由于正交相五氧化二铌(T-Nb2O5)为ReO3型层状结构，锂、钠离子可以在其(001)平面快速脱嵌，而在001]方向的传输一般较难。本研究通过原位透射电子显微镜(Transmission Electron Microscope，TEM)方法研究钠在T-Nb2O5纳米片(001)面内及001]方向的钠离子电化学嵌入行为，发现由于纳米片晶体存在大量的位错和畴界，钠离子可通过这些缺陷穿越(001)面扩散，并进而在深层的(001)面内快速扩散。同时，本研究还发现刚合成的T-Nb2O5纳米片在001]方向上存在调制结构，存在交替分布的压应变和张应变区域，而钠离子的嵌入可以调节这些应变分布。
1. School of Nano Technology and Nano Bionics, University of Science and Technology of China, Hefei 230031, China;2. Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, Jiangsu Province, China;3. Department of Physics, Tsinghua University, Beijing 100084, China
Abstract:
With the development of clean energy sources such as solar and wind power, large-scale energy storage technologies will play a significant role in the rational utilization of clean energy. Sodium ion batteries have garnered considerable attention for large-scale energy storage owing to their low cost and the presence of abundant sodium resources. It is particularly crucial to develop electrode materials for sodium battery with good rate capability and long cycle life. Orthogonal-phase niobium oxide (T-Nb2O5) exhibits good potential to be used as anode material for sodium-ion batteries owing to its high theoretical specific capacity (200 mAh·g−1) and high ionic diffusion coefficient. Furthermore, it demonstrates a better performance than that of graphite and exhibits a higher specific capacity than that of Li4TiO4 when used in sodium-ion batteries. However, its poor electrical conductivity has hindered its practical application. Recently, effective strategies such as coating with carbon materials or metal conductive particles have been developed to overcome this issue. Although the electrochemical performance of T-Nb2O5 has been improved, the sodiation mechanism of T-Nb2O5 is still unclear. It is considered to be similar to the lithium mechanism wherein lithium ions diffuse rapidly on the (001) planes, but exhibit difficulty in diffusing across the (001) planes. In this study, the electrochemical sodiation behaviors along the (001) lattice planes and the 001] direction of the T-Nb2O5 nanosheet are studied by in situ transmission electron microscopy (TEM). The results indicate that there are a large number of dislocations and domain boundaries in nanocrystals. Furthermore, it was observed that, sodium ions can diffuse across the (001) lattice planes through these defects, and then diffuse rapidly on the (001) planes. Meanwhile, we found a modulation structure in the 001] direction of the original nanosheet, in which alternating compressive and tensile strains were observed. These strain distributions can be regulated by the insertion of sodium ions, while the modulation structure is maintained. Moreover, the in situ TEM method used in this work can be applied to various energy materials.
