Preparation And Electrochemical Performances Of Metal Oxides And Nanoscale FeSn₂ As Anode Materials For Lithium Ion Batteries

With the development of the portable devices and hybrid electric vehicles (HEV), there is a demand for higher gravimetric and volumetric capacity electrode materials for lithium ion batteries which supply the power for them. Carbonaceous material is a major material for the negative electrode in the rechargeable lithium ion batteries, but it cannot meet the demand of the next generation lithium ion batteries due to the low theoretical capacity (372 mAh g^-1). In this present work, to improve the electrochemical properties of metal oxides and intermetallic alloys as new anode materials for lithium ion batteries, three aspects were studied. To improve the initial coulombic efficiency of metal oxides as anode materials for lithium ion batteries, to study how the morphology affects electrochemical properties of metal oxides as anode materials for lithium ion batteries, to find some facile and cheap methods to synthesize nanoscale intermetallic alloys as anode materials for lithium ion batteries.Almost all metal oxides as anode materials for lithium ion batteries have high theoretical capacity, but the initial irreversible capacity is large due to the irreversible reaction of metal oxides with Li⁺ involving the formation Li₂O. It has been reported that nanoscale transition metal particles have the catalytic activity to facilitate Li₂O decomposition. Ni-coated ZnO composite was synthesized by electroless plating and studied as anode material for lithium ion batteries. The nickel membrane plays several important roles in the electrochemical performance of ZnO. Nickel as a conductor can improve the high rate properties of ZnO, acts as a buffer to alleviate the stress during cycling, and has the catalytic activity to facilitate Li₂O decomposition. When ZnO was coated with Ni, initial coulombic efficiency was improve from 49.5% to 75% and the stable reversible capacitiy was improved from 130 mAh g^-1 to 490 mAh g^-1.The size and morphology of the transition metal particle have great influence on the electrochemical properties. In order to study the influence of morphology on the electrochemical properties of metal oxides, Cu₂O particles with different shape were synthesized by reducing the copper citrate complex solution with glucose and the electrochemical properties of the electrodes were systematically evaluated. The particle shape has an effect on the electrochemical performance, and the cubic Gu₂O is better than the star-shaped Cu₂O. The reversible capacity of the cubic Cu₂O is 390 mAh g^-1, and is able to sustain its capacity over 50 cycles with a slight fluctuation (less than 4%). The star-shapedCu₂O particles were pulverized after cycling, while the cubic Cu₂O particles could sustain their morphology after cycling, resulting in good cyclability.Recently, the investigation of nanoscale FeSn₂ intermetallic compound as anode material for lithium ion batteries to replace the conventional carbon based materials has received considerable attention. However, FeSn₂ powders were only synthesized by ball milling and melting methods which were high energy and time cost process, and the cyclability was poor. So it is necessary to find some facile methods to fabricate FeSn₂ intermetallic compound powder. Nanoscale FeSn₂ was successfully synthesized by chemical reduction process and solvothermal method as anode materials for lithium ion batteries. The FeSn₂ powder prepared by chemical reduction process presented quasi-spherical morphology with aggregation and the particle size were 30-70 ran, while the FeSn₂ powder prepared by solvothermal process was nearly monodisperse nanospheres, and the particle size was 80 nm with narrow distribution. The FeSn₂ intermetallic compound synthesized by solvothermal method showed better crystallinity than that prepared by chemical reduction process. The nanoscale FeSn₂ powders both delivered a high reversible discharge capacity (500 mAh g^-1). The FeSn₂ synthesized by solvothermal method presented lower initial discharge capacity and better cyclability than that prepared by chemical reduction process due to the lower specific surface area and better crystallinity.