Revealing Electrochemical Sodiation Mechanism of Orthogonal-Nb2O5 Nanosheets by In Situ Transmission Electron Microscopy

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.

Automated summary

With the development of clean energy sources, large-scale energy storage technologies have become important. Sodium ion batteries, as a potential storage solution, require good electrode materials. Orthogonal-phase niobium oxide has shown good performance, but its poor electrical conductivity has been a limitation. Recent studies have used carbon materials or metal conductive particles to improve its performance. In this study, in situ TEM was used to investigate the diffusion behavior of sodium ions in nanosheets and observed defects and a modulation structure.