Tunable oxygen defect density and location for enhancement of energy storage

Defect engineering is in the limelight for the fabrication of electrochemical energy storage devices. However, determining the influence of the defect density and location on the electrochemical behavior remains challenging. Herein, self-organized TiO₂ nanotube arrays (TNTAs) are synthesized by anodization, and their oxygen defect location and density are tuned by a controllable post-annealing process. TNTAs annealed at 600 °C in N₂ exhibit the highest capacity (289.2 mAh g⁻¹ at 0.8 C) for lithium-ion storage, while those annealed at 900 °C in N₂ show a specific capacitance of 35.6 mF cm⁻² and stability above 96% after 10,000 cycles for supercapacitor. Ex situ electron paramagnetic resonance spectra show that the surface-exposed oxygen defects increase, but the bulk embedded oxygen defects decrease with increasing annealing temperature. Density functional theory simulations reveal that a higher density of bulk oxygen defects corresponds to higher localized electrons states, which upshift the Fermi level and facilitate the lithium intercalation kinetic process. Meanwhile, differential charge density calculation indicates that the increase of surface oxygen defects in the anatase (101) plane leads to higher density excess electrons, which act as negative charge centers to enhance the surface potential for ion adsorption. This oxygen-deficient location and density tunable strategy introduce new opportunities for high-energy and high-power-density energy storage systems.