THE MECHANISM OF ELECTROINTERCALATION

The general mechanism of electrochemical intercalation of atoms into solid hosts is investigated. Such intercalation may be regarded as the solid-state analogue of mercury amalgam formation. The process is also similar to the electrochemical doping of polymers such as polyacetylene. The electrointercalation of lithium into both the layered and the cubic polymorphs of TiS2 was chosen as a model system. It is shown that the electrointercalation process may be described in terms of an adatom model in which a lithium ion in solution adjacent to the electrode becomes partially desolvated and adsorbed onto the surface of the TiS2 electrode; this process is accompanied by the insertion of an electron into the conduction band of the solid. Subsequently, the partially solvated Li+ ion diffuses across the surface to an intercalation site, where it becomes fully desolvated and enters the lattice. Ac impedance studies on the cubic and layered LixTiS2 electrode indicate that the diffusion of adatoms across the surface is very fast and that lattice incorporation is the major step limiting the rate of electrointercalation. The charge-transfer step appears to be almost potential independent.