Enhanced electrochemical performances of anatase TiO₂ nanotubes by synergetic doping of Ni and N for sodium-ion batteries

The main challenge of using TiO₂ as anode for sodium-ion batteries (SIBs) is its inherent low electrical conductivity, which leads to its unsatisfied capacity, short cycle life and low initial Coulombic efficiency. Herein, we report that synergistically doped anatase TiO₂ nanotubes with Ni and N via a simple sol-gel process, subsequent alkali-thermal reaction, and final thermal treatment in NH₃ can optimize the relationship among the phase structure, electrical conductivity and sodium-ion diffusion kinetic performance of anatase TiO₂ to enhance its sodium-ion storage and transport performances. The resultant Ni and N co-doped anatase TiO₂ nanotubes (Ni-N/TNTs) exhibit a high charge capacity of 303 mA h g⁻¹ after 500 cycles at a current density of 50 mA g⁻¹ with an initial Coulombic efficiency of 65% and even at a high current density of 5 A g⁻¹, a capacity of 143 mA h g⁻¹ is maintained after 8000 cycles with a capacity retention of ≥100%. The bandgap estimation, phase structure evolution, and electrochemical impedance spectroscopy analysis combined with the electrochemical test results indicate that the significantly improved sodium-ion storage and transport performances of Ni-N/TNTs should be mainly ascribed to the increased electrical conductivity, stable phase structure during the electrochemical processes, lower charge transfer resistance, and enhanced sodium-ion diffusion coefficient after Ni and N co-doping. The high capacity, improved Coulombic efficiency, excellent rate performance and long cycle life enable Ni-N/TNTs to be an applicable anode material for SIBs.