Facile synthesis routes of silicon nanowires for Li-ion battery applications using a zinc catalyst

The work presented in this thesis describes the development of new synthesis routes for Zn catalyzed Si nanostructures used for the fabrication of high-capacity Li-ion battery electrodes on stainless-steel current collectors. The results chapters are arranged as research articles with introductory summaries at the beginning of each.

As an anode material, Si boasts a capacity multiple times higher compared to graphite which currently is predominantly used for commercial Li-ion battery anodes. The high capacity of Si is facilitated by its ability to form Li-rich alloys where Li15Si4 is its fully lithiated state and results in a volume expansion of <300%. This enormous volume change associated with the lithiation/delithation of the active material results in pulverization and loss of contact with the current collector causing the cells cycle life to decrease dramatically. Si structures with nano-dimensions, in particular nanowires, have the ability to accommodate these volume changes without pulverizing and can therefore allow for a longer cycle life. One of the main challenges associated with the use of nanostructured materials for battery applications is their high material and productions costs. Catalysts commonly used to catalyze Si nanowires including Au and Sn are expensive while the PVD techniques employed to prepare the catalyst are limited in scalability.

Chapter 3 describes a fabrication method of NW electrodes whereby electrodeposited Zn is used to directly catalyse the growth of Si, Ge and Si-Ge axial heterostructure nanowires. The study demonstrates that LiBH4 is a suitable reducing agent for removing the Zn oxide layer that forms after air exposure which dramatically enhances overall nanowire growth density. It was also found that Zn is a highly suitable catalyst for Si-Ge axial heterostructure NWs which had an atomically abrupt interface. The Zn-seeded Si NW electrode exhibited an initial discharge capacity of 1772 mAh/g retaining 85.5% of it’s initial capacity after 100 cycles. Notably, we demonstrate that the Zn seeds actively participate in the cycling process causing them to alloy with the Si to form a Zn-Si mesh. Chapter 4 describes a solution-based synthesis method whereby ZnO is reduced in-situ to form a Zn catalyst which can facilitate the growth of Si NWs. The approach was shown to allow for Si NW growth using ZnO both in bulk and powder form. Following NW synthesis, the NWs were washed in acidified IPA to remove any residue ZnO as well as the Zn seeds. The use of ZnO powder allowed for the synthesis of up to 200 mg of NWs per reaction with a chemical efficiency of 25%. Chapter 5 seeks to apply the nanowire growth mechanism described in chapter 4 and use this to produce high mass loading Si NW anodes. This was achieved by electrodepositing ZnO platelets onto a stainless-steel current collector which are placed in the solution phase of a refluxing solvent and reduced to generate the Zn catalyst to facilitate Si NW growth. This allowed for production of mass loadings of up to 1 mg/cm2 on planar substrates. To mitigate the effects of delamination during cycling, the stainless-steel current collector was modified using an etching procedure capable of creating either a micro and nano textured surface both of which were tested using the 1 mg/cm2 mass loading electrodes and show improvements in cycling performance. In chapter 6 the ability to grow Si NWs via both the VLS and VSS growth mechanism by using reaction temperatures above or below its melting point of Zn is reported. A larger mean NW diameter is observed when synthesizing at sub-eutectic temperatures via the VSS mechanism compared to NW synthesized via the VLS mechanism. This was used to vary the diameter along the axial length of individual NWs by transitioning between VLS and VSS growth temperatures in single reactions.