Preparation And Characterization Of Cathodic Bifunctional Electrocatalysts For Non-aqueous Lithium-oxygen Battery

Non-aqueous lithium-oxygen batteries?LOBs?have the highest theoretical energy density?3505 Wh kg-1?compared with other novel rechargeable battery systems and would be sufficient to deliver a driver range of over 550 km by electrical vehicles,which consequently attracts tremendous attention and continuous endeavor to promote the practical application of non-aqueous LOBs.However,poor round-trip efficiency,bad rate capability and unsatisfactory cyclic performance limit their commercialization.Only by fully understanding and improving the discharge-charge process and designing high-performance electrocatalysts can we cope with the problems mentioned previously.In this thesis,we focus on bifunctional electrocatalysts for LOBs to improve the battery performance and the main research achievements have been stated as follows:RuO₂/MnOOH nanoflower electrocatalyst has been successfully synthesized through the two-step hydro-thermal reactions,during which octadecanoic acid and RuCl₃ was used as surfactant and source of ruthenium,respectively.The Li-O₂ battery containing RuO₂/MnOOH sustained 90 cycles with a capacity limit of 500 mAh g-1 at a current density of 50 mA g-1.Moreover,the capacity retention was 43.2%at a higher rate of 500 mA g-1 showing good cyclic performance and rate capability.Chronoamperometry and linear sweep voltammetry tests illustrated the high electrical performance which was attributed to the synergistic effect between RuO₂ and MnOOH.Ex-situ XRD,SEM and EIS characterizations also showed that the nanoflower structure of MnOOH was favor of the storage and decomposition of discharge products?lithium peroxide?.RuO₂/MnOOH electrocatalysts with different loading of RuO₂ were prepared by changing the mass of added RuCl₃.SEM and XRD results demonstrated that the more loading of RuO₂,the more changing from MnOOH to Mn3O4,accompanying with the gradual destruction of nanoflower morphology.Rotating disk electrode in alkaline solution and Li-O₂ battery tests found that when the mass ratio of MnOOH and RuCl₃ was 1:0.15,the obtained RuO₂/MnOOH showed the best electrochemical property.The Li-O₂ battery containing this RuO₂/MnOOH electrocatalyst sustained 87 cycles with a capacity limit of 1000 mAh g-1 at a current density of 200 mA g-1.From the contrast experiment and XPS,we could get the conclusion that the interaction between RuO₂ and MnOOH resulted in the morphological and structural change in the composite electrocatalysts as well as a higher valence than IV of ruthenium and a transition from crystalline to amorphous RuO₂,which made it possible to improve the whole electrochemical catalysis to ORR and OER.High-performance bifunctional RuO₂/Co₃O₄ electrocatalyst was synthesized by integrating the RuO₂ nanopaticles and two-dimension Co₃O₄ nanosheets,in which the latter one were prepared through surfactant self-assembly strategy by using block copolymer P123.XPS characterization confirmed much more oxygen deficient regions and Co3+ appeared after loading of RuO₂,which would contribute to oxygen absorption as well as providing more active sites for ORR and OER.Chronoamperometry and linear sweep voltammetry tests illustrated better electrochemical activitiy than simple Co₃O₄ nanosheets.Li-O₂ battery with RuO₂/Co₃O₄ electrocatalyst could maintain 140 reversible cycles with a limited capacity of 500 mAh g-1 at a current density of 50 mA g-1 and 90 cycles with a limited capacity of 1000 mAh g-1 at a current density of 200 mA g-1.The excellent cyclic performance and rate capability was ascribed to the effective formation and decomposition of larger lithium peroxide under the influence of the RuO₂/Co₃O₄ electrocatalyst,which concluded from SEM,XRD and in-situ EIS characterizations.As a consequence,transition metal complexes?MnOOH,Co₃O₄?are combined with RuO₂ nanoparticles to form high-performance bifunctional cathodic electrocatalysts for LOBs so as to accelerate the oxygen kinetics and reduce the polarization during discharge-charge process.A series of characterization has been conducted to find the reasons why the composite electrocatalysts can dramatically improve the comprehensive catalytic activities to ORR and OER.The research work in this thesis explores the synthesized electrocatalysts innovatively and will help provide new strategies for constructing bifunctional electrocatalysts for non-aqueous LOBs.