Ferrous Oxide Semiconductors As Anode Materials Of Lithium-ion Batteries

Metal oxide semiconductors are a kind of important semiconductor material, they generally have ionic crystal structure, relatively wide energy gap and low electron mobility. Due to their special electrical, magnetic and thermal properties, they have been widely used in electronic industry and other fields.Many metal oxide semiconductors have excellent lithium storage performance, their theoretical capacities are 2³ times higher than the traditional graphite material. Besides, metal oxide semiconductors own the advantages of easy preparation, without pollution and low cost, which make them become one potential candidate for anode material of the next-generation lithium ion battery（LIB）. However, their electrochemical performance at high current densities is inferior because of the low conductivity, and the drastic volume changes during lithium ion insertion and extraction severely restrict the cycle life of the batteries.This thesis mainly focuses on preparing iron oxide semiconductors（Fe₃O₄ and Fe₂O₃）, and aims at improving the power density and cycle life of the corresponding anodes. We designed nanostructures of iron oxides and composited them with carbon materials, which effectively reduced diffusion distance of lithium ions, enhanced mobility of electron, alleviated stress derived from volume changes and maintained integrity of the electrode. On the other hand, we attempted new electrode preparation process and investigated its influence on electrochemical performance.The main contents and conclusions are as follows.1. Carbon-wrapped Fe₃O₄ nanoparticle composite film prepared on Ni foam as an anode of LIBCarbon-wrapped Fe₃O₄ nanoparticles films on Ni foam were prepared by a hydrothermal synthesis combined with carbonization treatment. The as-prepared samples were directly used as binder-free anodes for LIBs which exhibited enhanced rapid charge/discharge capability and excellent cyclability. A reversible capacity of 543 mA h g-1 is delivered at a current density as high as 10 A g-1 after more than 2000 cycles. The superior electrochemical performance can be attributed to a good contact between the active material and current collector, and the formation of a thin carbon layer which constructs a three-dimensional network structure enwrapping the nanosized Fe₃O₄ particles. Such an architecture can facilitate the electron transfer and accommodate the volume changes of the active materials during discharge/charge cycling.2. Fe₂O₃ microcuboids with a caramel-treats morphology as an anode material of LIBFe₂O₃ microcuboids material was synthesized by a two-step hydrothermal procedure combined with an annealing treatment. The special caramel-treats morphology of α-Fe₂O₃ microcuboids can provide enough space to accommodate the volume changes during lithium ion insertion and extraction. When the electrode was fabricated by traditional coating process, it showed that the synthesized material owns excellent cyclability at high current density. A specific capacity reaches up to 513 m A h g-1 at a current densityof 5 A g-1 after 1000 cycles.3. Cheese-like Fe₂O₃@carbon microtubes composite as an anode material of LIBCarbon microtubes（CMTs） were fabricated by simply carbonizing cotton. A layer of cheese-like Fe₂O₃ particles were grown on the surface of CMTs by hydrothermal synthesis. It was shown that the prepared CMTs have a high degree of graphitization. When the as-prepared cheese-like Fe₂O₃@CMTs composite was used as an anode material for lithium ion battery, it exhibited excellent charge/discharge cycle stability. A reversible capacity up to 562 mA h g-1 is delivered at a current density of 1 A g-1 after 600 cycles.4. Fe₂O₃/graphene composite prepared by dip-coating as an anode material of LIBFirst, we synthesized graphite oxide and Fe₂O₃ nanoplates by Hummers method and template method, respectively. The porous Fe₂O₃/graphene composite electrode was prepared on Cu foil by a dip-coating. The prepared electrode exhibits remarkable cyclability at high current densities, its reversible capacity reaches up to 478 mA h g-1 at a current density of 5 A g-1 after 2500 cycles. The superior electrochemical performance can be attributed to the excellent contact between the active material and current collector. Besides, porous structure of the electrode was beneficial for the full infiltration of electrolyte to the active material, and it could accommodate volume changes of the active material during lithium insertion/extraction.