Preparation And Study Of The Highly Active Catalysts For Li-O₂ Batteries

Li-O₂ batteries are considered to be promising energy storage denices because of their high theoretical energy density,which is comparable to that of gasoline.In recent years,although many achievements for Li-O₂ batteries have been made,there are still some issues that limit their industrialization.One of the major challenges for Li-O₂ batteries is the sluggish kinetics due to the insoluble and inslulationg nature of Li₂O₂,leading to a large overpotential,which causes a low round-trip efficiency and a short cycle life.The employment of catalyst has been proved to lower the overpotetial and enhance the battery performance.Therefore,the main research work of this thesis includes the following respects.1.The development of self-standing and binder-free O₂ electrodes is significant for enhancing the total specific energy density and suppressing parasitic reactions for Li-O₂batteries,which is still a formidable challenge thus far.Here,a three-dimensional foam-like composite composed of Mo₂C nanorods decorated by different amounts of N-doped carbon?Mo₂C-NR@xNC?x=5,11,and 16 wt%??was prepared by freezing dryer and annealing,which was directly employed as an O₂ electrode without applications of any binders and current collectors.Mo₂C-NR@xNC presents a network microstructure with interconnected macropore and mesoporous channels,which is beneficial to achieving fast Li⁺migration and O₂ diffusion,facilitating the electrolyte impregnation,and providing enough space for Li₂O₂ storage.Additionally,the coated N-doped carbon layer can largely improve the electrochemical stability and conductivity of Mo₂C.The cell with Mo₂C-NR@11NC shows a considerable cyclability of200 cycles with an overpotential of 0.28 V in the first cycle at a constant current density of 100mA g^-1,a superior reversibility associated with the formation and decomposition of Li₂O₂ as desired,and a high electrochemical stability.On the basis of the experimental results,the electrochemical mechanism for the cell using Mo₂C-NR@11NC is proposed.These results represent a promising process in the development of a self-standing and binder-free foam-based electrode for Li-O₂ batteries.2.Advancing the understanding of electrocatalytic mechanism plays an important role in the selection of electrode materials and design of electrode architectures for Li-O₂ batteries,which is a still big challenge thus far.We present a self-standing O₂ electrode by growing MoS₂microspheres with sulfur deficiencies onto the carbon textiles?Def-MoS₂@CTs?.The 3D flower-like microspheres with hierarchical porous structure can contribute to achieving fast Li⁺migration and O₂ diffusion as well as to providing enough space for Li₂O₂ accommodation.In comparison to the cell performance of MoS₂@CTs?i.e.,without sulfur defects participations?,the cell with Def-MoS₂@CTs exhibits a much lower overpotential,a better cyclability,and a more favourable reversibility.The experimental and theoretical results provide a solid evidence for the introduction of sulfur deficiency relating to the catalytic activity and Li₂O₂ morphology.Additionally,the fundamental catalytic mechanism is proposed.Furthermore,the key factors for the steps involved in Li₂O₂ nucleation are also revealed.These results represent a promising progress in the development of O₂ electrode materials and the understanding of the catalytic mechanism for Li-O₂ batteries.3.Introduction of redox mediator?RM?is a promising approach to promote the decomposition of Li₂O₂ in Li-O₂ batteries.However,the redox shuttle can lead to the corrosion of lithium anode and reduce the persistence of RM.Here,we first report InBr₃ as a difuctional RM in Li-O₂ batteries.Br^-as a RM can lower the overpotential and thus enhance the electrochemical performance of cells.Equally,an indium-rich composite layer uniformly coated on the lithium anode surface during cycle,which is beneficial to suppressing the redox shuttle,improving the interface stability of lithium and electrolyte,and inhibiting the formation of dendrites.Compared with the Li-O₂ cell with LiBr,the cell with InBr₃ exhibits a higher round-trip efficiency and a better cyclability.We expect that the research can provide a new idea for the development of difuctional RM in Li-O₂ batteries.