Intercalation

Intercalation is a process in which guest species—atoms, ions, or molecules—are inserted between layers of a host material, typically a layered solid such as graphite, metal dichalcogenides, or clays. The insertion occurs without converting the host to a fundamentally different structure; the layers may separate or slide, increasing the interlayer spacing, and the guest species often reside in regular arrangements or stages.

Common hosts include graphite and other two‑dimensional layered materials like MoS2, TiS2, and other transition metal dichalcogenides, as well as layered silicates such as montmorillonite and vermiculite. Intercalation can be reversible, allowing the guest species to be inserted and subsequently removed, as in battery cycling, or more permanent, as in some dye intercalation compounds.

Mechanisms involve diffusion of guest species into galleries between layers, sometimes accompanied by charge transfer between guest and host. In graphite intercalation compounds, guests form staged structures where every nth layer is occupied. Intercalation can alter electronic, optical, or mechanical properties of the host.

Applications are broad. In energy storage, intercalation is central to lithium- and sodium-ion battery electrodes, as well as supercapacitors using reversible ion intercalation. In electrochromic devices, intercalation of ions changes optical properties. In biology and medicine, DNA can be intercalated by planar aromatic drugs and dyes, which slide between base pairs and disrupt replication or transcription.

Methods to achieve intercalation include electrochemical intercalation, chemical or redox-driven insertion, and thermal or solvational approaches. The field has a long history; intercalation compounds have been studied since the 19th century and remain important for materials science and energy research.