Structural Design Of Transition Metal(Mn,Fe,Ti,Zn) Oxides And Their Application In Lithium-Ion Battery Anode

The development and utilization of green and clean energy（especially solar and wind）is an important way to solve the problems of resource depletion and environmental pollution caused by excessive use of traditional fossil energy.Due to the intermittent and fluctuating characteristics of green clean energy,it is urgent to develop advanced energy storage systems.Among many energy storage systems,lithium-ion batteries（LIBs）have been widely used due to their high energy density,long cycle life and environmental friendliness.At present,commercial graphite anode of LIBs is difficult to meet the high-demand of the next generation high-energy density LIBs mainly due to their low theoretical specific capacity of372 m Ah g^-1and poor rate capability.Transition metal oxides(such as Fe₂O₃and Mn O₂,>1000 m Ah g^-1)and lithium metal(3860 m Ah g^-1)have attracted great concerns due to their high theoretical specific capacities,and are considered as promising commercial anode materials for LIBs.However,the Fe₂O₃and Mn O₂suffer from poor lithium storage properties due to their low conductivities and large volume changes during the discharge and charge process.Lithium metal is easy to form lithium dendrites during the cycling so that it can result the rapid capacity decay and serious potential safety hazards.In combination of the advantages of transition metal oxides,such as easy control on the structures of Fe₂O₃,Mn O₂and TiO₂,and high lithium affinity of Zn O,this thesis prepared a series of Fe₂O₃and Mn O₂with porous structures and TiO₂/Zn O/Li composite,and studied their anode performance for LIBs.The detailed research results are included as follow:（1）In terms of the rapid decay on the cycling performance caused by the large volume change of Fe₂O₃during the discharge and charge process,the porous structures of Fe₂O₃can effectively alleviate the volume change.The（NH₄）₃Fe F₆prepared by electrochemical method was simply annealed to formα-Fe₂O₃with octahedra and porous structures.As anode for LIBs,theα-Fe₂O₃maintains a high reversible capacity of 1187 m Ah g^-1after 300 cycles at a current density of 0.3 A g^-1.（2）In terms of the poor conductivity of Mn O₂and the rapid decay on the cycling performance caused by the volume expansion during the discharge and charge process,the use of highly conductive hollow carbon nanospheres（HCN）to effectively hybridize Mn O₂nanoparticles can significantly improve the conductivity of the electrode and suppress the volume change of the electrode.HCN obtained from m-aminophenol and formaldehyde as raw materials reacts with potassium permanganate（KMn O₄）through the redox reactions to form the Mn O₂@HCN with core-shell structures.Transmission electron microscopy characterization of Mn O₂@HCN reveals the evolution of the core structure of the carbon spheres from hollow to porous nanospheres.As anode for LIBs,the Mn O₂@HCN reaches a the reversible capacity reaches 604 m Ah g^-1after 200 cycles at 0.3 C,which is much higher than that of HCN with only 211 m Ah g^-1.（3）In terms of the battery performance decay and potential safety problems caused by the formation of lithium dendrites during the cycling,the composite nanostructures with stable titanium dioxide（TiO₂）nanotubes and great lithium affinity zinc oxide（Zn O）layers can effectively reduce the nucleation potential energy during lithium metal deposition and form a three-dimensional lithium metal skeleton,thus effectively suppress the formation of lithium dendrites.In order to obtain three-dimensional TiO₂/Zn O/Li composite,the molten lithium was deposited on the TiO₂/Zn O skeleton.The TiO₂/Zn O skeleton was fabricated by the thermal decomposition of zinc hydroxide to the TiO₂nanotube arrays,which was prepared by anodic oxidation method.When the lithium metal was used as the counter electrode for LIBs,the overpotential of TiO₂/Zn O/Li composite at a current density of 0.5 m A cm^-2after1000 cycles was about 45 m V.Compared with the traditional cathode material of lithium iron phosphate（Li Fe O₄）as the counter electrode for LIBs,the TiO₂/Zn O/Li composite still delivers a high reversible capacity of 133 m Ah g^-1after 300 cycles at 0.5 C,which is much higher than 8 m Ah g^-1from that using Li Fe O₄and lithium metal as the counter electrodes.