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.