Supercritical Fluid-Liquid-Solid Growth of Alloyed Si1-xGex Nanowires

Abstract

Silicon (Si) and germanium (Ge) have emerged as next-generation anode materials for Li-ion batteries due to their high theoretical capacities (Si:3579mAh/g, Ge:1384mAh/g) and are promising replacements for lower-capacity, graphite-based anodes (372mAh/g). However, one significant challenge of their practical implementation is the large volume change associated with the insertion and extraction of lithium ions, which results in mechanical pulverization and capacity fade. Nanostructuring active material morphology has been demonstrated as a strategy to compensate for volume change and improve capacity and cycle life in lithium ion batteries. Although Si has the highest theoretical capacity among elemental lithium ion battery negative electrodes and maintains distinct advantages as an electrode material, silicon’s rate capability is hindered by its lower ionic diffusivity and electronic conductivity in comparison to Ge. However, Ge is much more expensive, which could limit its widespread use in anodes for lithium ion batteries. In an effort to capitalize on the benefits of both materials, there has been recent interest in developing an anode that combines the excellent rate capability of Ge with the high capacity and lower cost of Si. Recent efforts have demonstrated the ability to synthesize alloyed Si1-xGex nanowires (NWs) via a solution-based mechanism, using a thin film of tin to seed nanowire growth. In order to improve the scalability of this process, here we report the supercritical fluid-based synthesis of Sn nanocrystal-seeded alloyed Si1-xGex NWs. By balancing reaction parameters such as precursor reactivity, the semiconductor precursor to metal seed ratio, Si:Ge precursor ratio, and reaction temperature, we demonstrate the ability to synthesize alloyed Si1-xGex NWs through a colloidal, supercritical fluid-based process for the first time.