Intercalation in 3D-Skeleton Structures: Ionic and Electronic Features
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INTERCALATION IN 3D-SKELETON STRUCTURES : IONIC AND ELECTRONIC FEATURES
Paul HAGENMULLER and Claude DELMAS Laboratoire de Chimie du Solide du CNRS, Universit6 de Bordeaux I, cours de la Lib~ration, 33405 TALENCE Cedex, France
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ABSTRACT The voltage of an electrochemical cell, i.e. the difference between the chemical potentials of the two electrodes, may play the role of a sensor which allows to display the structural modifications and the physical properties. The electrochemical processes involved in an alkali metal (A) intercalation electrode emphasize the influence of the ionic and/or electronic features. The Ak-lattice and A+-A+ interactions as well as electronic band-filling may lead to phase transitions or even limit the intercalation reaction. The shape of the cell voltage vs. intercalation rate curve depends on the number of vacant sites available for intercalation, the number and the oxidation state of the reducible cations, the band structure of the material and the covalency of the framework. Alkali ion intercalation in 3D-structures related to perovskite (Ln iNbO ), hexagonal tungsten bronze (LiW 09F) and Nasicon-type (AM2 04)3) is discussed from that point of view. In LnlNbO = La,is Nd) (i.e. 3 12 /Ln1./ -'j/6NbO 3) Li+ intercalation "n v~rious sites related to the'rareleI~rth z Two extra lithium atoms can be introduced into LiW 0 F in which four sites are available, but only one out of two is occupied in order to reduce the electrostatic interactions. Moreover the change in the discharge curves can be associated to the modifications with intercalation rate of the Li+-lattice interactions. Within the Nasicon derived structures of ATi (PO ) and Fe (MoO4) the intercalation process is limited by the lowest stabAý oxidation state of titanium or iron. In both systems the strong electronic localization leads to formation of large two phase-domains. The relevance of using 3D-intercalation electrodes in electrochemical power batteries will be discussed as far factors such as electrical behavior or absence of significant unit cell modifications of the positive electrodes during the intercalation process are essential for many cycle utilizations.
Introduction The intercalation/deintercalation reactions of lithium within a host structure can be represented by the well known reaction host + xLi+ + xe- t-
Li host
From a practical view point, such a process illustrates how electrical energy can be stored in a reversible way in a battery where the host material plays the role of a positive electrode. It is also a novel synthetic route for obtaining new materials. In the practice, as intercalation occurs near room temperature, the involved materials are often metastable and cannot be obtained by the classical synthesis processes of solid state chemistry. Mat. Res. Soc. Symp. Proc. Vol. 210. 01991 Materials Research Society
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The overall reaction being carried out electrochemically, both half reactions concerned occur in distinct parts of the cell. Thus the variation of the cell voltage during the electroch
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