Fluoride-assisted coaxial growth of SnO₂ over-layers on multiwall carbon nanotubes with controlled thickness for lithium ion batteries

We report a facile strategy to coaxially grow compact SnO₂ over-layers on multiwall carbon nanotubes (MWCNTs) with hierarchical structures and controlled thickness via a hydrothermal method, using NaF as a morphology controlling and directing agent. The thickness of the SnO₂ over-layers can be controlled from several tens of nanometers down to several nanometers by adjusting the ratio of carbon nanotubes to Sn precursor. When applied as anode materials for lithium ion batteries, carbon nanotubes with coaxially grown thin (∼10 nm) SnO₂ over-layers showed lithium storage performance with a reversible capacity of 431 mA h g^-1 after 50 cycles, which is two times better than that of thick (∼55 nm) SnO ₂ over-layers or mixtures of carbon nanotubes and separated SnO ₂ nanoparticles. The improved cyclic performance was attributed to reduced agglomeration, enhanced electronic conductivity and released internal strain during the lithium insertion/extraction.