Mn And Ru Bimetal Catalyst For Lithium-oxygen Battery Cathode

Among many secondary batteries,lithium-air battery is considered as a promising alternative for future energy storage systems owning to its high theoretical specific capacity and theoretical energy density and low cost.At present,the research on lithium-air battery is still in its infancy,the practical lithium-air batteries still suffered from side reactions,poor cycling ability,low rate capacity and high overpotential.Besides,the volatilization of electrolyte and the corrosion of lithium anode also hinder the further application of lithium-air battery.It has been well established that the electrochemical activity of cathode materials play a key role in the development of electrochemical performance of lithium-air batteries.Transition metal oxides are widely used as cathode catalysts for lithium-air batteries owing to its good ORR reactivity,low cost and simple manufacture process,but their poor OER activity needs further improvement.However,noble metals and their oxides possess high OER reactivity;carbon nanomaterials have been applied as substrate and catalyst of lithium-air battery because of its good electrical conductivity,huge specific surface area and well-developed pore structure.In this study,manganese dioxide and ruthenium/ruthenium oxide were in-situ grown on carbon cloth by electrochemical deposition or one-step hydrothermal method on multi-walled carbon nanotubes.XRD,Raman,TEM,SEM,XPS and other characterization methods were applied to confirm the composition,structure and morphology of the material.Subsequently,the electrochemical performance was studied by the constant current discharge/charge test,cyclicvoltammetry and electrochemical impedance spectroscopy.The morphology and composition of the discharge products after discharge and charge were analyzed in order to explore the effect of precious metal oxides and structure for the ORR and OER.The specific research contents are as follows:?1?In order to control the morphology of the one-step synthesized material,reaction conditions,the time of hydrothermal reaction,the hydrothermal temperature and feed ratio are explored.The SEM results shown that the optimal reaction conditions are hydrothermal temperature of 90?,hydrothermal time of 12 h and KMnO₄:RuCl₃mass ratio of 40:3.Ruthenium dioxide nanoparticles decorated manganese dioxide nanosheet were uniformly loaded on the surface of carbon nanotubes.Electrochemical studies show that this material possess a discharge capacity of 20910 mAh g^-1 at current density of 100 mA g^-1;the constant current test with limited potential window indicates that materials possess an excellent rate capability and good coulombic efficiency of94.5%;galvanostatic cycling study indicated the good cycling performance of this material,it could recharged for 90,94,100,120 cycles with limited capacity of 1000mAh g^-1 at the current density of 100 mA g^-1,200 mA g^-1,500 mA g^-1 and 1 A g^-1,respectively.It delivered over 110 cycles with limited capacity of 500 mAh g^-1 at 50mA g^-1.The overpotential was only 0.8 V,which results in an energy efficiency of 78%.Compared to the cyclic voltammetry curve of the carbon nanotubes loaded with only manganese dioxide,this material shows higher ORR activity and OER activity after the addition of ruthenium dioxide.According to the analysis of the electrodes after discharge and charge,the discharge product is mainly lithium peroxide,and the three-dimensional structure of this material provides enough space for the storage of lithium peroxide.The cathode catalyst shows good reactivity towards the decomposition of lithium peroxide.Finally,the cycling performances of the materials in different electrolytes were investigated.The results indicate that the electrolyte additives can reduce the overpotential of the first cycle significantly,but its effect on the cycling performance is limited.?2?In order to control the morphology of as-synthesized materials,different deposition conditions were investigated.The results show that the manganese dioxide illustrated the shape of nanoparticles at current density of 1,2,5 and 10 mA cm^-2 and deposition voltage of 0.5 V and 0.75 V,while it become nanosheet under the deposition voltage of 1.0 V and 1.25 V.The results showed that 1.0 V prepared materials under low overpotential,the material morphology remained after the deposition of Ru.The cyclic voltammetry results showed that the addition of Ru enhanced the ORR and OER reactivity a lot.The constant current cycling test results showed that the material possessed a good cycling performance.it maintains stably for 110 and 160 cycles at 200mA g^-1 and 500 mA g^-11 with limited capacity of 1000 mAh g^-1,respectively.it maintains stably for 175 cycles at 100 mA g^-1 with a capacity of 500 mAh g^-1.The cycling performance had a dramatic improvement owing to the binder-free nature of cathode.Lithium peroxide is confirmed as discharge product and can be decomposed effectively via the characterization and analysis of the cathode after charging and discharging,the three-dimensional structure shows an excellent ability of lithium peroxide storage.