Mixed-conducting host structures into which either cations or anions can be inserted

It has been known for some 25 years that charge can be stored in some impor tant battery electrodes by the insertion of ionic species from the electrol yte. Insertion reactions play an especially important role in current versi ons of lithium batteries, where lithium cations are typically the inserted species in both electrodes. Hydrogen cations (protons) are the guest specie s during the operation of both the Ni(OH)(2)/NiOOH electrode and the "MnO2" electrode in aqueous systems. Whereas most attention has been given to materials in which the guest speci es are cations, it is also possible to have anion insertion into some cryst al structures. Materials in which the structure can accommodate either cati ons or anions are especially interesting. The hexacyanometallates, with variations of the cubic ReO3 type structure w ith rather large intercell windows, represent an interesting example. They can accommodate a wide variety of guest ions of both charges. Cations can b e inserted into the structure at relatively low potentials, and anions can be inserted at more positive potentials. A number of interesting features a nd properties of this family of materials are discussed. (C) 1998 published by Elsevier Science B.V. All rights reserved.