Study of the Structural Changes Undergone by Hybrid Nanostructured Si-CNTs Employed as an Anode Material in a Rechargeable Lithium-Ion Battery

Silicon–carbon nanotube (Si-CNT) hybrid structures have been fabricated in a single step on Cu substrate by hot filament chemical vapor deposition (HFCVD). A mixture of straight chain saturated aliphatic polymer and Si nanoparticles was used as the seeding source. The material was analyzed by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS), and the Si content in the Si-CNT was estimated to be ∼15% wt % by thermogravimetric analysis (TGA). Thereafter, the films were used for lithium-ion battery (LIB) anodes, whose cyclic voltammetry studies show redox peaks for Si and C consistent with lithium insertion/extraction, indicating good reversibility over extensive cycling. Electrochemical tests showed that Si-CNT electrodes can deliver an initial high discharge capacity of ∼700 mAh/g and a reversible capacity of ∼500 mAh/g over 520 cycles. After electrochemical cycling, the Si-CNTs were analyzed and compared to pristine material. The cycled films showed an increment of Si-CNT diameter and negligible cracks, formed due to high volumetric expansion of the silicon upon lithiation. Micro-Raman spectroscopy performed before electrochemical cycling established the presence of crystalline Si nanoparticles (<10 nm), and amorphous Si particles still bound to CNT after cycling. These results were confirmed by X-ray photoelectron spectroscopy (XPS). After cycling, the films showed good contact with the Cu substrate, and delamination was not observed by electrochemical impedance spectroscopy (EIS). The Si-CNT hybrid structure grown in a single step represents a promising anode material for a rechargeable LIB with high energy density and long cycling stability.