Vanadium Oxides as Host Materials for Lithium and Sodium Intercalation.
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VANADIUM OXIDES AS HOST MATERIALS FOR LITHIUM AND SODIUM INTERCALATION. K. WEST*, B. ZACHAU-CHRISTIANSEN*, T. JACOBSEN*, and S. SKAARUP** * Institute of Physical Chemistry, **
Physics Laboratory III
The Technical University of Denmark, DK-2800 Lyngby, Denmark.
ABSTRACT
A number of vanadium oxides can function as host materials for the insertion of lithium and sodium. These materials are of interest as they can be utilized as electrodes in high capacity secondary batteries. Although most of these systems are not thermodynamically stable over the entire composition interval spanned by the inserted ion, the kinetic stability of the vanadiumoxygen lattice is sufficient to ensure reversible operation at temperatures below 200 300°C. However, after transgression of the reversible composition interval some electrode materials exhibit characteristic changes in the voltage vs. composition curves. Depending on the guest/host system the phase formed can be either a new crystalline host phase, or an amorphous, highly defect material characterized by a smooth emf curve. The materials studied were predominantly of two-dimensional nature: Vanadium pentoxide and its molybdenum substituted analogue, which have layerlike structures due to some of the V--O bonds being long and weak, and a series of layered trivanadates. Even in their highest oxidation state the latter host structures contain an amount of interlayer alkali metal ions, balancing the otherwise strong electrostatic repulsion between adjacent oxygen layers. INTRODUCTION
The term intercalation is in chemistry used to denote the reversible insertion of mobile guest species into a rigid host structure. In this sense intercalation is a special case of a topotactic reaction in which a clear structural relationship between the starting phase and the product phase persists. The characteristic property of intercalation reactions is that no strong bonds in the host structure are broken during the insertion of guest species, and this property is the basis of the reversibility of these reactions: On removal of the guest species, the host structure ideally reverts to its original state, even on a molecular level. The intercalation of metal ions can conveniently be carried out inan electrochemical cell with one electrode acting as the source of metal ions and with the intercalation material in question as the other electrode. In such a cell it is possible to change the composition of the intercalation electrode by control of the cell current or potential. This opens a range of applications for these materials in electrochemical energy storage, displays, sensors and heterogeneous catalysis [1]. A number of vanadium oxides are considered as candidates for electrodes in high energy density lithium batteries [2,31. These materials offer high voltages and the Mat. Res. Soc. Symp. Proc. Vol. 210. ©1991 Materials Research Society
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composition intervals accessible for lithium intercalation are wide - in several cases more than one lithium per vanadium atom can be reversibly inserte
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