Abstract
The increasing demand for advanced electronic devices and energy storage have stimulated significant interests in lithium ion battery development. Li based batteries are one of the most promising energy storage systems which as they are light-weight and energy-delivery efficient. Compared to the common graphite-anode system, Si is known to have highest theoretical specific capacitance making Si the most promising candidate for the next-generation anode materials for lithium batteries. However, large volume expansion and serious material pulverization after cycling lead to poor life times, and is the main stumbling block toward their commercialization. In this research, glancing angle deposition (GLAD) technique is utilized to deposit uniform and aligned helix Si nanostructures. By varying the rotation angle during GLAD, various helix Si nanostructures with differing porosities were deposited. With increasing numbers of rotation (3 to 48) the double layer capacitance, related to the surface area, increased to from 0.112 to 0.208 F/cm3. Additionally, the areal spacing also increases and results in occupation of Si nanostructure decreased from 77.5% to 73.77%. As a result, 48 cycle helix Si anode shows the best electrochemical performance with a volumetric specific capacity 846.55 mAh/cm3. Following a 100 cycle test, the anode is able to maintain 70% of its original volumetric specific capacity. However, the low conductivity of intrinsic silicon makes charge transfer of the electrons slow and also gives rise to incomplete alloying reactions with Li ions. To overcome this we annealed our anode with the aim of forming copper silicide, utilising the underlying copper substrate as a source. In doing so, the volumetric specific capacity was increased to 1706.68 mAh/cm3, using a 100 cycle test at charge/discharge rates as high as 0.25 C. Throughout this work detailed analysis was carried out, including X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), I-V characteristics (I-V) Electrochemical Impedance Spectroscopy (EIS) and Cyclic Voltammetry (CV), providing an understanding of our results and possibilities for future work. Furthermore, we believe that the adequate porosity and lower conductivity helps to minimize the enormous stress within the film structure by proving enough space for volume expansion, leading to a longer life time and better charge transfer and electrochemical performance.