Si-and Ge-based Nanomaterials:Synthesis And Application For Lithium- And Sodium-Ion Batteries

Owing to the thriving of portable electronic devices and electric vehicle industry, there is a great need of novel lithium-ion batteries with higher energy densities and higher power densities. As typical alloy-type materials, Si and Ge (group Ⅳ elements) are considered to be one of the most promising candidates for anode materials of the next-generation power batteries. However, both Si-and Ge-based electrodes suffer from rapid capacity decline caused by large volumetric change during Li insertion and extraction, which results in pulverization of the active materials.In this dissertation, we propose various synthetic methods (e.g. solution growth, RF magnetron sputtering, voltage-control technique, magnesiothermic reduction, electrochemical deposition) to obtain a rich variety of Si- and Ge-based composite materials, including Cu-Li2O@Si core-shell nanorod arrays, Cu-Li2O@Si core-shell nanowall arrays, porous Si particles, porous Si@C composite material, hollow Si@C spheres, Ni-Ge nanopyramid arrays,3D porous Ge electrode and 3D porous Si electrode. Ascribed to the advantages of the active materials and the unique structural properties, the above-mentioned Si- and Ge-based anodes exhibit improved electrochemical performance. The main innovative results are displayed as follows:(1) CuO@Si core-shell arrays with different morphologies are synthesized on the surface of the copper foil by simple oxidation and subsequent RF sputtering of Si. CuO irreversibly transforms into Cu and Li2O due to the voltage limit, thus enabling the fabrication of Cu-Li2O@Si core-shell nanorod arrays during electrochemical measurement, which exhibit excellent cycling stability and improved rate capabilities compared to planar Si electrodes. The voltage-control technique simplifies the synthetic route and can be extended to other transition-metal-oxide/silicon composite anodes since the reduction of the oxides is driven by electrochemical reaction dynamics.(2) By controlling the state of magnesium during the magnesiothermic reaction, either hollow Si spheres or 3D interconnected porous Si with uniform pores can be obtained using SiO2 spheres as precursors, both of which have been coated with carbon via a CVD method subsequently. The as-prepared hollow Si@C spheres and porous Si@C composite anodes exhibit long cycle life, high capacity and superior rate capabilities.(3) Ni-Ge nanopyramid arrays have been fabricated on copper foil through a facile electrochemical deposition and subsequent RF sputtering approach. The nickel nanopyramid arrays function as electron transport medium, structural support and inactive confining buffer, while amorphous germanium is the active material for sodium-ion storage. The nanostructured electrode exhibits high capacity, superior cycling stability and rate capabilities.(4) By preparing silica opal-like templates on copper foil, three-dimensional porous copper current collector have been synthesized through subsequent electrochemical deposition and chemical etching process. Amorphous Ge/Si or crystalline Fe3O4 has been deposited onto the surface of the inverse-opal-like current collector via RF sputtering or electrochemical deposition, respectively. The 3D porous Ge and Fe3O4 electrodes exhibit high capacity, superior cycling stability and rate capabilities, demonstrating the universality of the copper current collector.