UC Berkeley

Li-ion batteries are the most promising secondary battery to meet the demand of electric vehicles and intermittent renewable energy sources. However, improvements to capacity and performance are necessary. As graphite-based negative electrodes are nearing theoretical capacity limits, alternative anode technologies are being explored to increase energy density. Materials that allow for Li alloying are of considerable interest due to the formation of high Li concentration alloys. Furthermore, additives to Si are a powerful tool to tailor the electrochemical performance of Si. This dissertation explores the use of Si, the alloying type electrode with the highest theoretical capacity, and Si-derived alloy materials as high capacity anodes. We probe the Si-alloy systems using computational methods, including i) simulation of short-range order using ab-initio molecular dynamics, ii) evaluating thermodynamics using electronic structure calculations, and iii) assessing diffusion kinetics using AIMD. From phase diagrams, derived from the first-principles calculations, additive elements are classified as inactive and active elements based on the element's reactivity with Li, where active elements form stable binary compounds with Li while inactive elements do not. In general, binary Si-X compounds, formed with the active elements are not thermodynamically stable and will phase separate into distinct Si and X phases. Alloyed components are capable of tuning the equilibrium lithiation potential with a range of 0V-3.0V dependent on the alloying element and mole fraction. Phase separation of the inactive Si-X binary compounds is not expected but, the incorporation of inactive elements leads to dramatically lower capacity due to the combined effects of decreased active material and formation of X rich Si-X compounds which are electrochemically inactive. While there is no limit to alloying with active elements, alloys with greater than 33% inactive element yield extremely low capacity. Alloying of a third element is found to dramatically modulate the Li diffusivity through the modification of melting temperature. In the case of Oxygen, decreased Li-diffusivity is seen, while Sn increases Li self-diffusion rates. The methods and analysis utilized to predict electrochemical characteristics may be generalized for more alloyed components (ternary, quaternary, etc.) to rapidly screen the chemical space for Si-based alloy electrodes.