Design,Synthesis And Study Of Ti/Mo/W-Based Electronic Conductive Oxides As Non-Carbon Catalysts For The Cathode Of Lithium-Oxygen Batteries

Lithium-oxygen(Li-O2)battery is a promising next-generation battery with high energy density(11.4 kWh kg-1),and is very suitable for large-scale energy storage and as power in electric vehicles.However,there still exists many problems in Li-O2 battery system,including the protection of lithium anode,the stability of electrolyte and the reasonable design and synthesis of cathode materials.And the most important problem focus on the cathode materials.Traditional carbon-based cathode materials utilized in Li-O2 battery is not stable.Carbon-based materials can react with discharge products to form by-products.And the carbon itself is not stable under high voltage during charge process.Therefore,using non-carbon based materials to replace carbon as cathode of Li-O2 battery is a promising method.Not only that,the reasonable design of cathode structure is also very important.The porous structure can provide enough space for the storage of discharge products,and can also facilitate the transfer of electrolyte and oxygen which improves the rate performance at high current density.Based on above consideration,we focus on the Ti/Mo/W-based conductive oxides.By rational design,we fabricated non-carbon based cathode materials with nanoporous structure for high performance Li-O2 batteries.And we also study the battery performance and reaction mechanism.All the research content and main results are shown as follows:(1)For Ti-based oxides,we focus on Magneli phase Ti4O7 oxide with high electronic conductivity as support materials of cathode used in Li-O2 batteries.During the high temperature reduction reaction,the obtained Ti4O7 nanoparticles is large-scale.We use mesoporous SiO2 to coat on the surface of TiO2 and prevent the agglomeration.By compared with directly-synthesized Ti4O7 sample,our sample has higher specific surface area and porosity.The voltage gap of the Li-O2 battery,composed of MCO/Ti4O7 electrode,is significantly reduced as compared with the batteries composed of pure carbon,pristine Ti4O7 and C+MCO.The MCO/Ti4O7 hybrid exhibits enhanced specific capacity and improved cycling stability with respect to the Ti4O7 cathode.The high performance of the Li-O2 battery with the MCO/Ti4O7 cathode could be attributed to the high electronic conductivity of Ti4O7 and the high electrocatalytic activity of MCO nanoparticles.And the synergistic interaction between Ti4O7 and MCO can improve the catalytic activity.(2)We used sol-gel method combined with electrospinning technology,and then prepared free-standing Ti4O7 and Ru/Ti4O7 porous nanowires though carbothermal reduction reaction.The as-synthesized products can be a complete plate.The Ti4O7 supports provide efficient electron transfer pathway and a stable supporting framework structure.And this special macroporous structure can improve the transmission of electrolyte.When Ru nanoparticles were coated on the surface of Ti4O7,there would be a strong interaction between Ru and Ti4O7 and change the electronic structure,which improves the electrocatalytic activity.The Ru/Ti4O7 hybrid cathode exhibits excellent catalytic performance,including lower polarization and good cycle stability.And the free-standing electrode can be used as fabrication of flexible Li-air batteries.The batteries have good flexibility and be continuous discharge/recharge operation in air under bending state.(3)Based on the Mo-based conductive oxides,MoO2 nanosheets were chosen to grow on the surface of nickel foam and MnCo2O4 catalysts were coated on the surface of MoO2.This hybrid cathode material exhibits excellent cycling stability and rate performance,with a high energy efficiency of over 85%.These excellent battery performance can be attributed to the high conductivity,stability and outstanding electrocatalytic performance of the hybrid electrode material.We also found the MnCo2O4 grown on MoO2 surface is spinel type structure.Linear scanning shows that Co tends to distribute on the outer layers,which has a good catalytic effect on the reactions in batteries.Not only that,this hybrid cathode directly grows on the Ni foam and avoids the addition of carbon and binder,which eliminates the potential side reactions and improves the stability(more than 400 cycles)and efficiency of the battery(>80%).(4)By using the carbon protection strategy,we prepared a hybrid cathode material with MoOx oxide protective layer on the surface of carbon nanotube(MoOx/CNT)by solvothermal method.The MoOx layer has abundant oxygen defects,and the conductivity of MoOx is greatly improved compared with MoO3.Therefore,the transmission of electrons at interface will not be blocked.This MoOx/CNT hybrid has both the low mass density and high specific surface area.And the corrosion of carbon materials by intermediate products in discharge process is greatly reduced due to the surface coating of MoOx.Through density functional theoretical calculation,we found that the intermediate LiO2 can exists stably on the surface of MoOx,thus depositing and growing large-scale discharged products on the surface of the electrode by solution-phase reaction mechanism.This porous morphology is also very helpful to the decomposition of the charging process.However,the surface of MoO3 material is unstable,and LiO2 can be directly reduced to form LiO2,so it presents amorphous discharge products.(5)For W-based conductive oxides,W/Co/Fe ternary metal oxide nanowire network were synthesized by one-step solvothermal method on the surface of nickel foam(WCoFe@Ni).The WCoFe@Ni composite electrode exhibits high reversible capacity,good rate performance and excellent cyclic stability.The direct synergistic effects of W,Co and Fe,including the adjustment of valence states of elements and the morphology of electrode structure,can directly promote the catalytic process,the transmission of electrolyte and the storage of discharge products during battery operations.The addition of W formed a W18O49 conductive oxide support,provides an effective electron transport channel and regulates the valence states of Co and Fe.Fe exists in the form of high valence of Fe3+ in WCoFe catalysts,while Co exists in the form of Co2+,which results in a large number of materials similar to CoO oxides on the surface of WCoFe catalysts.This low-valent Co material has excellent catalytic activity,contributes to the effective decomposition of discharged products during charge process,and improves the performance and cyclic stability of batteries.Based on above study,we deeply discussed the application of conductive oxides as cathode materials in Li-O2 batteries.Through the reasonable selection and design of electrode structure,we obtained high battery performances and stability.At the same time,we can better understand the stability mechanism of the electrode surface by studying the surface passivation mechanism of cathode materials.These research results provide good ideas and effective solutions for solving the problems in the cathode of Li-O2 batteries,and are of great significance for the developments of commercial and high performance Li-O2 batteries in the future.