Oxygen Defect and Cl^--Doped Modulated TiNb₂O₇ Compound with High Rate Performance in Lithium-Ion Batteries

TiNb₂O₇ has attracted extensive attention from lithium-ion battery researchers due to its superior specific capacity and safety. However, its poor ion conductivity and electron conductivity hinder its further development. To improve the ion/electron transport of TiNb₂O₇, we report that chlorine doping and oxygen vacancy engineering regulate the energy band and crystal structure simultaneously through a simple solid-phase method. NH₄Cl was used to realize Cl^- doping and oxygen vacancy production. A Rietveld refinement demonstrates an effective substitution of Cl in the O sites of Nb-O octahedra, with an enlarged crystal plane spacing. The oxygen vacancies provide more active sites for lithium intercalation. The diffusion coefficient of Li⁺ is inceased from 2.39 × 10^-14 to 1.50 × 10^-13 cm² s^-1, which reveals the positive influence of Cl^- doping and oxygen vacancies on the promoted Li⁺ transport behavior. Charge compensation is introduced by the doping of Cl^- and the generation of oxygen vacancies, leading to the formation of Ti³⁺ and Nb⁴⁺ and the adjustment of the electronic structure. DFT calculations reveal that TiNb₂O₇ with Cl^- doping and an O vacancy shows a metallic property with a finite value at the Fermi level, which is conducive to electron transfer in the electrode material. Benefiting from these advantages, the modified TiNb₂O₇ presents superior rate performance with a commendable capacity of 172.82 mAh g^-1 at 50 C. This work provides guidance to design high-performance anode materials for high-rate lithium-ion batteries.