Direct Observation of Electrochemical Lithium–Sulfur Reaction inside Carbon Nanotubes

An ongoing challenge for next-generation energy storage systems is to maximize the battery performance of lithium–sulfur (Li–S) systems, which exhibit high theoretical capacity and high energy density. Despite the outstanding effects observed by nanoconfinement of sulfur within conductive porous media, few studies have elucidated the ideal nanospace for the Li–S reaction because nanoscale characterization of lithiated sulfur molecules is difficult. We present direct evidence of electrochemically lithiated sulfur molecules confined inside carbon nanotubes (CNTs) using Cs-corrected high-resolution scanning transmission electron microscopy with electron energy loss spectroscopy. For a certain diameter of CNTs, short sulfur chains were stabilized inside CNTs via the charge transfer interaction, exhibiting a unique electrochemical activity and stable cycle performance compared to those of long sulfur chains. Our findings reveal that optimal CNTs have the one-dimensional channels for smooth progress of the lithiation reaction.