Unveiling the Na-ions storage mechanism and sodiation-induced brittleness of multiwalled carbon nanotubes

Although the multiwalled carbon nanotubes (MWCNTs) are considered as ideal conductive network or buffer materials in the electrode materials due to their good mechanical property and high conductivity, their practical electrochemical performance and mechanical behavior in the sodium-ion batteries (SIBs) are still unclear. In this work, the sodiation/desodiation mechanism of MWCNTs and their mechanical properties are revealed by in situ transmission electron microscopy (TEM). It is found the solid electrolyte interface layer easily grow on the surface of sodiated MWCNT, and the fish scale-like sodium dendrites appear on the surface with further sodiation. There is an unconventional decrease in the width of MWCNT during the sodiation process, which can be due to the collapse of the crystal lattices of MWCNT caused by the intercalation of Na-ions and the loss of carbon caused by the Joule heating effect. In situ bending experiments show that the pristine MWCNTs show excellent mechanical property, while the MWCNTs after electrochemical process break easily even without external force. It is determined that the sodiation process aggravates the brittleness of MWCNTs. Our detailed in situ results provide direct evidence and understanding on the degradation mechanism of MWCNTs as electrode materials in SIBs.