Intercalant

An intercalant is a species, such as an atom, ion, or molecule, that inserts between the layers of a layered host material to form an intercalation compound. The guest species reside in the galleries between adjacent layers and can cause expansion of the interlayer spacing, charge transfer, or changes to the host’s electronic, optical, and catalytic properties.

Common hosts include graphite, transition metal dichalcogenides (for example MoS2 and WS2), layered oxides, and layered clays. Intercalants may be charged ions (such as Li+, Na+, K+), neutral molecules (such as water or ammonia), or larger organic or inorganic species. In graphite intercalation compounds, staging describes how many graphene layers separate consecutive intercalant layers, a concept important to understanding their structure and properties.

Intercalation processes can be electrochemical, chemical, or thermal. In electrochemical systems, such as lithium-ion batteries, Li+ ions insert into graphite during charging and retract during discharging, accompanied by electron transfer within the host. Intercalation can also modify conductivity, magnetism, catalytic activity, and optical behavior, enabling applications in energy storage, electrochromic devices, sensors, and catalysis. Some intercalated materials exhibit emergent phenomena, including superconductivity in certain graphite intercalation compounds synthesized with alkali or other species.

Synthesis methods include electrochemical intercalation, chemical intercalation by reaction with reactive species, and gas- or solution-phase approaches that insert guests into pre-formed layered hosts. The suitability of an intercalant depends on the compatibility with the host framework and the desired property changes.