Three-Dimensional Fast Na-Ion Transport in Sodium Titanate Nanoarchitectures via Engineering of Oxygen Vacancies and Bismuth Substitution

Layered sodium titanates (NTO), one of the most promising anode materials for advanced sodiumion batteries (SIBs), feature high theoretical capacity and no serious safety concerns. The pristine NTO electrode, however, has unfavorable Na+ transport kinetics, due to the dominant two-dimensional (2D) Na-ion transport channels within the crystal along the low energy barrier octahedron layers, which impedes the practical application of this class of potential materials. Herein, an interesting concept of opening three-dimensional (3D) fast ion transport channels within the intrinsic NTO frameworks is proposed to enhance the electrochemical performance through a combination of oxygen vacancy generation and cation substitution strategies, by which the interlayer spacing of the NTO frameworks is expanded for fast 3D Na-ion transport. It is evidenced that the oxygen-deficient and bismuth-substituted HBNTO (BixNa2-xTi3Oy, 0 < x < 2, 0 < y < 7, HBNTO) exhibits obvious enhancements on the reversible capacity (similar to 145% enhancement at 20 mAh g(-1) compared with NTO), the rate capability (similar to 200% enhancement at 500 mAh g(-1) compared with NTO), and the cycling stability (similar to 210% enhancement of retention capacity after 150 cycles at 20 mAh g(-1) compared with NTO). The molecular dynamic simulations and theoretical calculations demonstrate that the enhanced performance of HBNTO is contributed by the multiplied sodium diffusion pathways and the increased ion migration rates with the successful opening of 3D internal ion transport channels. This work demonstrates the effectiveness of the strategies in opening the 3D intercrystal ion transport channels for boosting the electrochemical performance of SIBs.

Automated summary

This study proposes a novel concept of enhancing the electrochemical performance by opening three-dimensional fast ion transport channels within the NTO frameworks, achieved through a combination of oxygen vacancy generation and cation substitution strategies. The oxygen-deficient and bismuth-substituted HBNTO exhibits significant enhancements in reversible capacity, rate capability, and cycling stability, attributed to the successful opening of 3D internal ion transport channels.