Superior sodium-storage behavior of flexible anatase TiO₂ promoted by oxygen vacancies

The miniaturization and rapid development of portable and wearable electronics require significant advances in new energy storage devices with high capacities and excellent mechanical properties. Along with flexible Li-ion batteries (LIBs), flexible Na-ion batteries (SIBs) are among the most promising power devices for portable and wearable electronics. Unfortunately, advances in the field of flexible SIBs have been relatively stagnant. In this work, we created a flexible anode for SIBs by applying a simple ex situ electrospinning method to introduce oxygen vacancies (Vo) into a TiO₂ structure (F–TiO_2-x). Thanks to the oxygen vacancies and inclusion of N-doped carbon nanofibers (CNFs), the F–TiO_2-x electrode can deliver an ultrahigh reversible capacity of 353 mAh g⁻¹ at 0.1C. Even at a relatively high current rate of 1C, a charge capacity of 331 mAh g⁻¹ can still be obtained after 1000 cycles. Furthermore, a sodium-ion full cell with F–TiO_2-x—Na_2/3Ni_1/3Mn_2/3O₂ full cell was assembled, delivering a high average voltage of 2.6 V (after 200 cycles, a reversible capacity of 158 mAh g⁻¹ can be retained). Combining both experimental and first-principles density functional theory (DFT) methods, we show that the introduction of oxygen vacancies can alter the electronic structure and enhance the electronic conductivity of the electrode. Additionally, the wrapped N-doped CNFs can further enhance the sodium diffusion kinetics of sodium storage. Overall, the study of preparing an oxygen vacancy-containing flexible electrode is expected to offer new insights into fabricating high capacity flexible electrodes for advanced secondary batteries including SIBs.