Scalable and non-intensive routes to silicon for lithium-ion battery anodes

Silicon has been highlighted as a promising anode material in lithium-ion batteries due to its step change in capacity verses conventional graphite. The cycling of silicon anodes within a lithium-ion battery (LIB) leads to degradation and capacity fade due to the 280% volume change of silicon. Many avenues of silicon synthesis have been explored to produce nanostructures which can withstand this change in volume.
Magnesiothermic Reduction (MgTR) of silica to silicon shows significant promise over other syntheses in scalability, economic and environmental aspects for producing porous silicon nanostructures. The problem with MgTR is a lack of understanding regarding the pore evolution of porous silicon based on reduction parameters and precursor material, which in turn limits predictive design for desired applications. Here we show for the first time that the pore structure of porous silicon is strongly related to the interconnectivity of silicon crystallites. We show that the MgTR is a thermodynamically driven equilibria which determines the purity of the silicon product. Higher temperatures also cause sintering of silicon nanocrystallites. We show that it is the interconnectivity of these crystallites determine the pore size and distribution within porous silicon. These findings apply to a wide variety of porous silica precursors and we show this mechanism is true for the introduction of pores into nonporous quartz after MgTR. Further, we show that by exploiting this mechanism, mesoporous silicon can be produced which has excellent promise for LIB applications with a capacity of 2170 mAh/g after 100 cycles.
As a second section of the thesis, we focused on the use of silica directly in LIBs. The use of silica has potential advantages over other silicon based active materials, upon reduction with lithium silicon is produced and contained within a supporting structure of inactive material. However, there is no detailed understanding of how the lithium-silica reduction reaction progresses and the chemical nature of the products. Here we develop a new method to effectively monitor the rate of electrochemical reduction of silica and propose a mechanistic understanding of this process. In addition, and for the first time, we characterise the existence of elemental silicon in the reduced structures. Our proposed mechanism is based upon the initial insulating nature of the silica active material and how electronic conduction pathways are formed in the reduced material. We apply the principles of this mechanism to reduce the length of the electrochemical reduction reaction to 13 hour compared with 400 hour reduction times reported in the literature.
The findings herein provide a significant step change and hence can be taken forward to design optimal materials for LIB applications. These results strongly support the potential for reduction in silicon costs for LIB in both economic and environmental terms as well as for a reverse engineering approach to design specific porous silicon and silica for desired applications.