Construction And Lithium Storage Performance Of Porous Iron-based Anode Materials

Lithium ion batteries?LIBs?have been widely used in smartphone,laptop,and new-energy vehicles due to their high efficiency and environmental protection.However,the energy density of LIBs still need to be further improved.Graphite,the conventional commercial anode materials of LIBs,can only provide a low capacity of³72 mA h g^-1,which seriously limits the improvement of energy density of LIBs.Therefore,developing new high-capacity anode materials is of great significance.Iron-based compounds,such as Fe₂O₃ and Fe₂N,are regarded as the promising anode materials due to abundant resources,low cost,simple synthesis,and high specific capacity.For the purpose of improving the cycle life of electrodes,we designed and fabricated nonhierarchical heterostructured Fe₂O₃/Mn₂O₃ porous hollow spheres to relieve the volume change and improve the sluggish kinetics of the iron oxides anode materials in this dissertation.Responding to the issue of general low mass loading of conversion-type anode materials,we also designed and constructed micrometer-sized porous Fe₂N/C bulk to improve the areal capacity of electrode materials.Meanwhile,the electrochemical performances of Fe₂O₃/Mn₂O₃and Fe₂N/C materials have been systematically investigated.Furthermore,the mechanism of lithium storage and the structure-performance relationship have also been explored.The main results are summarized as follows:Based on the solvothermal method,the nonhierarchical heterostructured Fe₂O₃/Mn₂O₃ porous hollow spheres have been fabricated through tuning the ratio of raw materials and reaction time.When used as the anode material of LIBs,the Fe₂O₃/Mn₂O₃ anode displays a high specific capacity of 852 mA h g^-1 after 600cycles at a current density of 1 A g^-1,corresponding to a high capacity retention of89.3%,which is much higher than that of the pure Fe₂O₃ and Mn₂O₃ materials.In addition,the results of electrochemical measurements indicate that the chemical diffusion coefficient of Li⁺in Fe₂O₃/Mn₂O₃ anode material is higher than that in pure Fe₂O₃ and Mn₂O₃ materials,and the reaction resistance of Fe₂O₃/Mn₂O₃ is lower than that of pure Fe₂O₃ and Mn₂O₃ materials,which suggest that this nonhierarchical heterostructure can effectively improve the cycling stability and kinetics of electrode materials.Based on the simple solution agitation and nitridation treatment,the micrometer-sized porous Fe₂N/C bulk material has been synthesized through controlling the ratio of raw materials and heat treatment process,and its tap density is higher than that of conventional commercial graphite anode material(1.03 g cm^-3).When tested as the anode material of LIBs at different mass loadings,the areal capacity of Fe₂N/C anode increases with the increasing mass loading.When the mass loading is up to 7.0 mg cm^-2,the Fe₂N/C anode delivers a high areal capacity of 2.59 mA h cm^-2 after 200 cycles at a current density of 0.89 mA cm^-2,and the corresponding capacity retention is 98.8%.The excellent lithium storage performance of Fe₂N/C anode at high mass loading benefited from the high electronic conductivity and high tap density of Fe₂N/C.Meanwhile,the randomly distributed pores in Fe₂N/C material also play an important role in facilitating the penetration of electrolyte and relieving the inner stress.