Intercalation compounds: Some recent developments and future trends in mixed conductors

Abstract

Intercalation compounds are formed by the injection of guest ions (e.g. Li⁺) and their charge compensating electrons into a solid host (e.g. TiS₂). Such compounds are mixed, ionic and electronic conductors.

Recent developments concerning three aspects of intercalation are discussed in this article.

(1) Ionic diffusion. Results are presented for the self-difusion coefficient of Li⁺ in both layered and cubic titanium disulphide which permits comparison of Li⁺ diffusion in two and three dimensions within a chemically identical environment. In both cases the activation energy and pre-exponential factor for self-diffusion exhibit a dramatic variation with Li⁺ ion content. These variations and possible interpretations are discussed.

(2) Thermodynamics of intercalation. The variation in the chemical potential of lithium in cubic TiS₂ is presented and interpreted in terms of interactions between both the ions and electrons; the interactions follow a cube root dependence on composition.

(3) New intercalation compounds. New sodium intercalation compounds are described and the future prospects for other monovalent and divalent cation intercalation compounds discussed.