Intercalation Chemistry

Abstract The term ‘intercalation’ refers to a process whereby a guest molecule or ion is inserted into a host lattice. The structure of the guest–host or intercalation compound is only slightly perturbed from the host structure and the reaction used to form the compound is reversible. In this chapter, the major classes of host lattice are classified by both dimensionality and electronic properties. In three‐dimensional systems, the sizes of the guest species are constrained by the dimensions of the host lattice. In lower dimensional systems, no such restriction exists and the strongly bonded layers or chains can adjust their separation freely to accommodate guest species of different sizes. With increasing guest size, the structural correlation between layers or chains is diminished and in the limiting case, complete separation of single layers or chains occurs in solution, resulting in the formation of colloidal dispersions. Insulating host lattices undergo reactions that involve ion exchange, acid–base chemistry, solvent exchange, and sorption of neutral molecules from the gas phase. The insulating hosts have a fixed concentration of ionic guests that remains unchanged throughout subsequent reactions. The conducting host lattices have the feature that the concentration of ionic guest species can be altered by oxidation or reduction of the host. The chemistry of most compounds involves reduction and cation intercalation with the major exceptions of graphite and some layered oxides, which can be both reductively intercalated with cations and oxidized with anion insertion. The present understanding of the mechanism of intercalation with particular emphasis on layered structures is reviewed. Reactions involve adsorption of guest species on host crystals, exchange or insertion at the host surface, the formation of intermediate stages in layered compounds, and transport within the host lattice. Macroscopic effects such as variations in crystal size, dislocations, stacking faults, and pore mouth blockage can strongly influence the reaction kinetics. The major classes of host lattices in each category and their chemical behavior are described. The insulating materials discussed include zeolites, clays, layered double hydroxides, acid phosphates, and layered oxides while graphite, C 60 , carbon nanotubes, transition metal chalcogenides, metal phosphorus trisulfides, metal oxyhalides, metal nitride halides, and metal oxides are the major examples of conducting hosts. In the case of metal oxides, examples of both reductive intercalation of lithium ions and oxidative intercalation of oxide ions are discussed. These two types of intercalation reactions are important for lithium batteries and high‐temperature superconductivity, respectively.