Defect-Assisted Selective Surface Phosphorus Doping to Enhance Rate Capability of Titanium Dioxide for Sodium Ion Batteries

Phosphorus doping is an effective strategy to simultaneously improve the electronic conductivity and regulate the ionic diffusion kinetics of TiO₂ being considered as anode materials for sodium ion batteries. However, efficient phosphorus doping at high concentration in well-crystallized TiO₂ nanoparticles is still a big challenge. Herein, we propose a defect-assisted phosphorus doping strategy to selectively engineer the surface structure of TiO₂ nanoparticles. The reduced TiO_2-x shell layer that is rich in oxygen defects and Ti³⁺ species precisely triggered a high concentration of phosphorus doping (∼7.8 at. %), and consequently a TiO₂@TiO_2-x-P core@shell architecture was produced. Comprehensive characterizations and first-principle calculations proved that the surface-functionalized TiO_2-x-P thin layer endowed the TiO₂@TiO_2-x-P with substantially enhanced electronic conductivity and accelerated Na ion transportation, resulting in great rate capability (167 mA h g^-1 at 10 000 mA g^-1) and stable cycling (99% after 5000 cycles at 10 A g^-1). Combining in/ex situ X-ray diffraction with ex situ electron spin resonance clearly demonstrated the high reversibility and robust mechanical behavior of TiO₂@TiO_2-x-P upon long-term cycling. This work provides an interesting and effective strategy for precise heteroatoms doping to improve the electrochemical performance of nanoparticles.