Design And Fabrication Of Core-shell Electrode For Enhanced Electrochemical Energy Storage

Lithium-ion batteries (LIBs) have been considered as the most promising secondary batteries with the advantages of superior security, high energy density, high operating voltage and long cycle life. The development of LIBs is dependent on the electrodes which show Li-ion storage capability. However, growing demand for energy consumption has advertised the incapacity of the currently used graphite negative material due to its low gravimetric capacity (372mA h g-1). On the other hand, new anode materials with much higher capacities such as Si and transition metal oxides, have not yet been applied to practical industry because of the fast fading of the capacities caused by the poor conductivities and severe volume change during Li-ion insertion and extraction.In this thesis, we designed and fabricated several core-shell nanostructures, aiming to improve the cyclic stability of the anode materials with high capacity. The morphologies, structures and electrochemical performances of the as-prepared core-shell materials have been systematically investigated. The core-shell structure design was found to be helpful to improve the cyclic stability of the anode materials. The main contents and results are listed as follows:(1) NiSix nanostructures were grown on low-cost Ni foam substrates by chemical vapor deposition (CVD). The influences of growth temperatures, growth times and growth pressures on the morphologies and structures of the NiSix were systematically investigated. The pressure-dependent morphologies were explained as competitive growth along axial and radial direction at various growth pressures. This work can provide basic experimental research of NiSix for the fabrication of NiSix-based composites.(2) NiO coated NiSix core-shell (NiSix/NiO) nanowires were prepared by a simple oxidizing of the Ni foam supported NiSix nanowires (NWs) at400℃for1h. The surfaces of the NiSix nanowires could be roughened by HF solution immersion, and the rough surface is found to be very useful to enhance the electrochemical performances of the NiSix/NiO nanowire electrode. For the30min HF-treated and then oxidized NWs electrode, high reversible capacity of1.28mA h cm-2were obtained after200cycles, which is3times higher than the one without HF-treatment. The contribution of capacitive effect induced lithium-ion storage was investigated. The metallic NWs can improve the electrode conductivity and provide stable supporting to the NiO coating layers during cycling, leading to improved electrochemical performance.(3) Si films were in situ coated onto the obtained NiSix nanowire arrays (NWs) and nanocone arrays (NCs) at low temperature by Inductively Coupled Plasma (ICP-) CVD, and the obtained NiSix/Si core-shell nanostructures were used as anodes for LIBs. For the Si films coated NiSi*nanowires (NWs/Si) and nanocones (NCs/Si) electrodes, high reversible capacities of2650and2400mA h g-1were achieved after50cycles at a current density of2.1A g"1. The reversible capacities maintain at1385and1220mA h g-1even cycled at8.4A g-1. The electrochemical performance of the NWs/Si electrode is superior to NCs/Si, which is ascribed to the formation of highly interconnected net structure after Si coating.(4) Light-weight, flexible and free-standing graphene foam (GF) was employed as a novel core material to replace the relatively heavy NiSix nanowires and Ni foam substrates. Si films were deposited onto the obtained GF by CVD. The hierarchical architecture of GF supported Si film (GF/Si) anode shows high reversible areal capacity of1.4mA h cm-2at a current density of0.22mA cm-2. The corresponding gravimetric capacity is as high as620mA h g-1by considering the whole mass of the electrode, which is41times higher than that of the NiSx/Si core-shell electrode. The greatly improved electrochemical performance of the GF/Si anode can be attributed to the well-designed composite structure in which the hollow GF could provide sufficient room to accommodate the great volume change of Si, as well as effective limitation to Si films.