The Design, Synthesis And Electrochemical Performance Of Nanostructured Anode Materials For Lithium-ion Batteries

Lithium-ion batteries are currently the dominant power sources for portable electronic devices and are also considered as promising power sources in hybrid electric vehicles and clean energy storage. However, the current lithium-ion batteries are approaching limits set by the electrode materials because of their low specific energy density and power density. Due to the low theoretical capacity (372 mAhg-1) of graphitic carbon, which is the most commonly used anode materials, intensive research has been conducted to research and development of novel electrode materials with higher specific energy density, higher specific power density, longer cycle life and lower cost. Nanostructured materials are currently of interest in this field, because they can greatly reduce the distances of lithium ion diffusion and electron transport, and provide large electrolyte/electrode contact areas, which can significantly enhance the performance of the battery. Therefore, it is a significative research topic for the synthesis of nanostructured electrode materials and systemical study of the optimized relationship between the nanostructured electrode materials and electrochemical performance. The research content of this paper is as following:1. A flower-like Fe3O4/carbon nanocomposite with nano/micro hierarchical structure was prepared by controlled thermal decomposition of the iron alkoxide precursor, which was obtained via an ethylene glycol-mediated solvothermal reaction. The nanocomposite was characterized by the assembly of porous nanoflakes consisting of Fe3O4 nanoparticles and amorphous carbon that is in-situ generated from the organic components of alkoxide precursor. When used as the anode materials for the lithium-ion batteries, the resultant nanocomposite showed high capacity and good cycle stability (1030 mAh g-1 at a current density of 0.2 C up to 150 cycles), as well as enhanced rate capability. The excellent electrochemical performance can be attributed to the high structural stability and high rate of ionic/electronic conduction arising from the synergetic effect of the unique nano/micro hierarchical structure and conductive carbon coating. Other nanostructures of Fe3O4/carbon composite, including single nanoflakes and hollow flower-like microspheres, can also be synthesized through adjust the reactant concentrations. And the formation mechanism was investigated. But the electrochemical measurements demonstrated that their performances are not good as the flower-like structure. 2. TiO2/C nanocomposites with diameter of 300～400 nm were synthesized through one-pot hydrothermal reaction from titanium glycolate spheres with glucose as carbon precursor. The weight fraction of carbon present in the final product can be readily tuned by varying the concentration of glucose used during the hydrothermal process. A systematic study has been carried out to examine the effect of carbon content upon lithium-ion battery performance. It is found that the TiI2/C nanospheres with 7% carbon can deliver a capacity of 165 mAhg-1 after 80 charge/discharge cycles at a current density of 0.2 C as well as good rate capability. The excellent electrochemical performance is attributed to the nanosturcturing of TiO2 and the suitable carbon coating around the TiO2 nanopaticles.3. SnO2 porous submicron tubes were synthesized by the thermal decomposition of SnC2O4 precursor that was obtained in an ethanol-mediated system in the presence of surfactant P123 at room temperature. The influence of reaction conditions on the morphology of SnC2O4 is discussed in detail. It shows that P123 induces the oriented growth along the one-dimensional direction and the hollow structure is formed during the ethanol washing. The as-prepared SnO2 porous submicron tubes showed good cycle stability and delivered reversible capacity of 370 mAhg-1 after 30 cycles, which might be related to the hollow and porous structure of the tubes that could alleviate the volume changes and mechanical stress during charge/discharge cycling.4. The carbon nanotubes (CNTs) were homogenously coated by pyrolytic carbon via thermal decomposition of methane at 950℃. The effect of coating time, carbon sources, introduction of hydrogen was investigated. The electrochemical results revealed that appropriate amount of carbon coating could improve the cycling stability at high current rate and rate capability performance of the CNTs. After 120 min carbon coating, CNTs can delivered a reversible capacity of 234,218,197,166 and 113 mAhg-1 at the current rates of 0.2C、0.5C、1C、2C and 5C, respectively. After 600 cycles at 1C, its capacity retention is 63.6%, while that of the initial CNTs is only 25.4%.