High-Performance Lithium-Ion Battery Anode Based on the Core-Shell Heterostructure of Silicon-Coated Vertically Aligned Carbon Nanofibers.

摘要

This study reports a high-performance hybrid lithium-ion anode material based on coaxially coated Si shell on vertically aligned carbon nanofiber (VACNF) core. The unique cup-stacking-like graphitic microstructure of the VACNFs provides abundant broken graphitic edges at the sidewall, making VACNFs an interesting Li+ intercalation medium and, more importantly, a robust and highly conductive core which can form good interface with the active Si shell for Li+ storage. Reversible Li+ intercalation and extraction with the capacity of 350-400 mAhg-1 has been obtained with bare VACNFs at C/2 and C/1 rates. Upon sputtering deposition of Si, it forms an interesting core-shell vertical nanowire structure with a tapered coaxial Si shell. The uniform vertical alignment enables Si to spread along the whole VACNF length, resulting in a much smaller shell thickness. With the 10 ?m long VACNF arrays, the core-shell nanowires remain well separated from each other even after depositing a thick layer of Si equivalent to 1.5 ?m nominal thickness on flat substrates. This open three-dimensional hybrid architecture allows the Si shell to freely expand/contract in the radial direction during Li+2 insertion/extraction. High-performance lithium storage with the specific capacity of 3000-3650 mAhg-1 (normalized to the Si mass) has been achieved, which changes little as the power rate is increased by 20 times from C/10 to C/0.5 (or 2C). The obtained capacity matches the maximum reported value for amorphous Si, indicating that the Si shell is fully active due to effective charge collection by the VACNF core and short diffusion path for Li+ insertion/extraction.