Preparation and electrochemical properties of novel silicon-carbon composite anode materials with a core-shell structure

Multi-component porous Si-SiO_x (pSi) consisting of Si, SiO and SiO₂ was formed by the pretreatment of SiO at 950 °C for 3 h in an inert atmosphere (He) using a disproportionation reaction. Hybrids of pSi and carbon nanofibers (pSi-CNFs) with a core-shell structure were prepared by catalytic chemical vapor deposition (CVD) using Fe-Ni species as the catalyst and a mixture of CO/H₂/C₂H₄ (volumetric ratio 3∶1∶1) as the reactant for 0.5, 1 and 2 h, and were characterized by SEM, TEM, EDS, XRD, Raman spectroscopy and XPS. Results indicate that the pSi-CNF particle sizes are 5−20 μ m with the diameters of the CNFs being 5−40 nm. The CNFs are uniformly coated on the surface of the pSi to form a core-shell structure. Electrochemical performance testing shows that the reversible capacity of the pSi-CNF (0.5 h) remains at 1 411 mAh^.g⁻¹ and the capacity retention is 74% after 100 cycles at a current density of 0.2 A^.g⁻¹. The reversible capacity remains at 735 mAh^.g⁻¹ at a current density of 1 A g⁻¹ after 300 cycles with a capacity retention of 86%. In the pSi, Si and SiO provide the electrochemical reversible capacity. The core-shell structure with the CNF coating effectively improves the conductivity of the composites, and also inhibits the volume expansion of silicon to maintain the integrity of the core shell structure.