Oxygen vacancies and phase tuning of self-supported black TiO2-X nanotube arrays for enhanced sodium storage.

• Defect and phase are tuned for the sodium storage of TiO 2-X nanotube arrays. • The oxygen vacancy and phase of TiO 2-X nanotube arrays show unique evolution rule. • The black TiO 2-X nanotube arrays show superior sodium storage performance. • The sodium storage mechanism is clarified by CV analysis and DFT calculation. Defect engineering and phase engineering are two powerful strategies for regulating the intrinsically sluggish electronic and ionic kinetics of TiO 2 to enhance sodium storage. However, synergetic control of the defects and phases in TiO 2 nanomaterials remains a major challenge. Herein, we clarify the phase and defect synergetic evolution regularities of self-supported TiO 2-X nanotube arrays (TNTAs) with different annealing processes, which amazingly demonstrate different characteristics from traditional TiO 2 nanoparticles. The phase composition and oxygen vacancy (OV) concentration tuning strategies are combined to systematically clarify how these two important synergetic factors influence the sodium storage properties of TNTAs. In particular, unique porous-wall black amorphous TNTAs annealed at 300 ℃ in air and core-shell black anatase TNTAs annealed at 600 ℃ in N 2 show the best sodium storage performance among all amorphous and anatase samples, respectively. Cyclic voltammetry (CV) kinetics analysis demonstrates that both pseudocapacitive and diffusion-controlled charge storage mechanism exist in the TNTAs. The correlations of the pseudocapacitive capacity and the diffusion-controlled capacity with the OV concentrations and phases are clarified in detail. Both the pseudocapacitive capacity and diffusion-controlled capacity are enhanced in samples with high OV concentration in the amorphous and anatase TNTAs. Density functional theory (DFT) calculations further indicate that a high OV density corresponds to a low sodium insertion energy and high capacity, which matches the experimental results. [ABSTRACT FROM AUTHOR]