Na-ion batteries have attracted enormous interest due to its low cost and the abundance of sodium in the earth, holding the potential to become the technology of choice for large-scale electrochemical energy storage. However, the investigation of suitable anode materials for Na-ion batteries is still in its infancy and few electrode materials meet the performance requirement for practical applications. Black phosphorous (BP), which possesses layered structure, is a promising anode material for sodium ion batteries due to its extremely high theoretical capacity (2596 mAh/g) and good conductivity. However, there are few studies on the electrochemical reaction between BP and sodium.^1,2 Here, we explored the structural evolution of BP during the sodiation reaction by employing a state-of-the art in situ (scanning) transmission electron microscopy electrochemical testing technique.^3,4 Our in situ TEM study indicates that the sodium ion transport pathway is highly orientation dependent inside the BP. An anisotropic expansion and multiple phase transitions  are also observed during the sodiation process.

[1] Park, C. M.; Sohn, H. J. Adv. Mater. 2007, 19, 2465-2468

[2] Sun, J.; Lee, H.-W.; Pasta, M.; Yuan, H.; Zheng, G.; Sun, Y.; Li, Y.; Cui, Y. Nat. Nanotechnol. 2015, 10, 980-985.

[3] Nie, A.; Cheng, Y. C.; Zhu, Y. H.; Asayesh-Ardakani, H.; Tao, R. Z.; Mashayek, F.; Han, Y.; Schwingenschlogl, U.; Klie, R. F.; Vaddiraju, S. Nano Lett. 2014, 14, 5301-5307.

[4] Nie, A.; Cheng, Y.; Zhu, Y.; Asayesh-Ardakani, H.; Tao, R.; Mashayek, F.; Han, Y.; Schwingenschlögl, U.; Klie, R. F.; Vaddiraju, S.; Shahbazian-Yassar, Reza. Nano let. 2014, 14, 5301-5307