The acupuncture effect of carbon nanotubes induced by the volume expansion of silicon-based anodes

The cyclic instability of Si-based anodes can be effectively alleviated by adding carbon nanotube (CNT) networks. However, the ion diffusion and electrochemical performance vary significantly depending on the type of CNTs added, particularly single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs), and the intrinsic mechanism remains unknown. Herein, we revealed that the large volume expansion of Si-based anodes leads to the acupuncture effect of short CNTs, with the compressive stress on the CNTs and the Li-ion (Li+) diffusion energy barriers in the solid electrolyte interphase (SEI) exhibiting a linear correlation. Both the SEI and carbon-coating are penetrated by short, thick CNTs with gigapascal (GPa)-scale compressive stress, thereby accelerating electrolyte decomposition and leading to a LiF-rich SEI and an increased Li+ diffusion barrier. In contrast, long, slender CNTs exhibit limited compressive stress, thus minimizing the acupuncture effect, and the formed SEI possesses a low energy barrier for smooth Li+ diffusion. Thus, long, slender CNTs are ideal for Si-based anodes. This work reveals the structure-property relationships among compressive stress, SEI components and Li+ diffusion energy barriers, providing a novel perspective on the development of high-performance electrodes. The acupuncture effect of CNTs induced by the volume expansion of silicon-based anodes: neither the SEI nor the carbon coating can withstand the GPa-level compressive stress on CNTs.