Synthesis Of Iron-oxide-based Nanocomposites And The Lithium Intercalation/Deintercalation Performances

Nowadays,rechargeable lithium-ion batteries（LIBs）have been widely used in various devices.Compared with the early-developed rechargeable battery systems,lithium-ion batteries show higher energy density in terms of weight and volume.In recent years,it has been widely used as a power battery in electric vehicles（EVs）.The theoretical lithium storage capacity of iron oxide exceed 1000 m Ah/g,which is much higher than the currently used graphite anode（372 m Ah/g）.At the same time,it features the advantages of abundant reserves,environmental friendliness,and high safety.However,it usually suffers from a short cycle life,which is largely due to the pulverization of the material structure and the destruction of the electrode structure caused by its obvious volume expansion（>200%）during the lithiation and delithiation process.On the other hand,the relatively low intrinsic conductivity also limits its rate performance.To solve the above problems,iron oxide-based nanocomposites with different structures have been designed and fabricated in this thesis,aiming to improve the cycle life and rate performance.The combined phases include carbon,other oxide,ceramic,etc.The special structure of these composites helps to alleviate the stress caused by the volume fluctuation,leading to higher stability of the electrode.at the same time,the synergistic effect between the combined phases is beneficial to enhance the conduction of ions and electrons in the composite,improving the rate performance of the electrode.Besides,an electrode preparation method,different from the traditional coating process was tried,of which feasibility in actual production was discussed.The main content of the thesis is summarized as follows:（1）Fe₂O₃@C NCs with coating structure have been prepared by DC arc plasma evaporation and a simple calcination.The highly graphitized coating on the surface protect the internal active Fe₂O₃ and endow the composite with strong electron transport capacity.In addition,its outstanding“pseudo-capacitance behavior”further enhance the diffusion of lithium ions and the conduction of electrons in the electrode,leading to an excellent electrochemical dynamic performance of the electrode.At a current density of 0.1 A/g,Fe₂O₃@C NPs electrode deliver a reversible capacity of 1031 m Ah/g after 100 cycles;at a higher current density of 5 A/g,its reversible capacity can reach 637 m Ah/g after 500 cycles.（2）Fe₂O₃/Ti O₂ NPs were prepared by DC arc plasma evaporation and a simple calcination.In the composite,Fe₂O₃ and Ti O₂ are closely combined,forming an intact nanoparticle,which can effectively buffer the strain produced by the lithium storage.At the same time,the synergy of them can promote the diffusion of lithium ions and the conduction of electrons.Compared with the bare nano-Fe₂O₃ electrode,the electrochemical performance of the two-dimensional Fe₂O₃/Ti O₂ NPs electrode is greatly improved,and the discharge specific capacity achives 804.3 m Ah/g after 100 cycles at a current density of 0.2 A/g;at a higher current density of 2 A/g,the reversible specific capacity maintains at 495.2 m Ah/g after 300cycles.（3）The Fe₂O₃@Si C NWs were prepared by DC arc plasma evaporation and a simple calcination process.Between these 1D Si C NWs,there are a lot of voids which can accommodate the volume fluctuation and enhance the accessibility of electrolyte.At the same time,the nanowires are interconnected forming a conductive network,resulting in an enhancement of the transportation of ion and electron.In addition,the high mechanical strength and chemical stability of Si C significantly help to improve the structural stability of the electrode.The reversible cycling capacities of Fe₂O₃@Si C NWs electrode are 775 m Ah/g and 576 m Ah/g after 500 cycles,at the current densities of 1 and 3 A/g,respectively.（4）Si O₂/Fe₂O₃/Fe integrated anode with a sandwich structure was prepared by laser ablation and simple calcination process.In the ablation process,the emission spectrum diagnosis technology is applied to realize the accurate control of the laser-ablation temperature.The multiple layers form new interfaces/surfaces,which promote the conduction of ions and electrons in the cycling process.In addition,the Si O₂ layer effectively relieve the stress generated during the cycles and enhance the structural stability of the anode.The design of the integrated electrode reduces the cost of manpower and material resources in the traditional electrode preparation,and provides a possibility for large-scale application.