A high-performance lithium-ion battery anode based on the core-shell heterostructure of silicon-coated vertically aligned carbon nanofibers

This study reports a high-performance hybrid lithium-ion anode material using coaxially coated silicon shells on vertically aligned carbon nanofiber (VACNF) cores. The unique "cup-stacking" graphitic microstructure makes VACNFs a good lithium-ion intercalation medium and, more importantly, a robust bush-like conductive core to effectively connect high-capacity silicon shells for lithium-ion storage. The vertical core-shell nanowires remain well separated from each other even after coating with bulk quantities of silicon (equivalent to 1.5 mu m thick solid films). This open structure allows the silicon shells to freely expand/contract in the radial direction during lithium-ion insertion/extraction. A high specific capacity of 3000-3650 mA h (g(Si))(-1), comparable to the maximum value of amorphous silicon, has been achieved. About 89% of the capacity is retained after 100 charge-discharge cycles at the C/1 rate. After long cycling, the electrode material becomes even more stable, showing the invariant lithium-ion storage capacity as the charge-discharge rate is increased by 20 times from C/10 to C/0.5 (or 2C). The ability to obtain high capacity at significantly improved power rates while maintaining the extraordinary cycle stability demonstrates that this novel structure could be a promising anode material for high-performance lithium-ion batteries.