Study of lithium-ion battery anodes based on silicon nanostructures
Date of Issue2014
School of Electrical and Electronic Engineering
Silicon is well known as a very promising anode material because it has an extremely high theoretical specific capacity of ~4200 mA h g-1 and some other advantages such as low discharge potential, non-toxicity and abundance, etc. However, it is found that anodes based on bulk or micron-sized Si exhibit very poor cycling performance due to the structural pulverization caused by significant volume changes (upto 400%) of Si during Li+ insertion/extraction processes. On the other hand, nanostructured Si has been shown to alleviate this problem because it has a large portion of surface atoms that could effectively release internal strain through the surrounding space. This dissertation focuses on the design and fabrication of lithium ion battery anodes based on Si nanostructures and the investigation of their charge/discharge behaviors. The first nanostructure demonstrated in this PhD project is a Si-Ni cone array comprised of supportive Ni particles rooted on a Cu foil current collector and electrochemically active Si coatings. In this structure, each Ni particle and its Si coating form a unique vertical cone rooted on the current collector. With the spherical Ni particle support, the Si coating is able to expand/contract along radial direction of Ni cores to release its internal stress through out-of-plane directions, instead of accumulating a large in-plane stress. Scanning electron microscopy images show that the cone array still adheres to the substrate after cycling and some cracks have developed along boundaries of the cones to help release internal stress, while the Si cones show no crack or pulverization. As a result, the composite anode shows quite good capacity retention and rate performance, significantly better than a conventional Si thin film anode with the same amount of amorphous Si loading. The second nanostructure demonstrated is carbon nanotube (CNT)-Si core-shell wire arrays rooted on a stainless steel current collector. The structure employs vertically aligned CNTs with controllable diameters and spacing as the core. Anodes based on the CNT-Si wire arrays with large core nanowire diameters and large inter-wire spacing demonstrate excellent electrochemical performance that is superior to most core-shell Si nanowires (SiNWs) previously reported. The good performance can be mainly attributed to two factors. First, small curvature of the thick CNT core effectively reduces the detrimental hoop stress in the Si shell. Second, the vertical alignment and large inter-wire spacing of the nanowire array provide sufficient space to accommodate radial volume change of the Si shell and promote access of electrolyte to the whole structure. The electrode shows well maintained morphology and almost intact core-shell structures after repetitive cycling, further confirming the advantages of the fabricated structure in effective strain relaxation. The CNT-Si core-shell structure is further improved by engineering the morphology of the Si shell such that its thickness gradually decreases along the CNT core from top to bottom of the array. The gradient Si shell eliminates structural damage caused by volume change of Si coating on current collector surface and at CNT roots. As a result, the capacity retention of the CNT-Si anode is further improved. During my PhD study, a novel method to fabricate SiNW arrays has also been developed. In this method, a monolayer of well-spaced metal particles is synthesized on Si substrate through a simple in situ dewetting process. With the metal particles as a sacrificial template, a large area catalyst metal mesh is then fabricated. Finally, SiNW arrays are obtained by performing solution etching with the metal mesh as the catalyst. Compared to previously reported methods to fabricate SiNW arrays with controllable diameters and spacing, our method is more repeatable, scalable and easier to control. The method is expected to be promising for many practical applications such as lithium ion batteries and solar cells.
DRNTU::Engineering::Electrical and electronic engineering