Controllable synthesis of one-dimensional silicon nanostructures based on the dual effects of electro-deoxidation and the Kirkendall effect

Abstract

In this study, we successfully synthesized silicon nanotubes (Si-NTs) and silicon nanowires (Si-NWs) in a controllable manner using a catalyst- and template-free method through the direct electrolysis of SiO₂ in a molten CaCl₂-CaO system, while also proposing a novel formation mechanism for Si-NTs. Si-NWs are formed through electro-deoxidation when the cell voltage is within the range of CaO decomposition voltage and SiO₂ decomposition voltage. By subsequently adjusting the voltage to a value between the decomposition potentials of CaCl₂ and CaO, in-situ electro-deoxidation of CaO takes place on the surface of the synthesized Si-NWs, leading to the formation of a Ca layer. The formation of Ca-Si diffusion couple leads to the creation of vacancies within the Si-NWs, as the outward diffusion rate of Si exceeds the inward diffusion rate of Ca. These differential diffusion rates between Si and Ca in a diffusion couple exhibit an analogy to the Kirkendall effect. These vacancies gradually accumulate and merge, forming large voids, which ultimately result in the formation of hollow SiCa-NTs. Through a subsequent dealloying process, the removal of the embedded calcium leads to the formation of Si-NTs. Following the application of a carbon coating, the Si-NTs@C composite showcases a high initial discharge capacity of 3211 mAh·g⁻¹ at 1.5 A·g⁻¹ and exhibits exceptional long-term cycling stability, maintaining a capacity of 977 mAh·g⁻¹ after 2000 cycles at 3.0 A·g⁻¹.