Fabricating Silicon Nanotubes by Electrochemical Exfoliation and Reduction of Layer-Structured CaSiO₃ in Molten Salt

Silicon nanotubes (SNTs) are very attractive in the fields of energy, catalysis, and sensors, but a facile template- and/or catalyst-free preparation method is still absent. Herein, we study a controllable and cost-effective approach for preparing SNTs by electrochemically reducing layer-structured calcium silicate (CaSiO₃) in molten CaCl₂/NaCl without any template and catalyst. The underlying mechanism of the SNT formation is uncovered: the layer-structured CaSiO₃ is first electrochemically exfoliated into SiO (0 < x < 2) sheets while releasing CaO into the molten salts, and then the SiO sheets are electrochemically reduced and simultaneously crimped into SNTs. The diameter (120-312 nm) and wall thickness (∼40 nm) of the SNTs can be tailorable by manipulating the reduction potential between -1.28 and -1.48 V (vs Ag/AgCl). Lastly, the electrolytic SNTs show a high lithium storage capacity of 3737 mAh g^-1 at 0.2 A g^-1, a high rate capability of 1371 mA h g^-1 at 10 A g^-1, and stable cycling with a capacity of 974 mAh g^-1 after 600 cycles at 1 A g^-1. Overall, the template- and catalyst-free electrochemical method provides a straightforward and facile way to prepare SNTs with a brand-new mechanism that can be applied to other tubular structure materials.