Efficient Surface Modulation of Single-Crystalline Na₂Ti₃O₇ Nanotube Arrays with Ti³⁺ Self-Doping toward Superior Sodium Storage

Although Na₂Ti₃O₇-based anodes have been widely investigated in sodium-ion batteries (SIBs), their Na⁺ storage properties especially high-rate capability and long-term cycling durability are far from practical application, because of their intrinsic low conductivity and unsatisfied Na⁺ diffusion resistance. Here, we report the surface engineering of Na₂Ti₃O₇ nanotube arrays grown in situ on Ti foil through a hydrothermal method and subsequent NH₃-assisted calcination. Benefiting from the effective surface modification, the as-derived free-standing electrode possesses highly crystalline surface with favorable Na⁺ diffusion kinetics and self-incorporation of abundant Ti³⁺ for improved electronic conductivity. These features enable the electrode to achieve remarkable reversible capacity (237.9 mAh g^–1), ultra-high rate capability (88.5 mAh g^–1 at 100 C = 17.7 A g^–1), and excellent cycling stability (92.32% capacity retention at 50 C after 5000 cycles), which are superior to the counterpart without surface modification, as well as almost all Na₂Ti₃O₇-based anode materials reported so far for SIBs. The outstanding electrochemical performance demonstrates the feasibility of proposed surface modulation in designing more efficient electrode materials for energy storage.