Tunnel-Structured K_xTiO₂ Nanorods by in Situ Carbothermal Reduction as a Long Cycle and High Rate Anode for Sodium-Ion Batteries

The low electronic conductivity and the sluggish sodium-ion diffusion in the compact crystal structure of Ti-based anodes seriously restrict their development in sodium-ion batteries. In this study, a new hollandite KxTiO2 with large (2 × 2) tunnels is synthesized by a facile carbothermal reduction method, and its sodium storage performance is investigated. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses illustrate the formation mechanism of the hollandite KxTiO2 upon the carbothermal reduction process. Compared to the traditional layered or small (1 × 1) tunnel-type Ti-based materials, the hollandite KxTiO2 with large (2 × 2) tunnels may accommodate more sodium ions and facilitate the Na+ diffusion in the structure; thus, it is expected to get a large capacity and realize high rate capability. The synthesized KxTiO2 with large (2 × 2) tunnels shows a stable reversible capacity of 131 mAh g–1 (nearly 3 times of (1 × 1) tunnel-structured Na2Ti6O13) and superior cycling stability with no obvious capacity decay even after 1000 cycles, which is significantly better than the traditional layered Na2Ti3O7 (only 40% of capacity retention in 20 cycles). Moreover, the carbothermal process can naturally introduce oxygen vacancy and low-valent titanium as well as the surface carbon coating layer to the structure, which would greatly enhance the electronic conductivity of KxTiO2 and thus endow this material high rate capability. With a good rate capability and long cyclability, this hollandite KxTiO2 can serve as a new promising anode material for room-temperature long-life sodium-ion batteries for large-scale energy storage systems, and the carbothermal reduction method is believed to be an effective and facile way to develop novel Ti-based anodes with simultaneous carbon coating and Ti(III) self-doping.