10.1021/nn500194q.s005 Qingmei Su Qingmei Su Gaohui Du Gaohui Du Jun Zhang Jun Zhang Yijun Zhong Yijun Zhong Bingshe Xu Bingshe Xu Yuehai Yang Yuehai Yang Suman Neupane Suman Neupane Wenzhi Li Wenzhi Li <i>In Situ</i> Transmission Electron Microscopy Observation of Electrochemical Sodiation of Individual Co<sub>9</sub>S<sub>8</sub>‑Filled Carbon Nanotubes American Chemical Society 2014 Situ Transmission Electron Microscopy Observation reaction front propagates Co 9S CNT microstructure evolution Na 2S matrix transmission electron microscopy sodiated Co 9S filler electrochemical sodiation process Individual Co 9S Carbon NanotubesThe comprehension Co 9S carbon nanotubes carbon shells increases 2014-04-22 00:00:00 Media https://acs.figshare.com/articles/media/_i_In_Situ_i_Transmission_Electron_Microscopy_Observation_of_Electrochemical_Sodiation_of_Individual_Co_sub_9_sub_S_sub_8_sub_Filled_Carbon_Nanotubes/2304124 The comprehension of fundamental electrochemical behavior and sodiation mechanism is critical for the design of high-performance electrode materials for sodium-ion (Na-ion) batteries. In this paper, the electrochemical sodiation process and microstructure evolution of individual Co<sub>9</sub>S<sub>8</sub>-filled carbon nanotubes (CNTs) have been directly visualized and studied using <i>in situ</i> transmission electron microscopy. Upon the first sodiation, a reaction front propagates progressively along the filling nanowire, causing the filled CNT to inflate. The filled CNTs behave differently depending on their structures and the magnitude of the sodiation voltage. For a Co<sub>9</sub>S<sub>8</sub>-filled CNT with an open end, the sodiated Co<sub>9</sub>S<sub>8</sub> filler shows a substantial axial elongation of 120.8% and a small radial swelling due to the extrusion of CNT walls. In contrast, the closed CNT shows a major radial expansion of 40.6% and a small axial elongation because of the mechanical confinement of the carbon shells. After sodiation, the spacing between the carbon shells increases from 3.4 to 3.8 Å due to the Na<sup>+</sup>-ion insertion and the single-crystalline Co<sub>9</sub>S<sub>8</sub> filler converts to numerous Co nanograins dispersed in a Na<sub>2</sub>S matrix. Compared with the gentle microstructure evolution of the CNT under small charging voltage, a strong electrochemical reaction accompanying drastic swelling and fracturing of CNT shells is observed for the CNT electrode under large charging voltage. Our observations provide direct evidence and important insights for the electrochemical process of CNT-based composite materials in Na-ion batteries.