Intercalants

Intercalants are atoms, ions, or small molecules that insert into the layered structure of another material, known as the host, without breaking its fundamental framework. The insertion typically occurs in the spaces between layers, such as the van der Waals gaps of graphite or the galleries of transition metal dichalcogenides and clays.

Common host materials include graphite, layered transition metal dichalcogenides (such as MoS2 and TiSe2), and other layered oxides or clays. Intercalants range from alkali and alkaline earth metals (for example Li+, Na+, K+), halogens, and metal chlorides to neutral molecules and solvent species. The intercalation process can involve redox reactions and partial charge transfer between host and guest.

Intercalation can be reversible, allowing the intercalant to be inserted and removed while the host lattice remains intact. In graphite, for example, lithium intercalants form compounds like LiC6 and alter electronic properties; in some TMDCs, intercalation can induce changes in conductivity or even superconductivity. Stage indexing describes the periodicity of intercalant layers, such as stage I or stage II.

Applications include energy storage (notably lithium and sodium ion batteries), electrochromic devices, gas storage, and catalysis. Intercalation is typically achieved by electrochemical methods (as in batteries), chemical intercalation, or vapor-phase techniques, each affecting diffusion and kinetics differently.

Challenges for intercalation compounds involve stability and cycling durability, as guest–host interactions, diffusion barriers, and lattice strain influence performance. The study of intercalants spans chemistry, physics, and materials science, linking fundamental bonding with practical functionality.