Preparation Of Tin And Iron-based Anode Materials And Their Applications For Lithium-ion Batteries

With many advantages such as high output voltage, low self-discharge, long cycling life, high specific energy, no memory effect and so on, lithium-ion battery has a wide application future and is considered to be the most important energy storage system in the 21 st century. The performance of lithium-ion battery mainly depends on the cathode material, anode material and electrolyte material. Therefore, it is a hotspot to develop low-cost and high-performance materials in recent years. For the anode materials, the commercial graphitic carbon has a low theotrtical capacity(372 m Ah g-1), which can not meet the demands of high-capacity lithium-ion battery. Among the numerous anode materials, Sn O2 and Fe2O3 are two kinds of potential materials. However, their practical application is limited by poor cycling performance resulting from poor conductivity and dramatic volume change during Li+ insertion/ extraction processes. As we know, carbon materials have good cycling performance. Meanwhile, the oxide materials have high specific capacity. It is possible to obtain the anode materials with high capacity and good cycling performance if we can combine carbon materials and the oxide materials. In this thesis, Sn O2/graphene, Sn O2/FGS, Sn O2/MCNF, Fe2O3/MCNF and Fe2O3/carbon foam composites with special nanostructures were synthesized and characterized. The electrochemical performances of the products were also studied.(1) The mesoporous Sn O2/graphene nanocomposite with a high specific surface area was synthesized by the redox reaction between Sn2+ and graphene oxide in ethanol without using any additive. The electrochemical testing results show that the sample exhibited good electrochemical performance. The good cycling performance can be attributed to the graphene nanosheets in the composite, which maintains the laminar structure of the nanocomposite and restricts the volume change of Sn O2 nanoparticles during the insertion/extraction processes of lithium ions. In addition, it is important that the porosity of the electrode favors the transportation of lithium ions and electrolyte.(2) The Sn O2/graphene nanocomposite was synthesized by a hydrothermal method without using any additive. The effect of loading and porosity for the electrochemical performance was discussed. The testing results show that the sample with a higher proportion of micropores exhibited good electrochemical performance.The above method was improved by a microwave-assistant method. The synthesis process can be completed within two minutes. The effect of microwave reaction time for the performance was studied.(3) A mesoporous carbon nanofiber(MCNF) with bimodal pore structure, graphitic pore wall, high specific surface area and high volume was prepared by an improved self-templated strategy according to the literature. By the thermal decomposition of nitrate method at low temperature, the Fe2O3/MCNF nanocomposite was prepared. The electrochemical test results show that the composite possesses excellent cycling performance, which can be ascribed to the special nanostructure of Fe2O3/MCNF. The most Fe2O3 nanoparticles were formed in the interconnected pores of MCNF, thus making them fully accessible to lithium ions in the electrolyte. Meanwhile, the MCNF can provide an internal void space to accommodate the volume expansion.The Fe2O3/carbon foam nanocomposite was prepared by the same method except using carbon foam as the matrix. The electrochemical performance of Fe2O3/carbon was compared with the Fe2O3/MCNF.(4) Two kinds of functional graphene sheets(FGS) with different oxygen-containing group content were preapared by thermal exfoliation of graphite oxide. By using FGS300 and FGS900 as the carrier, the Sn O2/FGS nanocomposite was synthesized through a hydrothermal method without any additive. The influence of functional groups on the performance of the product was discussed. The electrochemical test results show that Sn O2/FGS900 exhibited a good performance at a low current density, while the Sn O2/FGS300 exhibited a good performance at a high current density.The Sn O2/MCNF hybrid was synthesized by a melting impregnation-hydrolysis-thermal annealing approach. The test results show that the hubrid maintained the bimodal mesoporous structure of MCNF and exhibited a good cycling performance.