Ultrafast sodium storage in anatase TiO₂ nanoparticles embedded on carbon nanotubes

The main disadvantage of using transition metal oxides for Na⁺-ion batteries is the sluggish kinetics of insertion of Na⁺ ions into the structure. Here, we introduce nanosized anatase TiO₂ that is partially doped with fluorine (TiO_2-δF_δ) to form electro-conducting trivalent Ti³⁺ as an ultrafast Na⁺ insertion material for use as an anode for sodium-ion batteries. In addition, the F-doped TiO_2-δF_δ is modified by electro-conducting carbon nanotubes (CNTs) to further enhance the electric conductivity. The composite F-doped TiO₂ embedded in CNTs is produced in a one-pot hydrothermal reaction. X-ray diffraction and microscopic studies revealed that nanocrystalline anatase-type TiO_2-δF_δ particles, in which fluorine is present with TiO₂ particles, are loaded on the CNTs. This yields a high electric conductivity of approximately 5.8Scm^-1. The first discharge capacity of the F-doped TiO₂ embedded in CNTs is approximately 250mAh (g-oxide)^-1, and is retained at 97% after 100 cycles. As expected, a high-rate performance was achieved even at the 100C discharging rate (25Ag^-1) where the composite material demonstrated a capacity of 118mAhg^-1 under the 0.1C-rate charge condition. The present work also highlights a significant improvement in the insertion and extraction of Na⁺ ions when the material was charged and discharged under the same rate of 35C (8.75Ag^-1), delivering approximately 90mAh (g-oxide)^-1.