cycle life,and low rate capability,still need to be addressed before their practical application.Oxygen cathode is the place where electrochemical reactions take place,and its structure significantly affect the battery performance.During discharge,oxygen from ambience enters the O₂ cathode reacts with Li⁺to form LiO₂,follow by a chemical disproportionation reaction or a electrochemical reduction reaction to generate discharge product Li₂O₂?oxygen reduction reaction,ORR?.The Li₂O₂ will electrochemically decompose to oxygen and Li⁺during charge?oxygen evolution reaction,OER?.Because of the sluggish kinetics of ORR and OER of Li-air batteries,it is necessary to explore excellent catalysts for reducing the high over-potential and realizing the reversible reduction and generation?ORR and OER?of the oxygen in the li-air batteries.In addition,the insulated and insoluble Li₂O₂ will increasingly blocks the voids of the O₂ cathode and impede the transport of oxygen and electrolyte with battery cycling that eventually leading to the cell death.Therefore,an ideal cathode catalyst should not only be able to effectively catalyze the reversible reduction and generation of oxygen,but also have a reasonable porous structure to accommodate Li₂O₂ and promote oxygen transport.Based on the above analysis,we carried out the following works.1.Based on the concept of binder-free oxygen electrode,we have deposited a novel 3D hierarchically ordered N,S co-doped macro–mesoporous carbon catalyst on carbon paper?NSMmC/CP?by using functionalized fluidic acylonitrile telomer?ANT?as carbon source and doping agent.Firstly,Amino-Modified SiO₂ Nanospheres were assembled on the carbon fiber by electrostatic attraction.After that,the acetone solution of ANT were impregnated into amino-modified SiO₂ nanospheres template and covered on the surface.The ANT was further treated by pre-oxidation and carbonation,followed by etching silicon with hydrofluoric acid to obtained the 3D hierarchically N,S co-doped macro-mesoporous carbon?NSMmC/CP?.When used as a binder-free electrode for Li-O₂ batteries,the NSMmC/CP electrode exhibited excellent electrochemical performance.The initial galvanostatic discharge/charge profiles at a current density of300 mAg^-1 and voltage range of 2.0-4.5 V shows that NSMmC/CP electrode achieved a relatively high specific capacity of 6296mAh g^-1with a overpotential of 1.70V.In addition,when the limited capacity is 500mAh g^-1 and 1000mAh g^-1,NSMmC/CP cathode keep stable for more than 30 cycles and 12 cycles at a current density of 300mA g^-1,respectively.In spite of this,the NSMmC/CP electrode is not satisfactory for improving the performance of lithium-air batteries.The morphology of the electrode after discharge was analyzed by means of XRD and SEM,and the mechanism of electrochemical reaction of the electrode was explored.PdNi nanoparticles can be used as the active site for nucleation and growth of Li₂O₂,and guide the formation of cage structure,so as to reduce the charge overpotential.2.The ultrafine PdNi nanoparticles were homogeneously loaded on the 3D NSMmC/CP skeleton by simultaneous reduction using sodium borohydride as reducing agent.The composition and structure of PdNi-NSMmC/CP samples were characterized by XPS,SEM,TEM and XRD.TEM results show that NSMmC/CP can effectively disperse PdNi binary alloy nanoparticles.When used as a cathode of Li-O₂ batteries,the PdNi-NSMmC/CP cathode can obviously reduce the overpotential,increase capacity,improve cycle life and rate capability.PdNi-NSMmC/CP cathode keep stable for more than 120 cycles with a limited capacity of 500mAh g^-1.While the limited capacity is increase to 1000mAh g^-1,it remain its initial capacity over 70 cycles.The PdNi-NSMmC/CP electrode achieved a relatively high specific capacity of 9965 mAh g^-1 and low overpotential of 1.05V at a voltage range of 2.0-4.5V.Even when current density increases to 1400mA g^-1,the battery still has a specific capacity of 4412mAh g^-1,which represent a high rate capability.The morphology of the electrode after discharge is analyzed by means of XRD and SEM,and the mechanism of electrochemical reaction of the electrode is explored.3.We have successfully constructed a three-dimensional macroporous structure,which was composed of a three-dimensional macroporous carbon skeleton and a few layers of MoS₂ crystals embedded in the N,S co-doped carbon skeleton?MoS₂@NSC?,through the evaporation-induced self-assembly route.The TEM test results showed that the carbon skeleton was rich in MoS₂ crystals with layer number of 1-3 layers.This unique structure can effectively prevent the stacking of ultrafine MoS₂ nanosheets and maximize the catalytic activity of the MoS₂@NSC electrode.The Li-air battery with MoS₂@NSC-2 electrode run stable for more than 45 cycles with a limited capacity of500 mAh g^-1 at a current density of 200mA g^-1.When used as ORR catalyst in 0.1M KOH,MoS₂@NSC-2 exhibited a halfpotential of 0.82V and an initial potential of 0.93V.In addition,the oxygen reduction reaction catalyzed by MoS₂@NSC-2 demonstrated a four electron transfer reaction and have superior stability for methanol.