| Rechargeable non-aqueous Li-O2 battery is regarded as one of the most promising energy-storage technologies which may meet the demand of driving mileages of electric vehicles because of its theoretical high energy density.However,there are still many barriers impeding the utilization of Li-O2 batteries in reality,such as poor roundtrip ability,low energy efficiency and unpromising cathode performance as well as stability.The mechanism of Li-O2 batteries is based on a simple series of reactions between lithium ions and oxygen,namely,oxygen reduction reactions?ORRs?and oxygen evolution reaction?OERs?.In the discharge process,the ORR happens,meanwhile,oxygen is reduced to form Li2O2 with the lithium ions from the electrolyte.While the OER happens in the charge process,when the Li2O2 electrochemically decomposed to oxygen and lithium ions.As the discharge/charge process happens on the surface of cathode catalysts,the research of cathode catalysts becomes one of the most significant factor of Li-O2 batteries.Common used catalysts consists of carbon materials,precious metals,transition metal oxides and MOF-derived materials.Transition metal oxide catalysts are most widely used.Among transition metal oxides,cobalt oxide-based catalysts,for example,the widely studied Co3O4,can serve as the good bifunctional catalysts for Li-O2 batteries due to its low expense,high redox activity and favorable catalytic activity for both ORR and OERIn this study,freestanding cathodes and slurry coated cathodes are both studied and discussed.A list of characterization methods like XRD,XPS,SEM,TEM,TG,BET,and Raman are used to study the chemical statement,structure and morphology,specific surface area and thermal stability of as-prepared materials.Then the as-prepared cathodes are used to assemble Li-O2 batteries in glovebox and various text methods like CV and discharge/charge test are produces to study the electrochemistry property and roundtrip performance of Li-O2 batteries.The specific research contents are as follows:?1?In this work,Ru O2-Co3O4 with a 3-D web microstructure has been prepared via a simple hydrothermal method followed by impregnation and annealing treatment.The3-D web freestanding Co3O4 nanowires growing on the surface of Ni F could offer porous structure for O2 diffusion,provide large surface space for the loading of Ru O2nanoparticles as well as Li2O2 formation and decomposion.The uniformly distributed Ru O2particles could boost the catalytic activity for Li-O2 batteries.Hence,the 3-D porous structure of the freestanding Co3O4 nanowires loaded with Ru O2 can be maintained after Li2O2 deposition and the discharge products could be fully decomposed after charging.Therefore,Li-O2 batteries with the freestanding Ru O2-Co3O4/Ni F cathode has achieved a higher specific capacity and advanced cycling performance.When conducted at a limited specific capacity of 500 m Ah g-1 at current density of 100 m A g-1,the Ru O2-Co3O4/Ni F based Li-O2 battery can deliver stably 122cycles.To investigate the growth mechanism of discharge products of these two batteries equipped with different cathodes,ex-SEM characterizations of catalytic cathodes were implemented in both discharged and charged states.As for Ru O2-Co3O4,after 10th discharge,its knitted 3-D web structure are maintained with Li2O2 deposition.After charged,the knitted 3-D web nanowires are kept unbroken,with most of the Li2O2disappeared.While for Co3O4 electrode without Ru O2,Li2O2 can grow on the surface of Co3O4 nanowires as well as between the nanowires.The knitted 3-D web structure of nanowires is fully covered by Li2O2.After charging,most of the intermediates remain undecomposed and only a few decomposes.This phenomenon demonstrates that the loading of Ru O2 boosts the performance of catalytic cathodes.?2?Metal-Organic Frameworks?MOFs?derived materials possess excellent architecture,which is beneficial for Li-O2 batteries.In this work,ZIF-67 is used as precursor template and calcinated under different temperature to produce Co3O4 crystals.When the anneal treatment is under 350?,the Co3O4 nanocage holds the most complete skeleton and largest BET surface area.When pyrolysis temperature rises,the skeleton of catalysts start to break.According to the morphologies of catalysts heat treated at different temperature,the corresponding samples were named as Co-cage?350??,Co-collapsed cage?Co-polyhedron,500??and Co-particle?700??.In the electrochemical test,the Co3O4 nanocage stably delivers a large specific capacity of15500 m Ah g-1 as well as a long cycle-life of 132 cycles at limited discharge capacity of1000 m Ah g-1 under discharge/charge current density of 0.5 A g-1.While Co-polyhedron and Co-particle could only sustain a life time of 85 times and 50 times,respectively.To investigate the process of formation and decomposition of discharge products,SEM is carried out on air cathodes after 10th discharge and after 10th recharge.After 10th discharge,the surface of Co-cage nanocages is coated with Li2O2.Whereas,almost all Li2O2 on the surface has disappeared after recharge.However,for Co-polyhedron and Co-particle,the morphologies after charging and after discharging could not be distinguished,revealing the irreversible discharging products covered on the surface of Co-polyhedron and Co-particle.Through the controlling of the heat treatment conditions,the skeleton of ZIF-67 could be remained in Co3O4nanocage.And the excellent performance is ascribed to the unique structure of ZIF-67 derived materials.?3?In this work,a unique nanostructure of the nanoflower assembled through Co3O4nanosheets was synthesized by using 2-methylimidazole?2-MIM?as an effective structural directing agent.The XPS measurements revealed abundant oxygen vacancies on the surface of the Co3O4 nanoflower,which are beneficial for the uniform formation of Li2O2,ORR/OER process and long round-trip lifetime.However,when pyrolysis temperature rises,reaching 500?and 700?,the oxygen vacancies amount of 500Co and 700Co decreases,which means the oxygen vacancies comes from the precursor.On account of the synergistic effect of the unique structure,excellent OER performance and abundant oxygen vacancies,the Co3O4 nanoflower-based cells could deliver ultralong cycle life of 248 cycles with a limited discharge capacity of 1000 m Ah g-1 under charge/discharge current densities of 1 A g-1.In the same condition,500Co and 700Co could only sustain a life time of 131 times and 49 times,respectively.To investigate the process of discharge product decomposition on Co-flower electrode,SEM analysis is carried out in both discharged and charged states of the Co-flower cathode.At the end of 10th discharge,the whole petals of flowers are uniformly covered with discharge products Li2O2,and after 10th charging,all Li2O2 deposited on the Co-flower surface has disappeared.However,for 500Co and 700Co,the morphologies after charging and after discharging could not be distinguished,revealing the irreversible discharging products covered on the surface of 500Co and 700Co.This work proves the importance of oxygen vacancies on the surface of catalysts for the performance of Li-O2 batteries. |
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