Superimposed Effect of La Doping and Structural Engineering to Achieve Oxygen-Deficient TiNb<sub>2</sub>O<sub>7</sub> for Ultrafast Li-Ion Storage
Kangdong Tian, Zhongxiao Wang, Haoxiang Di, Haoyu Wang, Zhiwei Zhang, Shoubao Zhang, Rutao Wang, Luyuan Zhang, Chengxiang Wang, Longwei Yin
Abstract
TiNb2O7 (TNO) is a competitive candidate of a fast-charging anode due to its high specific capacity. However, the insulator nature seriously hinders its rate performance. Herein, the La3+-doped mesoporous TiNb2O7 materials (La–M–TNO) were first synthesized via a facile one-step solvothermal method with the assistance of polyvinyl pyrrolidone (PVP). The synergic effect of La3+ doping and the mesoporous structure enables a dual improvement on the electronic conductivity and ionic diffusion coefficient, which delivers an impressive specific capacity of 213 mAh g–1 at 30 C. The capacity retention (@30C/@1C) increases from 33 to 53 and 74% for TNO, M–TNO, and La–M–TNO (0.03), respectively, demonstrating a step-by-step improvement of rate performance by making porous structures and intrinsic conductivity enhancement. DFT calculations verify that the enhancement in electronic conductivity due to La3+ doping and oxygen vacancy, which induce localized energy levels via slight hybridization of O 2p, Ti 3d, and Nb 4d orbits. Meanwhile, the GITT result indicates that PVP-induced self-assembly of TNO accelerates the lithium ion diffusion rate by shortening the Li+ diffusion path. This work verifies the effectiveness of the porous structure and highlights the significance of electronic conductivity to rate performance, especially at >30C. It provides a general approach to low-conductivity electrode materials for fast Li-ion storage.