TiO_2–x Nanocages Anchored in N-Doped Carbon Fiber Films as a Flexible Anode for High-Energy Sodium-Ion Batteries

A novel flexible and freestanding anode material, consisting of oxygen-deficient TiO2–x nanocages anchored in N-doped carbon fibers (TiO2–x/NCFs), is fabricated for the first time by an electrospinning technology. As a rationally designed anode for sodium-ion batteries (SIBs), the resulting material not only features an advantageous hierarchical structure, especially ultrasmall TiO2 nanoparticles for a shortened Na+ diffusion path, hollow nanocages as a building block with enlarged surface area and adequate space to accommodate volume expansion, and a robust framework allowing for high mass loading of active materials, but also possesses improved conductivity originating from uniform N-doping and oxygen vacancies. Consequently, the hybrid TiO2–x/NCFs films exhibit a superior reversible capacity (210 mA h g–1 at 100 mA g–1), an excellent rate performance (120 mA h g–1 at 1000 mA g–1), a high areal capacity (0.10 mA h cm–2 at 1000 mA g–1), and an outstanding stability (98.7% capacity retention after 1000 cycles at 1000 mA g–1).