Co₃O₄-Based Nanowire As Anodes For Lithium-ion Batteries

Lithium-ion batteries（LIBs） have been widely used in various portable electronic devices due to their light quality, high energy density, good safety, long cycle life, no memory effect and environmentally friendly. In recent years, LIBs have been used as power batteries in electric vehicles, the energy density, power density and cycling life of LIBs should be further improved to meet human’s requirment. The electrochemical performance of LIBs is mainly depended on the anode and cathode materials, hence, developing electrode materials with high performance is a research focus in present. Howerver, as a commercial anode material, graphite with a low theoretical specific capacity（372 mAh g-1） has restricted the widely using of LIBs. Co₃O₄, a transition metal oxide, has a high theoretical specific capacity（890 mAh g-1）, and have been widely investigated as an anode material in the past few years. Hence, Co₃O₄ have been choosed as the study object in this thesis. The purpose of this thesis is develop high performance Co₃O₄-based anode materials to improve the energy density, cycling performance and rate capability of LIBs, especially for the cycling stability at high current densities. Improving the electrochemical performance of Co₃O₄-based materials through the structure design, doping and compositing with materials possessing high electronic conductivity. The research contents of this paper are as follows:（1） Grass-like Co₃O₄ nanowire arrays（NWAs） on Ti foil have been designed and synthesized by a facile hydrothemal method and subsequent annealing treatment. The Ti foil is uniformly covered with the as-prepared grass-like Co₃O₄ NWAs, constructed by numerous randomly oriented nanowires. The diameters of the nanowires are in the range of 70 to 100 nm, with the length of about 9 μm. The Co₃O₄ nanowire are composed by numerous nanoparticles with the size of 5-10 nm, and irregular pores are numerously disturbed in the Co₃O₄ nanowire. The grass-like Co₃O₄ NWAs exhibits excellent electrochemical performance including outstanding rate capability and long cycle life. The specific capacity of grass-like Co₃O₄ NWAs could maintain 1031 mAh g-1 at a current density of 500 mA g-1 after 100 cycles without capacity decay. A remarkable capacity of 662 mAh g-1 is achieved even at a high current density of 5000 mA g-1. The excellent performance is greatly attributed to efficient electron transport from the Co₃O₄ nanowires to the current collector, good solid contact of Co₃O₄ nanowire with the substrate and stable cross-linked structure formed after repeated cycles.（2） CoSx/Co₃O₄ NWAs have been synthesized through a chemical bath method, the Co₃O₄ nanowire in grass-like Co₃O₄ NWAs has been partly transformed to CoSx, and CoSx is uniform distributed in the whole nanowire. The CoSx/Co₃O₄ NWAs display a better rate capability than the grass-like Co₃O₄ NWAs at the high current density: CoSx/Co₃O₄ NWAs deliver reversible capacities of 458 and 388 mAh g-1, higher than that of the grass-like Co₃O₄ NWAs（349 and 174 m Ah g-1）. According to the EIS analysis, the good rate capability of CoSx/Co₃O₄ NWAs can be attributed to enhanced electronic conductivity with the introduction of CoSx.（3） Co₃O₄@polypyrrole nanowire arrays（Co₃O₄@PPy NWAs） have been successfully synthesized via a simple hydrothermal method and further an oxidative cationic polymerization process. The PPy coating layer is existed in the form of amorphous state with the thickness of 10 nm and the content of 15 wt%. The Co₃O₄@PPy NWAs exhibit excellent electrochemical performance. Co₃O₄@PPy NWAs deliver a reversible capacity of 700 mAh g-1 after 500 cycles at 3 A g-1. Even at a high current of 20 A g-1, the Co₃O₄@PPy NWAs can still maintain a capacity of 470 mAh g-1. The excellent electrochemical performance is mainly attributed to the PPy coating layer. The introduction of the PPy coating layer can improve the enhanced electronic conductivity of the Co₃O₄ array structure, which is beneficial for the rate capability. Meanwhile, the PPy coating layer can effectively relief the array structure destruction caused by volume change during cycling, ensuring the array structure stability and improving the cycling stability.（4） Co-CoOx core-shell nanowire arrays（NWAs） were obtained by a simple H2 reduction process of the grass-like Co₃O₄ NWAs. The H2 reduction process can introduce metallic Co into the inner part of Co₃O₄ nanowire, and the array structure can be well keeped after the H2 reduction, Co-CoOx nanowire still show a robust adhesion on the current collector. Benefitting from those designed structural features, the Co-CoOx NWAs exhibit superior rate capability and cycle stability. The Co-CoOx NWAs electrode maintains highly stable capacities of 990 and 740 mAh g-1 after 1000 cycles at 10 and 20 A g-1, respectively. At an ultrahigh rate of 50 A g-1, a high reversible capacity of 413 mAh g-1 is achieved. The newly introduced metallic Co pays a key role in achieving extraordinary battery performance. The metallic Co not only provides a fast electronic conductivity pathway in the integral array electrode, but also acts as a skeleton structure remitting the structural deterioration and enhancing the structural stability.