Optimization And Design Of Transition-based Catalyst For Lithium-air Battery Cathode

Traditional commercial lithium-ion batteries can no longer meet the needs of electric vehicles and other large-scale energy storage equipment in the future due to its limited theoretical energy density.Therefore,the development of new high-energy density secondary batteries is imperative.Lithium-air battery possesses an ultra-high theoretical energy density of 3505 Wh/kg,much higher than commercial lithium-ion batteries,showing great application prospects.However,at present,due to large polarization and poor cycle performance,lithium-air batteries are far from meeting the standards of practical application.The slow kinetics of conversion reaction between oxygen and Li₂O₂ during the discharging and charging processe result in large polarization and poor reversibility of the battery,thereby causing the rapid decay of the battery performance.Promoting the rapid and reversible formation and decomposition of Li₂O₂ using efficient solid catalysts have been proved as an effective way to improve battery performance.Therefore,the development of highly efficient and stable air cathode catalysts has become a hot but challengeable research topic in the field of lithium-air batteries.The transition metal-based catalysts are very attractive due to their many advantages,including high stability,low cost,and facile adjustability of their microstructure.In this dissertation,we focus on the design and synthesis of the transition metal-based catalysts by combining the strategies of nanocrystallization,porosization,defect engineering and compositing.The improvement of electrochemical performance,including the energy efficiency,cycle stability etc.were achieved and the structure-activity relationship of the catalyst was studied.The main content and research results are as follows:?1?Design and synthesis of Co/CoO/boron-doped graphene composites.Boron atoms and Co/CoO composites were introduced into the graphene lattice and onto the surface of graphene,respectively by using Co-B_i/graphene composites as a precursor and using Co-B_i as a boron source and a cobalt source.In the composite catalyst,the doping of boron element activates the?electronic structure,increases the adsorption of active oxygen,and activates the Li-O bond,improving the decomposition of Li₂O₂.The constructed Co/CoO composites greatly improve the ORR and OER bifunctional catalytic activity of the material through a synergistic catalytic effect.The assembled lithium-air battery exhibited a low discharge-charge polarization and good cyclability of 100 cycles.?2?Preparation and electrochemical performance study of two-dimensional TiO₂?B?and zero-dimensional Co₃O₄ composites?demoted as Co₃O₄-TiO₂?B??.The synthesized atomic thickness TiO₂?B?nanosheets show an extremely high specific surface area?313.43m²/g?,which can provide a large deposition space for discharge products,leading to a high discharge specific capacity?11000 m Ah/g?of the battery.Moreovre,ultrafine Co₃O₄ nanocrystals were successfully combined on the surface of ultrathin TiO₂?B?nanosheets and oxygen vacancies were induced at the same time,which provide abundant active catalytic sites.During the discharging and charging process,film-like Li₂O₂ can be formed and be decomposed quickly on surface of the catalyst,accommodating the battery high energy efficiency and good reversibility.The Li-O₂ battery with Co₃O₄-TiO₂?B?behaved 200 cycles at a limited capacity of 1000m Ah/g with low polarization?above 2.7 V for discharge medium voltage and below4.0V for charge medium voltage within 80 cycles?.?3?Synthesis and research of one-dimensional hollow tubes constructed by Co-CeO₂ nano-heterostructure?denoted as 1D-Co-CeO₂?.The composites were designed and synthesized using an electrospinning method followed by a two-step heat treatment.XPS results confirmed the strong interface effect of the Co-CeO₂ nano-heterostructure,which induces abundant oxygen vacancies in the composite.Compared with 1D-Co₃O₄-CeO₂ and 1D-CeO₂ electrodes,lithium-air batteries with 1D-Co-CeO₂ showed the lowest charge-discharge polarization and the best cycle performance?90 cycles with the limited capacity of 500m Ah/g at the current density of 100m A/g,based on the total mass of the catalyst and KB?.The unique microstructure characteristics of 1D-Co-CeO₂endow the battery better electrochemical performance.On the one hand,the one-dimensional hollow tubular morphology of the catalyst can promote electrolyte penetration,ion transfer,O₂ diffusion,and provide a large deposition space for discharge products.On the other hand,the rich oxygen vacancies induced by the interface effect can provide abundant catalytic active sites for discharge-charge reactions,promoting the formation and decomposition of Li₂O₂ and leading to a superior reversibility of the electrode.?4?Preparation and electrochemical performance study of defect-rich holey 2D Co₉S₈ nanosheets?denoted as H-2D-Co₉S₈?.The relatively low electronic conductivity of transition metal oxides limits the charge transfer rate of discharge-charge reactions of the cathode.As a defective phase of cobalt sulfide,Co₉S₈ has a high electronic conductivity and effective O₂-adsorption,and shows high potential in the field of oxygen cathode catalysis.The two-dimensional morphology of the catalyst can provide a large place for the deposition of Li₂O₂;The in-plane pores of the nanosheets can not only promote the transfer of oxygen and lithium ions,but also provide abundant edge catalytic sites for ORR and OER.The favorable nanostructure of H-2D-Co₉S₈could induce the formation of loose and thick discharge product film constructed by discrete nanoscale Li₂O₂.The discrete nanoscale Li₂O₂ could be decomposed more easily because of the improved charge transport and delithiation property derived from its smaller particle size and higher surface area compared with the bulk film-shape Li₂O₂.Therefore,the lithium-air battery based on the H-2D-Co₉S₈ exhibits a high discharge capacity?3500m Ah/g,based on the total mass of the catalyst and KB?and excellent cycling stability?100 cycles with the limited capacity of 500m Ah/g?.