Reversible and selective ion intercalation through the top surface of few-layer MoS₂

Electrochemical intercalation of ions into the van der Waals gap of two-dimensional (2D) layered materials is a promising low-temperature synthesis strategy to tune their physical and chemical properties. It is widely believed that ions prefer intercalation into the van der Waals gap through the edges of the 2D flake, which generally causes wrinkling and distortion. Here we demonstrate that the ions can also intercalate through the top surface of few-layer MoS₂ and this type of intercalation is more reversible and stable compared to the intercalation through the edges. Density functional theory calculations show that this intercalation is enabled by the existence of natural defects in exfoliated MoS₂ flakes. Furthermore, we reveal that sealed-edge MoS₂ allows intercalation of small alkali metal ions (e.g., Li⁺ and Na⁺) and rejects large ions (e.g., K⁺). These findings imply potential applications in developing functional 2D-material-based devices with high tunability and ion selectivity.