Nanostructure Tuning Of Manganese Oxides And Their Performance In Lithium Ion And Lithium-oxygen Battery

At present,although the energy density of commercial Lithium ion batteries can satisfy the applications of mobile telephone,laptop and small portable instruments,but still cannot meet the growing demand of the development of Electric Vehicles(EVs).Therefore,it is significant to explore and develop some new battery systems with higher energy density.Manganese oxides which are based on multi-electron conversion reaction are one of the most promising anodes of lithium ion battery,especially MnO,which is with the lowest standard potential in them.In order to mitigate the tremendous volume change and improve the cycle performance of MnO,in this article,we use the methods of nanocrystallization,structure control and carbon coating to enhance the electrochemical performance.Moreover,lithium-oxygen battery(Li-O2 battery)should be one of the most promising next-generation battery systems owing to its very high theoretical energy density.However,tremendous challenges also exist in the process of developing Li-O2 battery.The disadvantages of Li-O2 battery such as poor cycle life,low energy efficiency and poor rate performance restrict the commercial application of Li-O2 battery.To solve these above problems,it is important to understand and optimize the charge/discharge processes of Li-O2 battery.Based on rich experience on synthesis of manganese oxides in the research of anode of lithium ion battery,this article focuses on the design of the manganese oxides as electrocatalysts on the oxygen cathode.High efficient electrocatalysts can be constructed through tuning the surface structure and morphology of manganese oxides,leading to improving the reaction kinetics of oxygen electrode,decreasing the electrochemical polarization during charge/discharge processes and enhancing the electrochemical performance of Li-O2 battery.In additions,in-situ and ex-situ characterizations are combined to trace and analyze the charge/discharge processes of different electrocatalysts.The main research work is as follows:1.MnO anode material was studied owing to its lowest electrochemical motive force(1.032 V vs.Li/Li+)value amongst various transition metal oxide anodes which are based on multi-electron conversion reaction.Porous MnO/C materials of composite nanostructure consisting of nanorods and nano-octahedra(denoted as nRO-MnO/C)were synthesized through a one-pot hydrothermal procedure followed by thermal annealing using PEG6000 as a soft template.Morphology,crystal structure,carbon content and surface area of the as-prepared products were detailedly characterized by using SEM,TEM,XRD,TGA and BET test.Cycle performance and rate performance of the nRO-MnO/C electrode was tested as anode of lithium ion battery.Electrochemical test results demonstrated that the nRO-MnO/C electrodes could maintain a capacity of 861.3 mAh/g at 0.13 C after 120 cycles and a capacity of 628.9 mAh/g at 1.32 C after 300 cycles.They could still deliver a capacity of 302.5 mAh/g at a high rate of 4.16 C.The nR-MnO/C and nO-MnO/C were obtained to compare their electrochemical performance with the nRO-MnO/C.Electrochemical test results and EIS results of three electrodes both demonstrated the synergetic effect of nanorods and nano-octahedra in the composite nanostructure.The study demonstrated that the nanostructure of electrode material could make a difference on their electrochemical performances.2.In order to decrease the voltage polarization of lithium oxygen battery during the charge/discharge processes,manganese oxides were decorated by RuO2 nanoparticles to enhance their bifunctional catalytic activities.The np-RuO2/nr-MnO2 served as bifunctional electrocatalyst was synthesized through two-step hydrothermal reactions:firstly,MnO2 nanorods were prepared by hydrothermal reactions,and then RuO2 nanoparticles were dispersed on MnO2 nanorods by the next-step hydrothermal reaction.Morphology and structure of the as-prepared products were detailedly characterized by using SEM,TEM and XRD.BET results demonstrated that the surface area of np-RuO2/nr-MnO2 was 31.9 m2/g.Electrochemical test results demonstrated that the np-RuO2/nr-MnO2 could present both higher ORR and OER catalytic activities than MnO2 nanorods.Moreover,the Li-O2 battery with the np-RuO2/nr-MnO2 could maintain 70 stable cycles at a constant current density of 50 mA/g with a limit capacity of 500 mAh/g