| Lithium-ion batteries(LIBs) are widely used for portable electronic devices, such as cell phones, laptop computers, and digital cameras, because of their high energy density and low self-discharge rate. Nevertheless, their energy densities are still not sufficient for emerging electric transportation systems. As a key component, electrode material dominates the electrochemical properties of LIBs. Silicon and tin with many merits, including high theoretical capacities, low Li-uptake voltage, and environmental benignity, have been regarded as potential substitutes for commercial graphite anodes. Unfortunately, their huge volume expansions during the lithiation proce ss lead to the pulverization of silicon and tin and subsequently results in fast capacity fade of the electrodes. Furthermore, silicon and tin also suffer from the low intrinsic electric conductivity and low lithium diffusion rate. These shortcomings hinde r their practical implementation. In this thesis, nanostructured silicon and tin based composite materials have been synthesized to enhance the lithium storage capability. The detailed contents could be seen as follows:(1) Dual surface modification of porous silicon(p Si) by Ag nanoparticles and graphene nanosheets(GNS). Porous silicon was prepared through the magnesiothermic reduction of commercial MCM-41 and then etched by HF solution. The as-prepared p Si is very hydrophobic. Therefore, it is difficult to coat Ag nanoparticles on the surface of the p Si by the conventional silver-mirror reaction. In our experiment, the p Si was decorated by Ag nanoparticles through a simple Ag NO3 impregnation/thermal decomposition method. Finally, the Ag-p Si/GNS composite was obtained through the thermal reduction of graphene oxide(GO). The composite exhibited a distinctly high reversible capacity, good cycle stability and super rate capability. The superior electrochemical performance could be attributed to the following reasons. The porous structure can not only buffer the large volume change, but also facilitate the diffusion of Li+ and electrolytes into the electrode, whereas the dual surface modification can significantly enhance the electrical conductivity of Si and reduce the direct contact of Si particles with the electrolytes.(2) Surface binding of polypyrrole(PPy) on porous silicon hollow nanospheres(PHSi). Mesoporous silica hollow nanospheres(MHSi O 2) were prepared via a spontaneous self-transformation approach by dispersing the silica spheres generated by the St?ber method in water. Then, PHSi nanospheres were generated through the reduction of MHSi O2 by Mg at a temperature of 650 oC in H2/Ar. Finally, pyrrole was polymerized in situ on the surface of the PHSi in an ice/water bath, resulting in the formation of PPy@PHSi nanocomposite. Discharged/charged at a current density of 1.0 A g-1, the PPy@PHSi electrode demonstrates an excellent cycling stability with 88% capacity retention against at the 2nd cycle after 250 cycles. The discharge capacities of the nanocomposite are 2594, 1610, 1125, and 661 m A h g-1 at current densities of 1.0, 2.0, 4.0, and 8.0 A g-1, respectively. When the current density is decreased stepwise to 1.0 A g-1, a rebound in capacity with a slight decrease can be observed for the PPy@PHSi. The outstanding lithium storage properties stem from porous hollow structure and PPy coating. The hollow structure and porous channels in the shell can not only facilitate the diffusion of Li+ and electrolyte into the electrode, but also buffer the large volume change and reduce the diffusion-induced stress. The PPy coating can significantly improve the surface electronic conductivity of the PHSi nanospheres and stabilize the whole structure.(3) Incorporation of Sn/Sn O nanoparticles in crumpled nitrogen-doped graphene nanosheets(NGNSs). The crumpled NGNSs and Sn nanoparticles were mixed together and heated in Ar at a temperature of 250 oC. The molten Sn was gradually imbibed into the pores of NGNSs by capi llary forces and partly reacted with the surface functional groups of NGNSs, leading to the formation of the Sn/Sn O/NGNSs composite. The remaining empty spaces among the crumpled NGNSs can not only accommodate the huge volume change of Sn during cycling, but also facilitate the diffusion of Li+ and the electrolyte into the electrode. The introduction of NGNSs can obviously enhance the electronic conductivity of the electrode. The generation of Sn O can ensure a good contact between the NGNSs and Sn-based nanoparticles, avoiding the aggregation of the nanoparticles and retaining the structural stability of the composite during lithiation/delithiation processes. Discharged/charged at a current density of 1.0 A g-1, a high discharge capacity of 853 m A h g-1 is obtained after 250 cycles, approximately 79.0% of that of the first cycle, showing the excellent cycling stability of the composite. The discharge capacities of the composite are 1224, 874, 701, 561, 401 and 241 m A h g-1 at current densities of 0.5, 1.0, 2.0, 4.0, 8.0 and 16.0 A g-1, respectively. After the high-rate discharge-charge cycling, a discharge capacity as high as 634 m A h g-1 is recovered when the current density is decreased stepwise to 1.0 A g-1,displaying the excellent rate capability of the composite. |
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