Carbon-bonded, oxygen-deficient TiO₂ nanotubes with hybridized phases for superior Na-ion storage

TiO₂ shows great potential as anode materials for sodium-ion batteries (SIBs). However, its practical application has been deferred by the sluggish electronic/ionic transport. In this work, we report the controlled synthesis of ultrathin, carbon-bonded TiO₂ nanotubes with oxygen vacancies (V_o) and hybridized amorphous/TiO₂(B) phases via a hydrothermal reaction and heat-treatment. The introduction of V_o and carbon in TiO₂ by C-Ti bonding effectively boosts its electron transport. Meantime, the ultrathin TiO₂ nanotubes (with diameter of ∼10 nm and tube thickness of ∼3 nm) enable a large electrode/electrolyte contact interface with shortened Na⁺ diffusion distance. In addition, the formed coherent amorphous/TiO₂(B) junctions further promote the charge transport and transfer at heterointerface. These synergic effects endow the resultant TiO₂ material with superior Na⁺ storage capability in terms of high capacity (191 mA h/g at 0.2 C) and rate property (141 mA h/g at 10 C). Kinetics analysis further discloses a pseudocapacitive Na⁺ storage exerts a significant contribution to the total capacity. The proposed strategy based on synergic engineering of vacancy defects, chemical bonding and phase composition can pave the way for exploration of novel electrode materials for beyond lithium-ion batteries.