and 20 stable cycles at a constant current density of 200 mA/g with a limit capacity of 4000 mAh/g.In-situ high-energy XRD and ex-situ SEM images were combined to illustrate the formation of discharge products during the charge/discharge processes of the np-RuO2/rnr-MnO2 and to explain why the cycle life of Li-O2 battery changed at different depth of discharge.3.Electrocatalysts was fabricated by the method of tuning nanostructure to facilitate the dispersion and storage of Li2O2,leading to that Li2O2 could be efficiently decomposed on electrode surface.Conductive porous carbon(PC)was obtained by using nano-CaCO3 as template argent and sucrose as carbon source.MnO2 nanopartices were deposited uniformly on the surface and pore structures of PC to obtain the MnO2@PC.Morphology,carbon content,crystal structure and surface area of the as-prepared products were detailedly characterized by using SEM,TEM,.XRD,TGA and BET test.BET results demonstrated that the surface area of MnO2@PC was 70.2 m2/g.Electrochemical test results demonstrated that the MnO2@PC could exhibit excellent catalytic activities and Li-O2 battery with the MnO2@PC could maintain 80 stable cycles at a constant current density of 50 mA/g with a limit capacity of 500 mAh/g.The charge platform was maintained in 3.4 to 3.5 V and the discharge platform was maintained in 2.75 to 2.80 V in 60 cycles.The excellent electrochemical performance should be contributed to the good conductivity and pore structure of PC.In-situ high-energy XRD and ex-situ SEM images were combined to illustrate that the existence of PC can promote the dispersion of the Li2O2 and ex-situ EIS results also helped to prove this above conclusion.4.Combining the consideration of bifunctional catalytic activity and the ability to disperse and store the discharge products,tuning the nanostructure of electrocatalysts could further enhance the performance of lithium oxygen battery.Mn3O4@C was synthesized by using sphere-like graphite as carbon source to be oxidized by KMnO4 aqueous solution through hydrothermal reaction.Sphere-like graphite could expand to be flower-like structure with large pores after oxidation and a layer Mn3O4 film were attached onto the surface of carbon.RuO2/Mn3O4@C served as high efficient bifunctional electrocatalyst could be obtained by further dispersing RuO2 nanoparticles on the surface of flower-like Mn3O4@C.Morphology,carbon content,crystal structure and surface area of the as-prepared products were characterized by using SEM,TEM,XRD,elemental analysis and BET test.BET results demonstrated that the surface area of RuO2/Mn3O4@C was 26.4 m2/g.Electrochemical test results demonstrated that the RuO2/Mn3O4@C could exhibit high efficient catalytic activities and Li-O2 battery with the RuO2/Mn3O4@C could maintain 100 stable cycles at a constant current density of 100 mA/g with a limit capacity of 500 mAh/g with a charge platform of 3.9 V at 100th cycle.It could maintain 70 stable cycles at a constant current density of 200 mA/g with a limit capacity of 1000 mAh/g with a charge platform of 3.79 V at 70th cycle.The excellent electrochemical performance should be contributed to high efficient bifunctional catalytic activity of the RuO2/Mn3O4@C and its pore structures which can promote the dispersion of the discharge products.Moreover,the carbon supporter of the as-prepared catalyst which was consisted of graphite could deliver excellent electron conductivity.In summary,this work enhances the performances of manganese oxide in both lithium ion battery and lithium oxygen battery by the method of structure tuning.In the research of lithium ion battery,porous MnO/C with composite nanostructures which produce synergistic effect is constructed as anode to obtain excellent cycle performance and rate performance.In the research of lithium oxygen battery,the nanostructures of manganese oxides are tuning to construct bifunctional catalysts and promote the dispersion and storage of Li2O2.We fabricate three catalysts(np-RuO2/nr-MnO2,MnO2@PC,RuO2@Mn3O4/C)with optimized nanostructure and enhanced bifunctional catalytic activities.The results demonstrate that tuning the nanostructures of catalysts,especially increasing the storage room for Li2O2 and electron conductivity of the carrier,is an efficient way to improve the cycle performance of lithium oxygen battery.The study of this thesis is of great importance to the development and exploration of lithium ion battery and lithium oxygen battery.