Porous Si Nanowires from Cheap Metallurgical Silicon Stabilized by a Surface Oxide Layer for Lithium Ion Batteries

In the quest to develop next generation lithium ion battery anode materials, satisfactory electrochemical performance and low material/fabrication cost are the most desirable features. In this article, porous Si nanowires are synthesized by a cost-effective metal-assisted chemical etching method using cheap metallurgical silicon as feedstock. More importantly, a thin oxide layer (≈3 nm) formed on the surface of porous Si nanowires stabilizes the cycling performance of lithium ion batteries. Such an oxide coating is able to constrain the huge volume expansion of the underlying Si, yet it is thin enough to ensure good permeability for both lithium ions and electrons. Therefore, the extraordinary storage capacity of Si can be well retained in prolonged electrochemical cycles. Specifically, Si/SiO_x nanowires deliver a reversible capacity of 1503 mAh g^-1 at the 560th cycle at a current density of 600 mA g^-1, demonstrating an average of only 0.04% drop per cycle compared with its initial capacity. Furthermore, the highly porous structure and thin Si wall facilitate the electrolyte penetration and shorten the solid-state lithium transportation path, respectively. As a result, stable and satisfactory reversible capacities of 1297, 976, 761, 548, and 282 mAh g^-1 are delivered at current densities of 1200, 2400, 3600, 4800, and 7200 mA g^-1, respectively.